FOOTNOTE:

[I] "Submarine Warfare," by Commander S. Barnes, U.S.N.

CHAPTER III.
DEFENSIVE TORPEDO WARFARE—continued.

BY electrical submarine mines is meant those whose charges are ignited by the agency of electricity.

Submarine Mines during the Crimean and American Wars.—It was during the Crimean war (1854-6) that this description of defensive torpedoes was for the first time employed on actual service. Several of the principal Russian harbours were protected by this form of submarine mine, but owing to the smallness of their charges, and to the want of electrical knowledge on the part of the Russian officers and men in charge of them, none of the ships of the Allies were sunk, or even rendered hors de combat by this mode of harbour defence, though in several instances ground known to be covered with submarine mines was passed over by both English and French vessels of war.

Subsequently the Confederates, during the American civil war, employed electrical submarine mines in considerable numbers for the defence of their numerous harbours, rivers, &c.; but though in so far as the size of the torpedo charges was concerned, they did not make the same mistake as the Russians, yet, owing to the absence of proper electrical apparatus, and the want of any practical knowledge of the manipulation of electrical sea mines, on the part of the Confederate torpedoists, they were almost entirely unsuccessful in destroying the Federal warships; the Commodore Jones being the sole instance, out of the large number of vessels belonging to the Northerners which were sunk and severely injured by torpedoes, of a war steamer being sunk by means of electrical submarine mines.

In the Franco-German and Russo-Turkish wars which have lately occurred, electrical sea mines were very extensively used in coast defence, but with the exception of the loss of the gunboat Suna to the Turks, during the latter struggle, by this form of defensive torpedo, no other damage to vessels resulted from their use, yet owing to the vast moral power possessed by these submarine weapons, they were enabled to most effectually carry out the work of defence entrusted to their care.

Of late years many important discoveries have been made in the science of electricity, and vast improvements have been effected in electrical apparatus, to which causes may be traced the vastly improved system of electrical submarine mines as adopted by the English, American, and principal European governments at the present day, as compared with those that have hitherto been employed.

The certainty of action when required of electrical submarine mines, which is of course the desideratum of all torpedoists, has, by the improved mode and manner of ascertaining the exact electrical condition of each particular mine, and of the system as a whole, which is at present in vogue, been made almost absolute.

Advantages of Electrical Submarine Mines.—This form of defensive torpedo possesses numerous important advantages, the principal of which are as follows:—

1.—They are always absolutely under control.

Note.—By detaching or connecting the firing battery, which is effected by means of a plug, key, &c., they may be respectively rendered harmless, or dangerous. Thus friendly ships may pass over them in safety, whilst those of the enemy are debarred from so doing. On this account harbours, &c., protected by such mines are termed "Harbours of refuge."

2.—Fresh mines may be added to a system of such defensive torpedoes, thereby allowing an exploded mine to be replaced.

Note.—This is a very important point in connection with a system of defence by submarine mines, as in the case of a deep water channel, a hostile vessel being sunk by one of them, would not become an obstruction, as, were the channel a comparatively shallow one would most probably be the result, and therefore it would be necessary to put a fresh mine in the place of the exploded one; this would also apply were a mine to be prematurely ignited, or if any portion of its firing apparatus were injured.

3.—At night, or in a fog, no vessel can pass through a channel, &c., so protected without affording a means of ascertaining her presence.

Note.—This is also a very important advantage of a system of defence by electrical sea mines, affording as it does a complete safeguard against surprise.

4.—The power of obtaining proof, without going near it, by a system of testing that the electrical condition of the mine, &c., is perfect.

Note.—This again is an extremely important point. For were a charge to become wet, one of the electric cables of the mine broken, or damaged, &c., it would instantly be made apparent at the firing station, and could be at once remedied.

5.—They can be raised for examination, or removed when no longer required, with ease and safety.

Such are some of the chief advantages of employing the agency of electricity to effect the ignition of the charge in a system of defence by submarine mines.

Defects of Electrical Submarine Mines.—The following are the chief defects connected with the use of electrical mines:—

1.—The number of wires that are required to be used with them.

2.—The necessity of employing specially trained men in their manipulation.

In time there seems little doubt but that the former obstacle will be to a considerable extent overcome, but the latter must always be a flaw in an otherwise perfect system of coast defence by submarine mines.

Rules to be observed in using Electrical Submarine Mines.—In connection with a system of electrical submarine mines the following rules should be carefully observed:—

1.—They should be moored in deep channels, that is to say, where the larger class of vessels would in attempting to force a passage be obliged to go.

Note.—Mechanical submarine mines should never be used under these circumstances, as the difficulties of mooring them and keeping them in position would be very considerable, also a vessel being sunk in a very deep channel would not necessarily block it, and as a mechanical mine cannot be replaced, a gap would be left in the defence.

2.—They should be placed in the narrowest parts of the channel.

Note.—The object of this rule is evident, fewer mines being required, and consequently in the case of electrical ones, a far less number of wires are needed, which gives an increase of simplicity, and consequently more effectiveness. This point should be observed in connection with mechanical, as well as electrical submarine mines.

3.—They should where practicable be moored on the ground.

Note.—The advantages attendant on an observance of this rule are:—

a.—Increased vertical effect.

b.—Avoidance of mooring difficulties.

c.—Less liability of shifting from its original position.

d.—Less chance of its being discovered and rendered useless by an enemy.

e.—By far heavier charges may be conveniently employed.

4.—Where possible, no indication whatever should be given of the position of the mines by their circuit closers, or in the case of small buoyant ones, by the mines themselves.

Note.—In some instances this will be almost impracticable, as for example, where there is a very great rise and fall of tide. For instance, at Noel Bay in the Bay of Fundy, the rise is over fifty feet. Here, when circuit closers, or small buoyant mines are used, both of which ought never to be more than twenty feet below the surface, long before low water they would be found floating on the surface in full view. Many attempts have been made to overcome this difficulty, but as yet no really practicable means have been devised.

5.—The stations where the firing batteries, &c., are placed, should be in the defensive work likely to be held the longest, thus enabling the mines to be commanded up to the last moment.

6.—The electric cables should be laid in positions such that their discovery by the enemy would be extremely difficult, and almost impossible.

Note.—This may be to a certain extent effected by leading them from the mines to the firing and observing stations by circuitous routes, and by burying them in trenches.

7.—They should not be thrown away on boats.

Notes.—As they can in all cases be fired by will, even when circuit closers are used, this rule is easily observed. But to prevent an enemy's boats from rendering the mines useless, a line of small torpedoes might be placed in advance of the large ones, or the circuit closers themselves might be charged.

At night, or in foggy weather it will be necessary to employ guard-boats, electric lights, &c., to protect them against damage by an enemy's boats, &c.

In the foregoing pages of this chapter will be found the requirements and conditions essential to a perfect system of electrical submarine mines for the defence of a harbour, river, &c.; in the following pages a general description of the component parts of such defensive torpedoes, under the following heads—Form and Construction of Case; Electrical Fuzes; Electric Cables; Watertight Joints; Junction Boxes; and Mode of Mooring, will be considered.

Form and Construction of Torpedo Case.—The case of a submarine mine should be capable of fulfilling the following conditions:—

1. It must be able at great depths to withstand a great pressure of water, and remain perfectly watertight.

Note.—This in the case of a charge of gunpowder being an imperative necessity.

2. As a buoyant mine, it must be capable of affording a considerable excess of buoyancy, by which it may be rendered stationary when moored.

Note.—This is generally obtained by having an air space within the torpedo, thus requiring a much larger case in which the charge is enclosed than would otherwise be necessary, causing increased difficulties in transportation, mooring, and raising them for examination, &c.

3. When explosive agents which require a certain time for thorough combustion are used as the charge, such as gunpowder, picric powder, gun-cotton (not fired by detonation), &c., a much stronger case is necessary to obtain the full explosive effect than would be the case were detonated charges, under the same conditions, employed.

Note.—This is an extremely important point, for if a weak case is employed with a charge of gunpowder, &c., fired by a fuze primed with powder only, a portion of it on being fired would generate a sufficient quantity of gas to burst the case, thus blowing out the remainder of the charge before its ignition had been effected.

4. It should be of such a form that the complete ignition of the charge is obtained by the employment of the least number of fuzes possible to effect this result.

Note.—This point is especially to be observed when gunpowder is the explosive agent.

The various forms of defensive torpedo cases may be classed under the following heads:—

Spherical Shape.—This form of case is theoretically the very best one possible to devise, but on account of the difficulty of constructing it, and its comparative costliness, such a form may be put aside as being impracticable.

Cylindrical Shape.—Torpedoists in general have hitherto adopted the cylindrical form of case as being the best adaptable for both ground and buoyant mines containing a heavy charge.

The Confederates employed exclusively this shape for their electrical submarine mines, which were ground ones, and the Austrians in the war of "66" approved of this form of case for their electrical submarine mines, which were buoyant ones. [Fig. 19] and [20] represent respectively the American and Austrian mines.

In England the cylindrical shape has up to quite lately found most favour with her torpedoists for both buoyant and ground mines. At [Fig. 21] is represented a 100-lb. buoyant electrical mine, surrounded by a wooden jacket, e, and having its circuit closer, C, enclosed within it; and at [Fig. 22] is shown a 250-lb. electrical mine, which may be used either as a buoyant or ground one.

For large ground mines, the best form of torpedo case seems to be that of the turtle mine, which is shown at [Fig. 9]. A heavy charge may be contained in it; it forms its own anchor; and it would withstand an explosion of an adjacent mine without sustaining any injury. At present the cylindrical shape is the form generally used, though as far as retaining its position on the ground in a strong tide, it cannot be compared to the turtle form.

FORM OF CASE OF SUBMARINE MINES.

The Conical Shape.—Hitherto this shape of submarine mine case was only used in connection with mechanical mines, but now it is the form considered most suitable for all buoyant mines, electrical or mechanical. At [Fig. 23] is shown the conical shaped mechanical mine, employed by the Confederates for use with sensitive fuzes. The conical form of torpedo case lately approved of by the English torpedo authorities is somewhat similar to that one, the charge being contained in a kind of box hung from the top of the case, and the circuit closer is screwed into the bottom of the case; surrounding the upper part of the case is a thick buffer of wood, by which damage to the mine is prevented by the passage of friendly ships. This is altogether a very neat and serviceable form of torpedo case. This form of case is also more difficult to discover by dragging, and easier to retain in position.

Electrical Fuzes.—The fuzes employed in connection with electrical submarine mines may be divided into two classes:—

1. Platinum wire bridge fuzes.

Note.—That is where the evolution of heat is caused by a large quantity of the electric force flowing through a good conductor of large section, such as the copper core of electric cables, being suddenly checked by a very thin wire composed of a metal which compared with the conductor offers a very great resistance, such as platinum.

2. High tension fuzes.

Note.—That is where the evolution of heat is caused by the electric spark, or by the electric discharge taking place through a substance which offers very great resistance to the passage of the electric force.

Platinum Wire Fuze.—This is the form of electrical fuze most commonly used, and which will most certainly supersede altogether the high tension fuze.

There are numerous advantages accruing from the use of platinum wire fuzes, the chief of which are here enumerated:—

a.—Great facilities for, and entire safety whilst testing the circuit.

b.—Extreme simplicity of manufacture.

c.—Non-liability to deteriorate.

d.—Perfect insulation of the electric cables used in connection with submarine mines not necessary.

English Service Platinum Wire Fuze.—The following is a description of the platinum wire fuze of the form adopted in the English service, a section of which is shown at [Fig. 24]. It consists of a head of ebonite a, hollowed out, in which a metal mould is fixed, the wires which have been previously bared are inserted into holes in this mould, and firmly fixed thereto by means of a composition poured into the mould, whilst hot; this is shown at b. The two bared ends of the wires which project beyond the metal mould, as c, c, are connected by a bridge of platinum-silver wire ·0014" in diameter and weighing ·21 grs. per yard. This is effected as follows:—

A very fine shallow groove is made in the flat ends of the bare wires c, c, and the platinum-silver wire is laid across in the incisions, and fixed there by means of solder. The length of the bridge d is ·25."

A tube e, made of tin, and soldered to a brass socket f, is fixed by means of cement to the ebonite head a; in this tube is placed the fulminate of mercury, the open end of the tube g being closed with a pellet of red lead and shellac varnish; around the bridge of the fuze is placed some loose gun-cotton.

McEvoy's Platinum Wire Fuze.—Another form of platinum wire fuze, which has been devised by Captain McEvoy, formerly of the Confederate Service, is shown at [Fig. 25]. It consists of the head a, formed of a mixture of ground glass, or Portland cement, worked up with sulphur as a base: this mixture when hot is poured into a mould, in which the two insulated copper wires, b, b, have been previously placed; when cold, the mixture with the wires affixed is removed from the mould, and the platinum wire bridge c being secured to the bare ends of the copper wires, the whole is firmly fixed in a brass socket d, by means of cement; the space e is filled with loose dry gun-cotton, so as to surround the bridge c; a copper tube f, closed at one end, is partly filled with fulminate of mercury, and when the fuze is required for service, this tube is secured to the brass socket d by means of cement.

In this form of low tension fuze there is no liability whatever of any injury being caused to the bridge by the working of the wires in the head, or by damp even after lying in the water for a month or more. One peculiarity of this fuze is that the composition is run over the insulated wires without materially softening the dielectric, or affecting in the slightest degree the insulation of the wires.

High Tension Fuzes.—The high tension fuze was devised for use with electrical submarine mines, in the place of the platinum wire fuze, on account of the little knowledge possessed, in the early days of submarine warfare, in regard to the manipulation of Voltaic batteries.

Platinum wire requires a temperature of some 500° F. to heat it to incandescence, and therefore necessitates the use of a powerful Voltaic battery, both in intensity and power, to effect the ignition of gunpowder by this means at considerable distances.

The Grove and Bunsen pile were the only suitable form of Voltaic battery known at the period of the introduction of high tension fuzes, both of which possessed the defects of uncertainty and inconstancy, and also were by far too cumbersome and too difficult to keep in effective working order to be of any real practicable value.

High tension fuzes may be ignited by means of either an electro-magneto machine, an electro-dynamo machine, a frictional machine, or by a Voltaic battery, generating an electric current of high intensity. Various kinds of this form of electrical fuze have been designed, the principal of which are as follows:—

Statham's Fuze.—A section and elevation of this electric fuze are shown at Fig. 26; a, b is a gutta percha tube, with an opening cut in it, as shown in figure. The interior of this vulcanised gutta percha tube is coated with a thin layer of sulphide of copper, which coating is obtained by leaving a bare copper wire for some time in connection with the above-mentioned tube. The extremities of two insulated copper wires c, c, considerably smaller than the conducting wires, are uncovered, scraped, and then inserted into the tube a, b, with an interval of ·15 inch between them. The wires are then bent as shown in the figure, and the priming placed between the terminals. The whole is covered with a gutta percha bag, which is filled with fine grained gunpowder. The priming substance is composed of fulminate of mercury worked up with gum water. The objection to this fuze, which was used by the Allies in their destruction of the Russian fortifications at Sebastopol, is the want of sensitiveness of sulphide of copper, and the consequent necessity of a very powerful firing battery.

Beardslee's Fuze.—This high tension fuze is shown at [Fig. 27]. It consists of a cylindrical piece of soft wood a, which is about three-quarters of an inch in length and in diameter; two copper nails, b, b, are driven through this piece of wood a, in such a way that while the two heads come together as close as possible without absolutely touching, the pointed ends are some distance apart from each other, and project through the wood a; two insulated copper wires, c, c, are firmly soldered to these projecting ends, and a piece of soft wax, d, is pressed around the junction points. In a groove, across the heads of the copper nails, is placed a little black lead, to which is added a minute quantity of some substance, the nature of which is known only to Mr. Beardslee. Several folds of paper are wrapped round the wooden cylinder, forming a cylinder about 2-1/2 inches long, one end of which is tightly fastened round the insulated wires as at e. The other end of the cylinder is then filled with powder, f, and closed by a piece of twine. The whole fuze is then coated with black varnish. Though not highly sensitive, Beardslee's fuze is exceedingly efficient, and extremely simple.

Von Ebner's Fuze.—This form of fuze was devised by Colonel Von Ebner of the Austrian Engineers. A section and elevation of it is shown at [Fig. 28]. It consists of an outer cylinder, a, of gutta percha, and an inner one of copper, b, which latter encloses a core formed of ground glass and sulphur, c, which core is cast round the two conducting wires d, d in such a way that they are completely insulated from one another. In the first instance the wire is in one continuous length, the opening e being subsequently made, and carefully gauged, so as to ensure a uniform break, or interval in the conductor of each fuze. The priming composition, which consists of equal parts of sulphide of antimony and chlorate of potash, is placed in the hollow f, to which is added some powdered plumbago, for the purpose of increasing the conducting power of the composition. This mixture is put into the hollow, f, of the fuze under considerable pressure, the terminals being connected with a sensitive galvanometer, in circuit with a test battery, and the pressure applied so as to obtain, as far as possible, uniformity in the electrical resistance of each fuze.

The Austrians employed this form of high tension fuze in connection with a frictional machine for the electrical mines used in their defence of Venice, &c. during the war of 1866.

Abel's Fuze.—Mr. Abel devised a high tension fuze, which in 1858 was extensively experimented with; the Beardslee and Von Ebner fuze being based upon the principles applied for the first time in Abel's fuze.

ELECTRICAL FUZES.

Many modifications of it have been from time to time devised by Mr. Abel; a section and elevation of the more recent form of his fuze is shown at [Fig. 29]. It consists of b, b, a body of beech wood, hollowed for half its length, in which space the priming charge is placed; it is also perforated by three holes, one vertical for the reception of the capsule of sensitive mixture, the other two horizontal, in which the conducting wires are placed; a, a are two insulated copper wires, passing into the vertical hole, and resting on the sensitive mixture; in a cavity, d, of the body of the fuze is placed some mealed powder, which is fired by the ignition of the sensitive mixture on the passage of the electrical current.

The insulated wires used in connection with this fuze consist of two copper wires, about 2 inches long, and ·022 inch in diameter, enclosed in a covering of gutta percha ·13 inch in diameter, and separated about ·06 inch from each other.

At one end the wires are bared to 1·25 inch, at the other they are merely cut across by a very sharp pair of scissors. This end of the double covered wire is inserted into a paper cylinder c, c, which holds a small quantity of the priming mixture. This capped end of the wires is inserted into the wooden body of the fuze through the vertical hole i, and projects ·15 inch into the cavity d. The bare ends of the double covered wires are pressed into small grooves in the head of the cylinder e e, and each extremity is bent into one of the small channels d' d', which are at right angles to the vertical perforation. d' d' are two small copper tubes driven into these channels over the wire ends, to keep the wires in position, and to form the opening into which the conducting wires f are inserted and bent round, as at e'.

The priming mixture of Abel's original fuze, which was the one used by the Confederates, was composed of 10 parts of subphosphide of copper, 45 parts of subsulphide of copper, and 15 parts of chlorate of potash. These ingredients reduced to a very fine state of division, and intimately mixed, in a mortar, with the addition of a little alcohol, are dried at a low temperature and preserved in bottles until required for use. The sensitive mixture used by Mr. Abel more recently for his submarine electrical high tension fuzes, is composed of an intimate mixture of graphite and fulminate of mercury. By the process of ramming, the electrical resistance of the fuze is regulated.

Extempore Fuzes.—It may be necessary in some cases, when a specially manufactured fuze is not attainable, to make a fuze on the spot. The following is a neat and simple method of constructing an extempore high tension fuze.

Fisher's Extempore Fuze.—This form of fuze was devised by Lieutenant now Captain Fisher, R.N. It consists of a small disc of gutta percha, through which the ends of two wires are inserted about 1/4 inch apart, their ends terminating in small copper plates formed by hammering down the wire. These flat ends should be fixed parallel, and in the first place in contact with one another, also should be level with the surface of the gutta percha. The other two extremities of the wires are then placed in circuit with a sensitive galvanometer and a test battery; the needle of the former deflects violently, there being a complete metallic circuit; the flat ends of the wires or poles of the fuze are then separated very carefully, until the needle just ceases to deflect. In the space thus formed, a little scraped charcoal is placed, and rammed in by a piece of wood. By the application of pressure, any degree of sensitiveness may be attained, merely observing the deflection of the galvanometer needle. Over the charcoal a little powdered resin is shaken, and pressed down, by which means the charcoal is fixed in position, and owing to the inflammability of the resin, the ignition of the gunpowder priming is ensured. The disc of gutta percha is then placed in an empty Snider ball cartridge, &c., and by the application of a little warm gutta percha applied externally, the holes where the projecting ends of the wires pass are closed, and the disc is fixed and insulated. The case is then filled with some mealed powder and fine grained powder, on the top of which is placed a little cotton wool, and the whole pressed down tightly with the finger, the open end of the case being then choked, as in Beardslee's fuze and Abel's extempore one. The apex is then covered with some warm gutta percha, and the whole of the fuze coated over with red sealing-wax dissolved in methylated spirits.

Insulated Electric Cables.—For the work of defence by electrical submarine mines, the wires along which the electric current flows have, on account of their being led underground and through the water, to be covered with some substance which shall prevent the current during its passage from escaping to earth, or in other words, they (the wires) must be insulated.

The substances in general use for such purposes are as follows:—

Gutta Percha.—This substance was used by Messrs. Siemens in the cables manufactured by them for the Austrian government in 1866, and is to some extent still employed, though Hooper's material or vulcanised india rubber, has been found to be more suitable. The dielectric, gutta percha, possesses the following advantages:—

a.—It can be put on the conducting wire, as an unbroken tube.

b.—It only absorbs 1 per cent. of water.

c.—It has the property of clinging to the metallic conductor, by which is meant, that should it (conductor) be cut through, and any strain be brought on the cable, there is a tendency on the part of the gutta percha to cling to the conducting wire, thereby not increasing the fault.

The defects of such an insulator are:—

a.—Its liability to become hard and brittle when exposed to dry heat, and consequently it requires to be stored under water.

b.—It becomes comparatively a bad dielectric at 100° F.

c.—It becomes plastic at high temperatures, which causes the conducting wire to alter its position.

In some particulars ordinary india rubber is a better insulator than gutta percha, but this substance is equally inferior to Hooper's material, &c. The advantages possessed by this substance are:—

a.—It is not easily affected by a dry heat.

b.—It is a very excellent dielectric.

The defects of this mode of insulation are:—

a.—It must be put on the conducting wires in a series of jointed pieces.

b.—It does not cling to the conducting wire, so that if the electric cable be cut, and any strain be brought on it (cable), the previous fault is increased.

c.—It absorbs 25 per cent. of water.

Hooper's Material.—This insulating material consists of an inside coating of pure india rubber, then another similar coating in conjunction with oxide of zinc, which is termed the separator, and an outside coating of india rubber combined with sulphur. The use of the separator is to prevent any damage to the conducting wires by the action of the sulphur. The three coatings are then baked for some hours at a very high temperature, which fuses the whole into a solid mass, and vulcanises the outer coating. The properties of the pure india rubber which is in contact with the metallic conductor are thus preserved, while any decay of the outer covering is prevented by the vulcanising process.

The advantages claimed by Mr. Hooper for this mode of insulating electric submarine cables, are:—

a.—High insulation.

b.—Flexibility.

c.—Capability of withstanding the bad effects of dry heat.

The qualifications essential to a perfect insulated electrical cable for use with submarine mines are as follows:—

1.—Capacity to bear a certain amount of strain without breaking.

2.—Perfect insulation, or at least as nearly so as it is possible to obtain, and composed of a substance capable of being readily stored, and kept for a considerable length of time without being injured.

3.—Pliability so that it may be wound on, or paid out from, a moderately sized drum without injury.

4.—Provided with an external covering capable of protecting the dielectric from injury when used in situations where there is a rocky or shingly bottom, &c.

The insulated wire of a submarine cable is technically spoken of as its core.

By a cable is meant to be understood any piece of covered wire.

Several forms of submarine electrical cables have been devised, all of which more or less possess the qualifications enumerated above. The following are some of the most effective:—

Siemens's Cable.—This form of cable is represented at [Fig. 30]. It consists of a strand a, which is composed of three or more copper wires formed by laying up the several single copper wires spirally, several layers of gutta percha, or india rubber, b, two coverings of hemp, saturated with Stockholm tar, c and d, and several plies of copper tape e, wound on, so that each strip overlaps the preceding one, as shown at [Fig. 30]. The conductivity of the copper employed for the strand is equal to at least 90 per cent. of that of pure copper.

This exterior covering of copper tape is a patent of Messrs. Siemens Brothers, and when once laid down, the cable so covered is very efficiently protected, and of course it is little affected by the action of the sea water. This mode of protection has one great defect, viz., that in the event of a kink occurring in paying out the line, and at the same time a sharp strain being applied, the copper tape at that point is extremely likely to destroy the insulation by being drawn in such a way as to cut through the dielectric. On this account great care must be observed in handling this form of cable.

In practice precautions must be taken to prevent the copper tape covering from being brought into contact with any iron, for were such to happen, electrical action would at once ensue, causing the iron to corrode with enormous rapidity.

In some of Siemens's cables, vulcanised india rubber replaces the gutta percha insulation. Iron covered cables, either galvanised or plain, are manufactured as well as the copper tape covered ones by that firm.

Hooper's Cable.—This form of cable is represented at [Fig. 31]. It consists of a metal conducting wire, generally copper, a, covered with an alloy to protect it from chemical action, the insulating substance b, known as Hooper's material, previously described at [page 39], a covering of tarred hemp c, and an outer covering of iron wires (No. 11 B. W. G.), each of which is separately covered with tarred hemp and wound on spirally, d.

Gray's cable is very similar to the one just described, the chief difference in it as compared with Hooper's being the absence of the separator.

Silvertown Cables.—The following is a description of the core of an electrical submarine cable, which is used by the English government, and is supposed to contain all the advantages of the foregoing, and none of their defects. It consists of a strand conductor of four copper wires (No. 20 B. W. G.) of quality not less than 92 per cent. of pure copper, and possessing an electrical resistance of not more than 14 ohms per nautical mile. This strand is tinned and insulated with vulcanised india rubber to a diameter of ·24 inch, and then covered with a layer of felt, and the whole subjected to a temperature of 300° F. under steam pressure. This forms the core of the various kinds of cables employed in connection with a system of defence by electrical submarine mines, which are enumerated as follows:—

1.—Single core armoured cable.

2.—Multiple cable.

3.—Circuit closer cable.

4.—Single core unarmoured cable.

5.—Special cables for firing by cross bearings.

Single Core Armoured Cable.—This form of cable is used in connection with each mine of a group or system, and also to connect forts, &c. across an arm of the sea. Over the core, which has been fully described, is laid a spiral covering of tanned, picked Russian hemp, over this are laid ten galvanised iron wires (No. 13 B. W. G.), each one of which is covered with a similar hemp, which is laid in an opposite spiral to the former similar covering, with a twist of one revolution in about thirteen inches; in order to prevent these wires from gaping when the cable is kinked, a further covering of two servings of hemp passed spirally in opposite directions is laid, and the whole passed through a hot composition of a tar and pitch mixture. Exterior diameter of this cable is 7/8 inch. Its weight in air is 27-50/112 cwt., and in water 14-40/112 cwt. per nautical mile. The breaking strain of a cable thus manufactured is 62-1/2 cwt., and its cost about £47 per nautical mile. A diagram of this cable is shown at [Fig. 32].

Multiple Cable.—This form of cable is employed in cases where it is necessary to carry a large number of cables into the firing station, &c. It consists of seven single cores formed into a strand, over which a padding of hemp fibres is laid longitudinally, and over this again is laid an armouring of sixteen (No. 9 B. W. G.) galvanised iron wires, each one of which is covered with a layer of tarred tape put on spirally with a twist of one revolution in 15 inches. The exterior covering consists of two layers of hemp and composition, which is laid on with a short twist, and in opposite directions. The external diameter of this cable is 1-1/4 inch. Its weight in air and water is 78-25/112 cwt., and 45-32/112 cwt. respectively per nautical mile. Its breaking strain is 135 cwt., and cost about £357 per nautical mile. This form of cable is used in connection with a junction box, from which the single armoured cables leading to the different mines radiate, and is shown at [Fig. 33].

Circuit Closer Cable.—This cable, which connects the mine and circuit closer, has been found to be subjected to exceptional wear and tear, and therefore requires a special form of exterior protection. The core of this cable is the same as the one described at [page 41], also it is covered with a similar padding of hemp, but instead of the iron wires as in the case of the multiple cable, &c., nine strands, each of which is composed of fourteen No. 22 Bessemer Steel Wires, are wound on, each such strand being covered with hemp, which is put on with a twist of one revolution in every 7-1/2 inches, the external covering being the same as in other cables.

This form of armouring for an electric cable possesses the qualifications of pliability, lightness, and great tensile strength. Its weight in air is 52-106/112 cwt., and in water 28-4/112 cwt. per nautical mile. Its breaking strain 65 cwt., and cost about £127 per nautical mile.

Single Core Unarmoured Cable.—This form of cable is used in a system of defence by submarine mines to connect the detached works of a maritime fortress, &c., for the purpose of telegraphing.

It consists of the ordinary service core, over which are laid two servings of tarred hemp, put on spirally. The weight of this cable in air is 4-13/112 cwt., and in water 1-36/112 cwt. per nautical mile; its breaking strain is 7-1/2 cwt., and its cost per nautical mile is about £35.

Special Cables.—In firing electrical submarine mines by means of cross bearings, a special cable is employed. As a general rule there would be three lines of mines placed to converge on one of the stations.

Each of these lines would be provided with a conducting wire in connection with the firing arrangements, while one line of wire in connection with the firing station would be required for telegraphing. For the purpose in question a four cored cable is used.

Land Service Cable.—The cable employed for this service consists of a core formed similar to that of the multiple cable, described at [page 41]; over which is laid a padding of hemp, and finally two servings of tarred hemp laid spirally in opposite directions are wound on. Its weight in air is 16 cwt., and in water 4-50/112 cwt. per nautical mile. Its breaking strain 17-1/2 cwt., and cost per nautical mile about £137.

Sea Service Cable.—This consists of a similar core to the land service cable, and padding of hemp, over which is laid an armouring of fifteen No. 13 galvanised iron wires, each one being covered with tarred tape, and finally the ordinary servings of tarred hemp. Its weight in air is 49-101/112 cwt., and in water 25-109/112 cwt. per nautical mile. Its breaking strain 65-100/112 cwt., and cost per nautical mile about £202.

When frictional electricity is used to fire high tension fuzes, it has been found by experiment that if several lines of insulated cables are laid in the same trench for a few hundred yards, the inductive effect of the electrical charge generated by a frictional machine is so great that its discharge through one cable is sufficient not only to fire the fuze in immediate connection with it, but by induction every other fuze in connection with the remaining wires laid in the trench. And this effect equally occurs when the electric cables are some feet apart, provided they run parallel for a few hundred yards, and whether the shore ends of the cables, the fuzes in connection with which are not intended to be fired, are insulated, or put directly to earth, the connections beyond the fuzes being to earth, or even insulated, provided a very few yards of conductor exist beyond the fuze.

The length of wire which it is necessary to use between the mine and its circuit closer would be quite sufficient for the purpose of effecting ignition by induction. With platinum wire fuses there is no danger whatever of the above happening, nor in the case of high tension fuzes is there so much danger of ignition by induction, when a constant instead of a frictional electric battery is used to generate the current.

Another mode of protecting an insulated cable is to place it, as it were, in the core of a hempen cable. In forming the rope on the cable, great care is necessary to prevent any serious amount of torsion, or tension coming on the insulated wire, either of which would most assuredly result in injury to the cable. This form of cable might in connection with obstructions, &c., be of great use, as on account of its closely resembling an ordinary rope, it would be very unlikely to excite suspicion, and so would most probably be cut, the result of which, by previous arrangement, would be an explosion of a mine, or by means of a galvanometer, &c., an indication that the obstructions, &c., were being interfered with.

Jointing Electrical Cables.—This is a very important point in connection with a system of defence or offence by electrical torpedoes. In many instances it will be found necessary to join either two lengths of cable, or an insulated wire and a cable, together, in both of which cases, great care must be used in making the joints, so that the insulation and the continuity of the circuit may be perfect.

ELECTRIC CABLES, EXTEMPORE CABLE JOINTS.

Many species of junctions have been from time to time devised, the most practical and generally employed of which are:—

India rubber Tube Joint.—This form of joint is a very useful one for extempore purposes, being easily and quickly made, and being very effective. At [Fig. 34] is shown a sketch of such a junction. About 1·5 inches of the copper conductor of the two insulated cables are laid bare and connected together by means of Nicoll's metallic joint, as shown at [Fig. 36], or by turning one of the conductors round the other, their ends being carefully pressed down by means of pliers, to prevent any chance of the india rubber tube being pierced; over the splice thus formed serve some twine, and over the whole put a coating of india rubber cement, grease, &c., then draw the vulcanised india rubber tube, which has been previously placed on one of the insulated cables, over the splice a, as shown at b, and secure it firmly by means of twine, c, c, and then to prevent any strain being brought on the joint, form a half-crown as shown in [Fig. 35] at A.

In forming the splice, it is very important that the metallic ends should be perfectly clean. The danger to this mode of jointing of the piercing of the tube by the ends of the conductors is entirely removed by employing the Nicoll metallic joint, which is formed as follows:—

Nicoll Metallic Joint.—One of the conducting wires, as a, [Fig. 36], is formed into a spiral twist by means of a very simple instrument, and the other wire b, which is left straight, is inserted into the spiral, the whole being placed on an anvil, and pressed closely and securely together by a single blow of a hammer.

Mathieson's Joint.—This somewhat complicated, though very effective mode of jointing, which is adopted in the English torpedo service, is shown at [Fig. 37], in elevation and section. It consists of two ebonite cylinders a, a, through which the cables to be connected are passed. Within these cylinders an ebonite tube b, b is placed, the ends of which are wedge-shaped, and which press against two vulcanite rings c, c; in the interior of this tube b, b is the metallic joint d of the two cables. The centre of the tube b, b is of square section, and fits into a hollow of similar form in the cylinders a, a, the object of this being to prevent any twisting of the wires during the process of screwing up, which would be liable to injure the metallic joint d.

The manner of making this joint will be easily understood from the figure. With this, as with all other temporary joints, it is advisable to form a half-crown in the cable, including the joint.

Beardslee's Joint.—This form of temporary joint when used with strand conductors, which are composed of a number of small wires, has been found to be exceedingly useful and effective, the only defect of such a joint being the liability of straightening the wires of the conductors should a direct strain be brought upon the wire extremities. [Fig. 38] represents a section of this joint; it consists of an ebonite cylinder a, one end of which is solid, and the other open and fitted with a screw thread, into which is screwed a plug b; through both the plug b, and the solid end of the cylinder a, perforations are made just large enough to admit the insulated wires c, c; about half an inch of the extremities of these wires are bared and cleaned, and then passed, the one through the plug b, a disc of vulcanised india rubber d, and a metal disc e, and the end of the strand conductor turned back on the face of this metal disc, the other through the perforation in the solid end of the cylinder a, then through similar discs d and e, and the end of the strand conductor treated in the same manner as the former one; then by means of the screw plug b, the two metallic discs b, b, and consequently the bare extremities of the strand conductors are brought into close metallic contact.

McEvoy's Joint for Iron Wire covered Cable.—This form of joint is shown in section at [Fig. 39]. Two brass caps a, a are slipped over the ends of the cables required to be joined, then the iron wire and other coverings of the cables down to the insulating substance are removed, the former being bent back close against the bottom of the caps a, a, as shown in [Fig. 39] at b, b; the cores of the cables are then joined by an india rubber temporary joint c, which has been described at [page 45]: the whole is then placed in the body of the joint, and the brass caps a, a screwed up, jamming the bent back iron wires against a solid piece of brass d, d, by which means a firm and perfect joint is made in the cables.

PERMANENT JOINTS FOR ELECTRIC CABLES.

[Fig. 40] represents a section of a McEvoy temporary joint for single cored unarmoured cables, which seems to fulfil all the conditions necessary to a perfect joint of that description. This joint is, with the exception of there being two screw plugs instead of one, very similar to Beardslee's joint described at [page 46]; this alteration is a great improvement, remedying as it does the one defect of Beardslee's joint, viz., the liability of the cables to be drawn apart due to any great tension being brought on them.

A permanent joint in electrical submarine cables, which from its nature requires to be an exceptionally good one, is a somewhat difficult and troublesome operation, and also requires a considerable time to form a thoroughly reliable one.

Siemens's Methods of Jointing.—The following methods, and instructions for forming such joints, are those adopted by Messrs. Siemens Brothers in connection with their telegraph cables, and which will be found generally applicable to all insulated cables.

The Formation of a Joint in the Conductor of an Insulated Cable.—The conductor is either covered with a gutta percha or an india rubber dielectric. In both cases cut off the dielectric so as to bare the conductor-wire for a length of about three inches, taking care never to cut at right angles to the conductor-wire, for fear of injuring it with the cutting-knife or scissors.

Then clean the wires forming the strand with file-card and emery-paper, and solder them into a solid bar for a length of about one inch.

Having soldered the wires, forming the ends of the two lengths of conductors to be joined, into two solid rods, file each of them off in a slanting manner, so that they will form a scarf-joint when put together.

Place the two ends of strand in the two small vices on a stand which is supplied for the purpose, so that the two scarfed ends overlap each other, and bind them round with a piece of fine black iron wire, in the shape of a spiral, so as to keep the ends close together, then solder the two ends together by applying a hot soldering iron.

Then remove the iron binding wire and clean up the joint, filing off all unnecessary solder.

And make a band of four fine tinned copper wires, and bind them tightly side by side round the joint, covering the whole length of the scarf, and then solder the band and joint solidly together.

Then make another band of four fine tinned copper wires and bind them round the joint in the same manner as before, but extending about a quarter of an inch beyond each end of the other binding wire, the parts only of this second binding which project beyond the end of the first binding are to be soldered, so that the centre part remains loose and may keep up a connection between the two ends by forming a spiral between them in the event of the scarf giving way and the two ends of the conductor separating slightly.

This form of joint is called the "spring" joint.

The finished joint should be washed with spirit of wine and brushed, so as to take away all particles of soldering flux, and to avoid oxidation of the wire. The washed joint should then be dried with a piece of cloth and exposed to the flame of a spirit lamp to dry it thoroughly. A cable conductor ought never to be jointed with the help of soldering acid, but with that of resin, sal ammoniac, or borax only, so that any chance oxidation, and consequently destruction, of the conducting wire may be avoided.

There are other modes of jointing conductors, such as the twisting and scale joint, but the foregoing method will sufficiently explain this part of electric cable work.

The Formation of a Joint in an India rubber Insulated Cable.—In making a joint in any insulated cable, the very greatest care must be taken to keep the hands, tools, and materials clean and dry.

Remove the felt for about twelve inches from each end of the core by soaking it with mineral naphtha and then rubbing it off clean with the file-card. The cleaned surface sear with a red-hot iron, to burn off all remaining fibres of the felt. Wash these seared ends clean with naphtha.

Then cut off about four inches of the insulating material (taking care never to cut at right angles to the conducting wire for fear of injuring it) so as to leave enough of the conductor bare to join and solder in the manner described at [page 47].

After the conductor is jointed and soldered, clean again the seared parts of the insulator with the glazed side of the squares of cloth moistened with mineral naphtha, so as to leave a clean adhesiveness only; taper again the insulating material down to the conductor for about two inches on each side of the conductor-joint with a pair of curved and very clean scissors.

The tapering must be completed in such slanting way that the different layers of the dielectric are so far exposed as to enable a secure laying on of the new jointing material.

India rubber core consists chiefly of three layers of insulating material: the first layer next to the strand is called the pure or brown; the second layer is the white or separating; the third layer is the light red or jacket rubber.

Coat the conductor with a pure (brown) rubber tape tightly laid on in a spiral form, commencing at the spot where the separator (white) ends, across the corresponding place on the opposite side of the joint and back again in a contrary direction. The ends are fastened down by pressing a clean, heated searing-iron or a heated knife on them. By doing so the band will stick; the remaining portions of the band to be cut off with the scissors.

Lay on tightly the separating india rubber tape in the same manner, but beginning where the jacket or outer layer of rubber ends. One lap will be sufficient.

Complete the insulation by lapping on tightly two layers of red india rubber tape: the last lap must cover each end of the core to four inches on each side of the conductor-joint, or extend to the searing or tackiness, but not beyond it.

Lay on three tight bindings of the cloth tapes, all in the same direction, care being taken to avoid wrinkles. The ends of the cloth tapes are cemented down with a thin coating of india rubber cement.

Immerse the joint in the jointing-bath at 150° to 200° F. and gradually raise the heat so that in half an hour the temperature will be 320° F., at which temperature keep the joint for twenty minutes: then take it out and let it cool in the open air.

The Formation of a Joint in a Gutta percha Insulated Cable.—Having jointed the conducting wires in the manner described at [page 47], clean and dry the joint well and cover the bare conductor with a thin layer of compound. This is best done by heating a small stick of compound to nearly its melting point, and rubbing it over the bare conductor, which has been previously heated with the flame of a spirit-lamp.

Heat the gutta percha covering of both ends gently until it is quite soft, without, however, causing it to bubble or burn. Draw, then, with the fingers, the gutta percha coverings of both ends down, tapering them off until they meet in the middle of the joint; heat them sufficiently to make them adhere together.

Apply a layer of compound on the tapered-off gutta percha in the same manner as described for coating the bare conductor, and cover it with a first coating of gutta percha sheet to about half the thickness necessary to finish the joint. This is done by heating a small sheet of gutta percha, of about one-eighth of an inch in thickness, until it is quite soft, and by pressing it in that state round the joint to the required size; the greatest care to be taken to expel all the air.

The projecting lips are then cut off with a pair of curved scissors. The seam thus produced is to be rubbed with a hot iron until it is completely closed and the joint well rounded off.

Apply another layer of compound and a second layer of gutta percha in exactly the same manner as described for the first layer; care, however, is to be taken to get the seam in this second layer of gutta percha not over, but as nearly as possible right opposite to, the seam in the layer underneath.

The whole to be worked as cylindrical as possible, and to a size not exceeding the original core. The joint, so far finished, is then to be cooled with water until the gutta percha is quite consolidated.

Another, the overlapping gutta percha joint, is made in the following manner:—

Cut off the two ends of the core, so that the gutta percha and the conductor-wire are flush. Warm the gutta percha for a distance of about three inches from each of the ends with the flame of a spirit lamp, and, when sufficiently soft, push it back until it forms an enlargement. The two ends of the conductor are then to be soldered according to instructions for making joint in conductors.

To have a perfectly clean surface of the two gutta percha enlargements, remove all impurities by the way of peeling them with a sharp knife. Warm gently both knobs and the copper joint, and cover the whole length of the bare wire with compound, planing it with a warm smoothing-iron.

Draw then with the fingers one of the warmed and softened knobs carefully up to the other knob or enlargement, leaving on its way a perfect tube of gutta percha upon the wire, decreasing gradually to the thickness of the copper strand towards the other knob. Any superfluous gutta percha is removed. This scarf is finished with a warm smoothing-iron, so as to unite it to the compound on the wire strand, and a thin layer of compound is also put over the scarf in the same manner as before.

The other knob is then warmed and drawn in the same way over the tube already formed, which is at the same time heated sufficiently to make the two adhere.

Apply a layer of compound on the second scarf of gutta percha, covering it in the same manner as described for coating the bare conductor, and cover it with a small sheet of gutta percha in the same manner as described above, so as to make the finished joint to the size of the core as manufactured.

Rules to be observed in forming Joints.—The following rules must be carefully observed in forming either a temporary or permanent joint:—

1.—In laying bare the conductor, the dielectric should be warmed and then pulled off, so preventing any chance of it being damaged, which might be the case were the dielectric to be cut off.

2.—For a perfect junction, soldering is necessary.

3.—The wires before connection should be carefully cleaned, and the hands of those performing the work must be dry.

4.—Gutta percha should not be given too much heat, for it then becomes oily and will not, in that state, properly adhere.

5.—Grease and dirt must be scrupulously avoided.

Great care is absolutely necessary in making junctions, as they are the principal sources of defect in the insulation of electrical submarine cables.

Junction Boxes.—When it is necessary to employ a multiple cable, a junction box is used to facilitate the connection of the several separate wires diverging from the extremities of such a cable. In one angle of such a box the multiple cable is introduced, while the separate cables make their exit on the opposite sides and pass to the different mines. Different views of a junction box are shown at [Fig. 41], where A is a plan of the top or lid, B a plan of the bottom, with the lid off, C an elevation, and D a section of the box.

The manner of using the junction box is as follows:—

The multiple cable is put in at a, and secured there by means of a nipping hook, shown at [Fig. 42], which hook passes through the bottom of the junction and is made secure by means of a nut. The single core cables radiating from the junction box pass through the openings b, b, b on the sides, and angle opposite to where the multiple cable a enters. Each multiple cable is composed of seven cores, and each of these is connected by means of joints with the mine cables within the junction box, and each of these seven cables is secured by means of a nipper similar to, but smaller than, the one shown at [Fig. 42], which are also secured by means of nuts, as in the case of the multiple cable nipping hook. When all the connections are made, the lid A is placed so as to rest on the studs c, c, c, and firmly secured by a bolt d, which is made water-tight by means of a washer and nut.

By means of the nipping hooks, which take any strain that may be brought on the cables, the connections within the box are ensured against injury by such a cause.

To enable the whole to be lifted together for the purposes of examination of the cables, &c., a buoyed rope is connected to the eye-bolt e. For this service a dummy circuit closer is the best form of buoy, it having great buoyancy and resembling in appearance an active circuit closer.

A junction box should be placed in such a position as to be easily attained, even in the presence of an enemy, and its buoy should, if possible, not be seen. It is also very essential that it should be in a safe and guarded position, for any injury to the junction box or multiple cable would be fatal to the group of mines in connection.

In the following cases, special junction boxes are used:—

1.—A seven cored armoured cable to be connected direct to another length of the same.

2.—A single armoured cable to be connected as in foregoing instance.

3.—A T junction box for the branch system of electrical contact mines.

Junction Box for Multiple Cables.—At [Fig. 43] is represented a plan of lower half of this form of junction box. It consists of a pair of cast iron plates of precisely similar form to the one shown at [Fig. 43], and so made as to be capable of being fastened tightly together by means of four bolts and nuts passing through the holes a, a. The grooves b, b at the two extremities are just large enough to grip the armoured cable firmly, when the upper and lower parts are screwed together. A larger space is provided in the hollow for the joint.

Junction Box for Single Cored Cables.—For this purpose a junction box similar to, but smaller than the one above described is employed.

T Junction Box.—This form of junction box is employed when the system of electrical contact mines on branches from a single cable is used. This system is dependent on the use of a platinum wire fuze in connection with a platinum wire bridge in each branch close to its junction with the main cable.

This form of junction box, which is shown at [Fig. 44] is very similar to the one used for the connection of two multiple cables, only differing in its shape, which is that of a T. a is a disconnector, which will be described further on; b, b, b' are the armoured electric cables, b, b being the main, and b' the branch cable in connection with the forked joint formed within the T junction box; c, c, c are Turk's heads formed to prevent any strain being brought on the forked joint. This form of Turk's head is made by turning back the wires of the cable armouring, and frapping them round with spun yarn until the necessary size and shape is attained.

McEvoy's Turk's Head.—Another form of Turk's head, devised by Captain McEvoy, is shown at [Fig. 45]. It consists of two separate pieces of brass, a and b, the former screwing over the latter. The mode of using it is as follows:—

Slip the piece of brass b over the cable c, and turn back the wires of the cable d, d, &c., so that they lie against the shoulder of the brass piece b, then slip the other piece of brass a over the cable and screw it on the piece b, firmly jamming the turned back wires d, d, &c. This is a very neat and quick method of forming a Turk's head, and it should be invariably used in preference to the foregoing method, which is clumsy, and which takes some time to form.

The section of a disconnector is shown at [Fig. 46]. It consists of an iron cover, or dome a, which is provided with a screw fitting on to another screw on the ebonite body b of the apparatus. When the dome a is screwed tightly down on the washer i, the whole is made perfectly watertight. c, c are insulated terminals for connecting the cores of the branch and main cables after their armouring has been removed, as shown at [Fig. 44]. d, d are two copper conducting wires (No. 16 B. W. G.) passing through the centre of the ebonite body b, and projecting into the interior of the apparatus. These wires are held in position and insulated by means of a composition formed of a mixture of pitch, tallow, beeswax and gutta percha. This composition is put on whilst hot and allowed to cool gradually, when it becomes hard and durable. Great care is necessary to ensure the cavity within the ebonite body b being completely filled, as otherwise a leakage might occur, owing to the great pressure of water at depths where the disconnection would be generally used. f is a boxwood cover which is slipped on, and fits fairly tight to the ebonite body b; g is a piece of thin platinum wire, weighing 1·6 grains to the yard, and being 4/10 inch in length; h is an ebonite pin, which passes through two small holes in the boxwood cover f, into which it fits tightly, and in such a position as to be directly beneath the platinum wire bridge g, when the boxwood cover f is fixed on. The pin h is pushed through the holes in the cover f from the outside, so as to pass beneath the bridge g after the priming has been inserted, and the cover has been placed on.

When prepared for use, the platinum wire bridge g is surrounded by some loose gun-cotton priming, sufficient in quantity to blow off the boxwood cover f, without destroying the dome a; the cover f being blown off, carries the ebonite pin h with it, and through the platinum wire bridge g, thereby rupturing it, and breaking the continuity of the circuit. The object of so doing is to cut off the connection of an exploded mine, so that the full amount of the firing current is available for the other mines, and not suffered to be wasted by passing through the exposed wire of the broken circuit, which, were the disconnector not employed, would be the case.

When any particular mine of a system is struck, the current passes through the main cable b, the disconnector a (which is in connection with that mine), and branch cable b' to the fuze, and so explodes the mine, and destroys the platinum wire bridge g of the disconnector at practically the same instant. The effect of the latter operation would be to cut off and insulate the branch cable of the exploded mine, and so prevent any loss of the electrical current, when another mine of that system is required to be fired.

The platinum wire bridge g is 4/10 inch long, while that of the fuze is 3/10 inch, the object of this difference in length of the bridges being to ensure the former one g being fired, and thus the insulation made doubly sure. Many other forms of disconnectors have been devised, but none have proved in practice so effective as the one just described.

JUNCTION BOXES. MECHANICAL TURK'S HEAD.

Mooring Electrical Submarine Mines.—This is one of the most difficult problems to be solved in connection with a system of submarine mines. The objects to be attained in mooring are as follows:—

1.—The mines should preserve the exact positions in which they are laid down.

Note.—From the comparatively small radius of destructive effect, of even heavily charged submarine mines, it will be understood how absolutely essential, in the case of mines fired by judgment, it is that this object should be attained.

2.—The mooring chains, or ropes, must be so arranged that no twisting whatever should occur, as otherwise fracture of the insulated wire would be likely to happen.

3.—In the case of buoyant mines, their distance from the bottom must be so adjusted, that at no time shall a vessel passing over them be out of their vertical range of destruction, nor shall they be visible.

The difficulties attendant upon the efficient mooring of submarine mines are immense, as will be understood when the action of gales of wind, and strong tides, which latter vary continually in their direction and in their rise and fall, are taken into consideration.

The foregoing remarks apply more particularly to a system of buoyant submarine mines, as those placed on the ground are comparatively easy to moor.

Several modes of mooring buoyant submarine mines have been suggested, the most practicable of which are as follows:—

Ladder Mooring.—This is a method of mooring, which in places where it may be necessary to place the anchors far apart will be found useful.

The circuit closer is connected to the mine by two ropes which lead thence to two anchors, the ropes being separated by wooden rounds, or spreaders, 1 to 3 feet long, by which the tendency to twisting is prevented.

The anchors are placed some 12 feet apart.

The only defect of the ladder mooring is the quantity of sea-weed, &c., that is liable to be lodged on the rounds, thus causing the circuit closer to be drawn out of its proper position.

Fore and aft Mooring.—This mode may be advantageously employed in a tideway where the current runs very strong, that is to say, five knots per hour, or more. It consists simply of two anchors, one of which is moored up, and the other down the stream.

Austrian Method of Mooring.—This method of mooring, adopted by the Austrians during the war of 1866, is shown at [Fig. 47]. It consists of a wooden triangular platform on which several heavy weights a, a, a are placed; the mine is attached to this platform by means of three wire ropes b, b, b, connected to the angles of the latter, and fastened to three chains, which by means of a catch holds the mine at the position required.

This catch consists of a pulley attached to the extremity of the wire rope of the platform, through which the mooring chain of the mine is passed, and fastened by a key at the required depth by means of a self-acting arrangement.

This key, which is of considerable weight, slips down as the mine is being hauled into position, but the moment the chain is slacked, two arms catch into a link of the chain, and so hold the mine in position. The weight of such a key is about 60 lbs. It is fitted with nuts, &c., to enable it to be taken to pieces.

This plan of mooring proved very effective in the harbours of the Adriatic, where there is hardly any tide or current to twist the mooring ropes, or otherwise disturb the mines. The Austrians have lately adopted the mushroom sinker in place of the wooden platform and weights, for their anchor.

Single Rope Mooring.—This simple method of mooring has after numerous exhaustive experiments been adopted as the most practicable and effective of all others. Whenever possible, a wire instead of hempen cable should be used to connect the mine and its circuit closer to the mooring anchor, as the former is less liable to twist, kink, or wear from friction than the latter.

A ground mine with circuit closer attached is represented at [Fig. 48], where a is the wire mooring rope, b the electric cable leading from the mine to the circuit closer, C, and c the cable leading from the firing station to the mine; d is the oblong sinker attached to the mine, and e the tripping chain leading to the shore, to which the cable c is attached at intervals, so that by underrunning the electric cable, the tripping chain may be easily picked up, and the mine raised.

MOORINGS FOR SUBMARINE MINES.

At [Fig. 49] is shown a buoyant mine. The only difference in the mooring of this and the one before described, is that instead of resting on its anchor on the ground, it is moored at a certain distance above its anchor d, to which it is secured by a chain e.

[Fig. 50] represents an electro contact mine. M is the mine with circuit closer enclosed, a the wire mooring rope, d the mushroom anchor, and b the electric cable leading from the mine to the disconnector D.

The mushroom sinker or anchor, which is undoubtedly the most effective of all other forms of mooring anchors used for the purposes of anchoring submarine mines, is shown at e, Fig. 49; the legs are added for use on rocky or hard bottoms, under which circumstances the weight of the anchor should also be increased.

For ground mines the form of sinker shown at d, [Fig. 48] is employed; it is of an oblong shape, and hollowed out in the centre to allow of its being lashed close up to the mine.

Large blocks of stones with their bases slightly hollowed are useful as extempore moorings, so also is the one shown at [Fig. 51], which consists of a strong heavy wooden shaft a, with a number of wooden arms b, b attached to its base; this form of extempore sinker was considered very efficient by the American authorities.

The wooden weighted platform, which was described at [page 56], is also a very useful form of extempore sinker.

For dead weight moorings, pigs of ballast, heavy stones, &c., may be used.

The weight of the anchor or sinker for mooring submarine mines is a very important consideration. It will depend on the amount of buoyancy of the mine, on the strength of current, and on the nature of the bottom, also whether the mines are to be hauled down to, or moored with the anchor.

Stotherd uses the following formula:

W = 2√B2 + P2

where B is the excess of the flotation over the weight of the charge of a given submarine mine;

P is the pressure exerted by any given current on the same buoyant mine;

W the weight of sinker necessary to overcome the tendency of the mine to move. In still water P becomes nothing, and therefore W equal to 2 B, that is, in still water double the buoyancy of a mine is a sufficient weight for its anchor.

The value of P may be found from the formula P = 4·085 × V2, where V is the velocity of the current in miles per hour.

From this equation P will be found in terms of pressure in pounds per square foot of flat surface, which is nearly double that on the curved surface of a cylinder.

In regard to the amount of buoyancy of a submarine mine, it has been found by actual practice that in the case of a mine moored in still water it should certainly be not less than the weight of the charge, whilst if subjected to the lateral pressure due to a current, it should be not less than three times the pressure exerted by the current.

It is always necessary to allow an excess of buoyancy over the calculated amount to counteract any leakage, or other disturbing cause which might otherwise materially affect the efficiency of the mine.

There are two modes of placing a mine in position; either by attaching the anchor, with the cable necessary for the depth of water, to the mine, and lowering both together, or by placing the anchor first, and then hauling the mine down to it, and by means of a catch, fastening it at the required depth.

The first mode is exceedingly simple, but except under very favourable circumstances cannot be relied on when firing by observation is the means adopted to explode a system of submarine mines. The second plan is practically easy to carry out, and by it a mine may be placed more accurately. To enable either of the above methods to be properly carried out, specially fitted steamboats, &c., are requisite.

At [Fig. 52] is represented a 42 feet launch fitted for laying down a submarine mine by the first of the two modes enumerated above.

[STEAM LAUNCH FOR MOORING SUBMARINE MINES.]

a is the mine; b is the electric cable carried from the drum c to the charge, and connected for use; d is the circuit closer, which is attached to the mine by its electric cable and mooring rope; f is the mushroom sinker attached by means of its mooring chain to the mine, it is suspended by a slip rope g, which passes over a small crutch fitted with a sheave h; i is a hollow iron derrick, and k the tackle and fall for lifting mine into boat; this derrick is formed of an iron tube about 3 inches diameter, 3/8 inch thick, and 10 feet 6 inches long; it is attached to an iron tube mast of similar diameter and thickness to the derrick, but 12 feet 3 inches long, an iron chain 6 feet 6 inches long and 5/8 inch diameter, connects the derrick to the mast; m is a leading sheave to keep the cable clear whilst it is being paid out; l is a crab, for working the tackle k, &c., and c is the drum on which the electric cable is wound.

In connection with the defence of a harbour by a system of electrical submarine mines of large size, it will be necessary to employ a service of steamtugs, steamboats, mooring-barges, &c., specially fitted for such work. One of the great advantages of the hauling down method of placing mines in position, is, that the anchors, with the cables connected thereto, may be carefully and accurately got into position during the time of peace, and the mines themselves, which should be kept in store ready fitted for immediate use, need not be placed in position until they are actually required. The drums used for reeling a multiple cable on, are capable of holding half a nautical mile in length. That used for a single core armoured cable is similar to but smaller than the aforesaid drum, and is capable of stowing one nautical mile of such a cable. For transportation wooden drums are ordinarily used.

CHAPTER IV.
DEFENSIVE TORPEDO WARFARE—continued.

CLOSING the Electric Circuit.—In connection with the system of coast defence by means of electrical submarine mines, there are two distinct methods of effecting the closing of the electric circuit, and consequently, the firing battery being connected, the explosion of the mine or mines, which methods may be used separately, or in combination, and are as follows:—

During the early days of submarine defensive warfare, the latter method alone was used, owing to the absence of anything like a practicable form of self-acting apparatus; but within the last few years, the former has almost entirely superseded the latter method, except in very exceptional cases; this revolution being due to the vast improvements that have been, and still are being effected in the system of firing electrical submarine mines automatically.

Use of Circuit Closers.—Electrical submarine mines may by means of an apparatus, termed a circuit closer, be rendered self-acting; that is to say, by the action of a vessel coming in contact with such an apparatus, which may be either within the mine itself, or within a buoy attached to the mine, the electric circuit is closed, and the mine in connection with the circuit closer so struck, exploded. The essential feature of such a mode of closing the electric circuit is, that electrical submarine mines may be rendered either active or harmless, at the will of the operator, which is effected by the putting in, or taking out of a plug, by which means the firing current is either thrown in, or out of the circuit.

Circuit closers.—Many different forms of circuit closers have been devised, among which the following seem the most suitable and are those generally used:—

Mathieson's Circuit Closer.—This form of circuit closer has been adopted by the English government in connection with their system of defence by electrical submarine mines.

The details of this apparatus are shown at Pl. xiii.

[Fig. 53], a is a gun-metal dome screwed on to a metal base b, its foot resting on a gutta percha washer c, so as to exclude any water; d is a cap screwed on to the top of the dome, and made watertight by the leather washer e; f is a guard cap screwed into the cap d, this is to keep the spindle of the circuit closer steady during transport, and would be removed when the apparatus is prepared for service; g is the ebonite base plug through which pass the insulated wires E and L; h is an hexagonal collar, working in the metal base plate b, by means of which, and the brass collar i, and the leather washer k, the base plug is secured, and water is excluded from the interior of the circuit closer; l, l, l are brass columns supporting a circular ebonite piece m; n is a metal bridge screwed on to the base plate b, into which is screwed the spindle p, both of which are prevented from moving after being screwed up by the set screws r and s.

The spindle p carries a leaden ball t, which is supported upon the rest v, and is secured in position by the screw nut w; x is an india rubber ring, the object of which is to prevent any damage being done to the spindle should the ball when set in action by a heavy blow from a passing vessel be brought into contact with the dome; 2 is a brass disc attached to the spindle carrying an ebonite disc 4, connected to it by screws; 6 is a brass contact ring also fixed to the ebonite disc 4, provided with a screw 8, for the attachment of one of the base plug wires, and with platinised projections 3, 3, 3, [Fig. 56]. The contact ring 6 is completely insulated from the spindle and brass disc 2. Three contact springs 5, are attached to the circular ebonite piece m, and the faces opposite to the platinised projections of the disc 2 are also platinised. 7 shows the contact screws of the connecting pieces, which serve also as adjusting screws to regulate the sensitiveness of the apparatus, the points of which as well as their bearings on the springs are platinised.

The springs are connected together by means of the wires 9, [Fig. 55], one end of which is secured to the connecting piece by the screw 10, and the other passes through to the top of the ebonite piece, and is attached to the top of the spring next in succession to that to which it is fixed below.

One terminal of a coil of 1000 ohms resistance (which is used for testing purposes) is attached to the line L, terminal of the ebonite base plug, which latter is also connected to the screw 8, on the circumference of the contact ring 6; the other terminal of the resistance coil is connected to the earth, E terminal of the base plug.

A bare copper wire of No. 16 B. W. G. connects the top of the last contact spring with the set screw s; a piece of similar wire jointed to it is passed round one of the brass collars and connected to the screw r. As a precaution against bad contact, the contact springs are connected together by bare wires A, B, C. This completes the connections for the signalling circuit, the earth being formed by the body of the instrument; D is a hole left in the metal base for the passage of the insulating wire which connects the earth plate to the earth E terminal of the base plug.

Testing Current.—For testing purposes the current from the test battery arrives by the line wire L, and passes thence through the resistance coil to earth by means of the wire E, which is attached to a zinc earth plate placed in a recess in the jacket of the circuit closer.

Action of the Circuit.—The action of the apparatus is as follows:—

Closer.—On the circuit closer being struck, the weight of the lead ball t causes the steel rod p to be deflected and brings the brass ring 6 in contact with one of the springs 5; the signalling current which up to this moment has been passing through the 1000 ohms coil to earth, then passes to the contact ring 6 (avoiding the resistance coil) thence to the spring which is in contact with it, and from there by means of the wire connections to the set screws s and r, and so to earth through the metal body of the apparatus; the effect of the resistance coil being thus eliminated, is to strengthen the signalling current, and thus enable it to work the shutter apparatus, by which means the firing current is thrown into circuit and the mine exploded.

MATHIESON'S CIRCUIT CLOSER.

Circuit Breaker.—By altering the mode of connecting the wires, the above apparatus may be used as a circuit breaker, that is to say, the signal may be given, and the mine exploded by the cessation of a passing current, instead of by the closing of the electric circuit. This system was specially designed for use with platinum wire fuzes, but is rarely used.

Circuit Closer of Electro Contact Mines.—When the inertia circuit closer is employed in connection with electro contact mines, the circular ebonite piece m is replaced by a similar shaped piece of brass, and which is in metallic connection through the brass pillars l, l, l with the mass of the metal of the apparatus which forms the earth plate.

The insulated wire of the base plug is connected to one pole of a platinum wire fuze, the other pole of which is connected by another wire to the outer metal rim of the disc of the spindle. As long as the circuit closer remains undisturbed, a break will remain in the circuit, which is due to the ebonite insulation between the spindle and the outer metal rim of the disc; but the moment the apparatus is struck, which causes the spindle to vibrate, the outer metal rim will come in contact with one of the springs completing the circuit, through the circular metal portion and the pillars of the circuit closer to earth.

Adjustment of Circuit Closer.—The sensitiveness of Mathieson's inertia circuit closer is determined by the distance between the disc 4 and the springs 5, 5, 5, which is regulated by means of the adjusting screws 7, 7, 7, which press against the inner faces of the springs. Owing to the great weight of the leaden ball, when by any cause the circuit closer is inclined for a length of time, a permanent set is given to the spindle, thereby destroying the adjustment of the instrument.

Improvements in the Inertia Circuit Closer.—To remedy this very serious defect, a cylinder of india rubber is substituted for the leaden ball; a circuit closer so fitted is also less affected by the action of counter mines, which is a very important advantage.

Mathieson's Spiral Spring Circuit Closer.—A sectional elevation of this form of circuit closer is shown at [Fig. 57]. It consists of a brass base a, provided with a grooved flange for carrying a gutta percha washer, and it has also an hexagonal projection for the purpose of screwing the circuit closer into the gun-metal mouth of its air-tight cylinder, or buoy; b is a brass dome enclosing the apparatus for the purpose of protecting it from injury, and also by means of india rubber washers to prevent an ingress of water, should the circuit closer case become injured, and leak; c is a brass collar to which the brass contact springs i, i are attached, and which are regulated by the set screws j, j; a brass spiral spring d carries a metal rod e, which supports a brass ball f, surrounded by an india rubber band h. A contact disc g is secured to the base of the spindle e, but insulated from it by an ebonite boss; k is an ebonite base plug with two channels in it, through which the wires m, m1 pass.

An Improvement on the Inertia Circuit Closer.—This instrument is a vast improvement on the inertia apparatus previously described, being more simple and more certain in its action, a desideratum in all circuit closers; but notwithstanding, up to the present time Mathieson's inertia apparatus has been used by our government, to the exclusion of all other instruments of a similar nature, some of which were proved to be far superior when subjected to the crucial test of actual practice.

Austrian Self-acting Circuit Closer.—This form of circuit closing apparatus, which is purely a self-acting one, that is to say, a mine so fitted cannot be fired at will, is shown at [Fig. 58].

It consists of several buffers a, a, a, which by means of strong springs are held in position, their heads projecting outside the torpedo case b; on being pressed in by the contact of a passing vessel, the ends of these buffers would be forced against a ratchet wheel c, which is also kept in position by means of a spring. Several strong pieces of wood d, d within the case keep the buffers and their attached arms in the proper direction, and also afford rigidity to the torpedo case. The brass ratchet wheel c being put in motion carries round with it a central arrangement e, the lower part of which is shown at [Fig. 58], A.

This portion consists of a cylinder of brass f divided into two parts insulated one from the other by a piece of ebonite g; on one side of this cylinder there are three arms of brass, h, i, and k, and on the other there are two arms, l and m, all of which are insulated from each other.

AUSTRIAN CIRCUIT CLOSER, MERCURY CIRCUIT CLOSER.

The arm h is close to, but insulated from a metal plate n, which latter is permanently connected with the conducting wire leading from the firing battery, and thus while in a state of rest is electrically charged; beyond the arm i is a spring o, which is connected with the earth, and in such a position that when the central portion is moved round, this spring o comes in contact with the arm i, and the plate n with the arm h simultaneously, and the circuit is thus completed through earth to the battery, but the current of electricity does not pass through the fuze. The arms k, l on the opposite sides of the cylinder, and consequently insulated one from the other, are connected with the fuze, and the arm m is connected with the earth.

On a further pressure of the vessel on the buffer, the arm i is pushed beyond the spring, and in contact therewith, and consequently the circuit by earth to the battery is broken, while the contact of the arm h and plate n is still retained, and the current is passed by the arm k through the fuze to the arm l, and then to earth through the arm m, thus completing the electric circuit of the firing battery through the fuze, and to exploding the mine.

The spring acts as a circuit breaker, and by means of an intensity coil in connection with the firing battery, the current is only passed through the fuze when at the point of greatest intensity.

By detaching the firing battery, the channel defended by such submarine mines may be rendered safe.

Fuze only in Circuit at Moment of Firing it.—One of the principal objects to be gained by the employment of such an arrangement for the closing of the electric circuit in connection with submarine mines, is the prevention of premature explosion from induction which might be caused by the proximity of any atmospheric electricity, the fuze in this system being entirely cut out of circuit until the moment when it is necessary to fire it.

The Austrians employed this form of circuit closing instrument during the war of 1866, and still continue to use it in connection with their coast defence by submarine mines.

McEvoy's Mercury Circuit Closer.—At [Fig. 59] is represented a longitudinal section of a circuit closer of this construction.

It is placed in the mine in such a manner that when undisturbed it maintains an approximately upright position.

It consists of a metal tube a into which the cup b of vulcanite, or other insulating material is fixed. The cup is contracted at some distance from the top by the perforated plug c, which is also of insulating material; d is a metal pin fixed into the bottom of the cup b, it is connected with the wire e, which is insulated and passes to the battery; f is a metal plug closing the tube a and the cup b at the top; g is a wire attached to the plug f, and passing from it to an earth connection. The cup b is filled with mercury up to the level of the plug c. By the contact of a passing vessel the instrument would be tilted sufficiently to cause the mercury to flow into contact with the metal plug f, thus completing the electric circuit and exploding the mine.

This form of circuit closer, though not generally adopted, would, on account of its being less liable to derangement by the motion of the waves, or by the explosion of an adjacent or counter mine, seem to fulfil the many requirements of a circuit closer for general service.

McEvoy's Weight Magneto Circuit Closer.—This form of circuit closer, which is shown in section and plan at [Fig. 60] and [61], is one of the most important improvements that has ever been effected in such apparatus, and bids fair to become universally adopted.

A heavy metal conical shaped weight a (Fig. 60), hollowed out in its base and working in a ball and socket joint b, rests on a solid brass base c, and is so arranged that on the apparatus being struck, the weight a will fall over, pivoting on one of its supports d, d; e is a band of india rubber, encircling the weight a, for the purpose of preventing a jar on its falling against the sides of the brass cylinder f, which contains the weight a and joint b. A brass rod g, connected to the ball and socket joint, passes through the base c, through a strong spiral spring h (which latter rests on an adjusting screw k), through a piece of ebonite l, which supports the bobbins and core m, m1; then between these bobbins m, m1 through an armature n, which is pivoted at p; and lastly through a slight spiral spring o, which is kept in position by the adjusting screw i.

The armature n is fitted with a small piece of brass r, so arranged that when it (the armature) is in the position shown in [Fig. 60], this piece of brass r does not make contact with the two strips of metal, s, s, between which it, r, works; but when the armature n is in contact with the cores of the bobbins m, m1, then the piece of brass r makes contact with the metal strips s s, and so makes a short circuit for the electric current. An ordinary telephone t, [Fig. 61], in which some small shot, bells, &c., are placed, is fixed to the top of the brass cylinder f.

Action of Circuit Closer.—The action of this apparatus is as follows:—

On the mine carrying this form of circuit closer being struck by a passing vessel, the weight a is caused to fall over towards the side of the brass cylinder f, thus allowing the strong spiral spring h to act on the brass rod g in an upward direction, by which means the armature n is brought into contact with the soft iron cores of the bobbins m, m1.

Mc.EVOY'S MAGNETO ELECTRO CIRCUIT CLOSER.

The connections of the wires are made as follows:—

The line wire w is led through the base of the apparatus and connected to a piece of brass under the ebonite support l, in connection with one of the wires of the bobbin m, the other wire of which is attached to the metal strip s; the wires of the bobbin m1 are connected, the one to the metal strip s1, the other to a piece of brass under the ebonite support l; from this latter piece of brass a wire w1 is led to the brass screw x. The wires w2, w3, from the fuzes are led, the one to the brass screw x, the other to a screw y, which forms through the metal of the apparatus the earth plate. One of the wires of the telephone t is connected to the brass screw x, the other w4 is connected to the piece of brass to which the line wire w is also attached. While the circuit closer remains in a state of rest, the current from the signalling battery flows along the line wire w, up the telephone wire w4, through the telephone which has a high resistance, then by the wire w2 through the fuzes, and to earth by the wire w3.

On the circuit closer being struck, by which cause the armature n is brought up to the cores of the bobbins m, m1, and the piece of brass r in contact with the metal strips s, s1, the signalling current, instead of circulating through the high resistance of the telephone t, passes round the bobbin m, down the metal strip s, across the brass piece r, up the metal strip s1, round the bobbin m1 (thus forming an electro magnet of m, m1), and by the wire w, direct through the fuzes to earth, and so explodes the torpedo. The effect of the telephone resistance being cut out, is to strengthen the signalling current, and enable it to work the shutter apparatus and so throw the firing battery in circuit and explode the mine.

The advantages of this circuit closing apparatus are:—

1.—Simplicity.

2.—Compactness.

3.—Increased certainty of action, due to the sustained contact of the armature n, on the apparatus being struck.

4.—Additional means of testing a system of electrical submarine mines, which is afforded by the telephone:—

When this form of circuit closer is put in action by a friendly vessel coming in contact with it, or when experiments are being made, the signalling current must be reversed, so that no doubt may exist as to the armature n having dropped, on the apparatus coming to rest.

The telephone test indicates whether the circuit closer is in position or not, the shot, &c., within the telephone being shaken about by the movement of the buoyant circuit closer, the noise so created is readily distinguished by the receiving telephone at the station.

Another form of submarine mine is that known as the "Electro Mechanical" mine. The difference between this form and an ordinary mechanical mine is, that the exploding agent is electricity, and that it may be converted into an electro contact mine if desirable.

Description of a Russian Electro.—The electro mechanical mine, used by the Russians during the late Turco-Russian war, is shown in elevation and section at [Fig. 62] and [63].

Mechanical Submarine Mine, used by them during the late Turco-Russian War.—A is the conical shaped case; B the loading hole; C the base plug; D, D, &c., are five horns, screwed into the head of the case A; these are composed of a glass tube A, containing a chlorate of potash mixture, enclosed in a lead tube B, over which is screwed a brass safety cylinder C; when ready for action this latter tube C is removed; directly beneath each of the horns A, on the inside of the case, as at E, is a thin brass cylinder, closed at one end by a piece of wood d, and containing several pieces of zinc and carbon, arranged in the form of a battery, the zinc and carbon wires z and x being led through the piece of wood d; F is a copper cylinder containing the priming charge of gun-cotton g, and detonating fuse f; the terminals of the fuze are connected to two insulated wires, w and w1, the former of which is led direct to the loading hole B, and attached on the inside to the five zinc connecting wires z, &c.; the latter is attached to one end of a safety arrangement S, the other end of which is connected to the wire w2, which is attached on the inside to the carbon wires x, &c.; the safety arrangement S consists of an ebonite cylinder, containing a brass spiral spring fixed to one end of it, and pressing against a brass plate at the other, thus preserving a metallic connection between the wires w1, and w2; the mine is rendered inactive by pressing the spring down, and inserting a piece of ebonite between it and the plate.

Its Action.—The action of this form of electro mechanical submarine mine is very simple; the brass safety cylinders c, c, &c., being removed on a vessel striking either of the horns, D, D, &c., the lead tube b is bent, causing the glass tube a to be broken, and the mixture contained therein to flow into the cylinder E, instantly generating a current of electricity in the zinc carbon battery, and exploding the mine.

Mode of Converting into an Electro Contact or Observation Mine.—To convert this mine into an electro contact one, it is only necessary to connect the wires w1 and w2 to other wires leading from the shore; also by replacing the horns D, D by solid brass screw plugs, the mine may be converted into an ordinary observation one. In this case the two wires w and w1 attached to the fuze f, terminals would have to be connected to the observation instruments on shore.

Turkish Vessel sunk.—It was by means of one of these electro mechanical mines, that the Turkish gunboat Suna was sunk at Soulina.

Firing by observation, that is to say, effecting the ignition of an electrical submarine mine at the precise moment of a hostile vessel being vertically over it, through the agency of one or two observers stationed at a very considerable distance from the mine, should, with the very perfect self-acting circuit closers that exist at the present time, be resorted to only in very exceptional cases, or in connection with the self-acting system.

There are two defects, which are common to all methods of firing submarine mines by observation, and these are:—

1.—At night time, or in foggy weather, it cannot be employed.

2.—It is necessary to employ at least two observers, at a considerable distance apart, who to effect a proper action at the right moment, must work in perfect unison. These defects alone are sufficient to explain the preference given to a self-acting method of closing the electric circuit at the precise moment of a vessel being in position over a mine by those governments who have adopted electrical submarine mines as a means of coast defence.

Methods of Firing by Observation.—There are several methods of firing by observation, of which the following are the ones principally used:—

Intersection by Pickets or Range Stakes.—In narrow channels and at short distances, this system of ascertaining the relative position of a hostile vessel and a submarine mine may be used, provided that skilled and careful men are employed to work it. Two or more pickets or stakes are arranged in front of the firing station in such a manner that a vessel passing up the channel on the prolongation of these stakes will be over a mine. This arrangement should of course always be considered as an extempore one; it was used on several occasions by the Confederates during the American civil war.

Firing by Cross Bearings.—The simplest method of so determining the relative position of a vessel and a submarine mine, and exploding it at the right moment, is that in which observers are placed on the prolongation of the mines. This mode is shown at [Fig. 64], where m1, m2, m3, &c., and n1, n2, n3, &c., are the mines; A and B, the points in prolongation of the mines where the observers are stationed; D the firing battery, and s, and s1 two hostile vessels.

At the stations A and B firing keys are placed, at the former one for each separate mine, perfectly distinct and insulated from each other, at the latter a single key. The pivot points of the series of keys at A are connected by separate wires to one pole of the firing battery D, the other pole of which is connected by a single cored insulated cable to the pivot point of the key at B; the contact points of the series of keys at A are connected by separate line wires as A m1, A m2, A m3, &c., to the different mines, while the contact point of the key at B is put to earth. Thus it will be seen that, in the case of the row of mines, m1, m2, &c., unless the key at B, and the key at A, of either of those mines are both pressed down at the same instant, no current can pass, and therefore none of those mines can be exploded.

[RUSSIAN SUBMARINE MINE, FIRING BY OBSERVATION.]

In the case of the vessel S, though at C, she is on the prolongation of the line A m5, C, and therefore the key of the mine m5, is pressed down at A, yet not being on the prolongation of the line B, E, the key at B is not pressed down, therefore the firing battery is not thrown in circuit, or the mine m5 exploded, but when the vessel s reaches the position N, that is over the mine m3, she being on the prolongation of the lines A m3, and B E, the key (m3) at A, and the key at B would both be pressed down, and therefore the mine m3 exploded, and the ship destroyed. In the case of a vessel passing through an interval between any two mines at such a distance as to be out of the radius of destructive effect of either of the mines belonging to the first row (which is shown at s1,) only the key at B would be pressed down, and thus the vessel enabled to pass safely through, but only to come to grief at the second or third row of mines, provided they have been properly placed, and separate though similar arrangements as in the case of the line of mines, m1, m2, &c. have been made.

Firing by a Preconcerted Signal.—At [Fig. 65] is represented a somewhat similar, though a much simpler plan of the foregoing system, by employing a preconcerted signal at the station B in the place of the firing key and insulated cable, as in the former case. The only material difference in the arrangement of these two methods, is that in the latter case the pole of the firing battery at A, which in the former case was connected to the firing key at B, is put direct to earth. As will be readily understood, this latter system requires great coolness and nerve on the part of the operator at A, who has not only to watch the vessel passing across his intersections, but also to be on the alert to receive the signal from the observer at B. Should it ever be necessary to adopt this latter system, it will be found advisable to employ two men at station A, one to watch station B, the other to attend to the firing key and intersections. A separate signal-flag for each line of mines, and also a separate firing arrangement, would be required. As in many cases it would not be practicable to have a station in such an advanced position as at B, in [Fig. 64] and [65], on account of the danger of its being cut off by an enemy, another combination becomes necessary. In this instance the station B is placed on the opposite side of the river, &c., to that on which the station A is placed, and a series of firing keys, instead of a single one, is here used, necessitating a multiple cable between the stations A and B, in the place of single cored cable; the manner of manipulating this method is very similar to that previously described.

Firing by Intersectional Arcs fitted with Telescopes.—The foregoing methods of firing by cross bearings are replete with many serious defects, to remedy which, to a considerable extent, special arrangements have been devised, that is, the employment of intersectional arcs fitted with telescopes at the stations A and B.

[Fig. 66] and [67] show the arrangements of these arcs, the former being the one used at the firing station A, the latter at the converging station B. At each station one arc is provided for each row of mines placed in position. The firing arc [Fig. 66] consists of a cast iron frame a, with three feet b, b, b, these being provided with levelling screws.

To ascertain when this frame is level, a circular spirit level is attached thereto, a telescope d provided with one horizontal and three vertical cross wires, supported on Y's, admitting of vertical motion and attached to an upright e. A mill-headed screw f enables the telescope d to be raised or lowered; the telescope, which is rigidly connected to a vernier g, traversing over a graduated arc h, can be moved rapidly in a lateral direction by means of a rack and pinion arrangement i, and it can be clamped in any position by means of the screw h. Sights are fixed on the telescope in a vertical plane passing through its axis. To the outer rim of the frame of the arc, which is smooth, are secured the sights l l (shown on a large scale at Fig. 68), to give the direction of the mines. These sights are provided each with a brass point of V form, m, and a binding screw, n, in metallic connection with each other, but insulated by means of an ebonite plate from the rest of the metal of the sight. One end of a short piece of insulated wire is attached to the binding screw n, and the other passes through a hole in the base of the sight and projects below it; o is a brass tube rigidly connected to and moving with the upright carrying the telescope d, and projecting in front of this latter. A brass spring p (see Fig. 69) is attached to, but insulated from the outer extremity of this tube, and is so arranged as to make contact with the V point m on the sight, by means of a corresponding projection fitted to its under side. An insulated wire passing the tube o, the outer end of which is connected to a screw on the spring p, forms a metallic connection between this projection and the firing key.

At [Fig. 68] is shown an enlarged view of the front of the sight; in addition to the V projection m, and binding screw n, it is fitted with a capstan-headed screw to bear against the inner rim of the frame, and a thin wire upright t for giving the alignment of the mine, to which a disc is attached, on which the number of the mine is affixed.

When the distance between the station and the mine is only about one mile, an ordinary eyepiece is used in the place of the telescope d.

At [Fig. 67] is represented the arc employed at the converging station, which with the exception of there being no tube o, and only one sight, is precisely similar in construction to the one used at the firing station, and which has been described.

APPARATUS FOR FIRING BY OBSERVATION.

Application of the Intersectional Arc Method.—The application of the method of firing by observation, by means of intersectional arcs fitted with telescopes, is shown at [Fig. 70]. C, D, and E are three of the larger kind of arcs, one being used for each row of mines at the firing station A. At the converging station B, one of the smaller arcs is used for each row of mines, as shown at F, G, and H. S, S1, S2, are the signalling apparatus, the F terminals of which are connected to the sights l, l, l, [Fig. 69], of arcs C, D, E. Firing keys a, a, a at station A are connected to each arc, and to three of the cores of the cable connecting the two stations A and B, respectively. At the converging station B, three firing keys b, b, b are connected to earth and to three cores of the connecting cable respectively. The remaining core of this cable is connected to the recording instruments d, e. The action of the arcs, &c., will be readily understood from the diagram at [Fig. 70].

This arrangement does not interfere with the action of the circuit closer, as all that is effected by the observing arc circuit is to put the signalling battery current at the converging station B to earth instead of at the circuit closer.

Prussian System of Firing by Observation.—The principle on which this system is based, depends upon the proposition that if c d, in the triangle shown in [Fig. 71], be always kept parallel to H B, then A c, c d, d A bear exactly the same proportion to each other as A B, B H, H A do to one another; so that by means of the small triangle A d c, the lengths of the sides of the large triangle A B H can be obtained, and hence the position of the point H, the base A B being of course known. In [Fig. 71] at A there is a slate table representing the roadstead, and upon it the exact position of every torpedo is laid down, corresponding to their position in the roadstead. At A and B, 500 yards apart, telescopes having cross wires are placed; at A a long narrow straight-edged strip of glass A d is arranged to move in unison with the telescope at A; and by the application of dynamo electricity, a similarly constructed piece of glass c d moves in exact unison with the telescope at B, and having its pivot at C; that is to say, C d keeps parallel with B H, the line of sight of the observer at B.

Then if the observers at A and B have got a ship in their telescopes, the point of intersection d of the two pieces of glass A d and C d gives the position of the ship on the slate table at A, and when this point d comes over the position of any one mine on the slate, it is known that the ship is over that particular mine in the harbour, and she may be destroyed accordingly, by throwing the firing battery into circuit.

By the employment of electricity and a mirror, the great defect of this method, viz., the necessity of employing four people to manipulate it, would be remedied. The foregoing is a modification of Siemens's method of ascertaining distances at sea, &c.

Rules observed in Planting Mines.—In placing a system of submarine mines in position, the following are some of the chief points to be attended to, this work depending in a great measure on local circumstances, and on the method that is to be adopted in exploding and mooring them:—

1.—The plan of defence must be carefully laid down on a chart, on a scale of not less than six inches to the mile, and on this plan are to be marked the sites of the observing stations, the positions of each mine, circuit closer, and junction box, with their corresponding numbers, and also of the electric cables.

2.—The position of each mine having been determined, should be marked off by buoys.

3.—The utmost care should be taken to lay the electric cables, so that they shall be as far as possible away from the mines in the vicinity of which it may be necessary to take them, so as to lessen the liability of injury to them, by the explosion of the latter.

4.—The electric cables should be laid parallel, and never be allowed to cross directly over each other, otherwise the operation of underrunning them will be much complicated, also a certain amount of slack should be allowed to facilitate in picking the cables up for repair, &c.

5.—Every manner of device is to be used to conceal the electric cables, such as laying dummies, making detours inland, &c.

6.—All marks indicating position of the mines to be removed, after the mines have been placed in position.

7.—The identity of each cable and mine to be very carefully preserved throughout, by means of a number.

8.—A number of electro contact mines should be placed in advance of the leading line of mines, at irregular intervals, to prevent the enemy, having once ascertained the position of one mine of a line, from knowing within limits the position of the others of that line.

SYSTEMS OF DEFENCE BY SUBMARINE MINES.

In connection with a system of defence by electrical submarine mines, the following batteries are required:—

Firing Battery.—The firing battery should be suited to the nature of the fuze employed, and should possess considerable excess of power to enable it to overcome accidental defects, such as increased resistance in the various connections, or defective insulation in the line wire, &c.

As platinum wire or low tension fuzes are now universally adopted as the mode of ignition for submarine mines, it will be only necessary to describe those electrical batteries which are most suitable as an exploding agent in connection with such fuzes; these are as follows:—

Siemens's Low Tension Dynamo Electrical Machine.—This instrument consists of an electro magnet and an ordinary Siemens armature, which, by the turning of a handle, is caused to revolve between the poles of the electro magnet. The coils of the electro magnet are in circuit with the wire of the revolving armature, and during rotation the residual magnetism of the soft iron electro magnet cores at first excites weak currents which pass into the electro magnet coils, increasing the magnetism of the core, thus inducing still stronger currents in the armature wire. This accumulation by mutual action goes on until the limit of magnetic saturation of the iron cores of the electro magnets is reached.

By the automatic action of the machine, the powerful current so produced is sent into the leading wire or cable to the fuze to be exploded.

In this apparatus the electric current passes continuously through the line wire until a sufficiently powerful current is generated to heat or fuze the bridge of the fuze, and so ignite the gun-cotton priming. The coils of the armature and electro magnets are wound with wire of large diameter, to a total resistance of 8 to 10 Siemens units, or 7·6 to 9·5 ohms, in about 2,000 windings.

With a platinum wire weighing 1·65 grains per yard, 6-1/2 inches can be fuzed on short circuit, and 14 inches can be heated to redness.

The total weight of this machine, which is manufactured by Messrs. Siemens Brothers, is about 60 lbs.

Advantages of Siemens's Dynamo Electrical Machine.—The advantages of such a machine over Voltaic apparatus are:—

1.—The absence of chemical agents.

2.—There is less liability to get out of order.

3.—No special knowledge is required to work them, or to keep them in order.

4.—Greater durability.

The great defect of this and all similar machines is that the electric force has to be developed by turning a handle for a certain time before it is possible to generate a current sufficiently powerful to ignite a fuze, which defect, in connection with a system of defence by self-acting submarine mines, particularly at night, renders them inferior to Voltaic batteries, as under such circumstances, an apparatus is required that will cause an electric current to flow at any moment when the circuit is completed.

The application of steam power would to a certain extent remedy the above-mentioned defect, but the cost of such a method, compared to that of a Voltaic arrangement, would be far too great to allow of its superseding the latter arrangement.

Von Ebner's Voltaic Battery.—This form of Voltaic battery, which may be considered as a modification of that known as Smee's, was designed by Baron von Ebner, colonel of the Austrian imperial corps of engineers, for use in connection with the Austrian system of submarine defence, by self-acting electrical mines.

A section of one of these cells is shown at [Fig. 72]. It consists of a glass vessel a, to contain the diluted sulphuric acid, within which is suspended a plate b of platinised lead, which is bent round into a cylindrical form to fit close around the inner surface of the glass vessel. In the centre of this latter is hung a porcelain perforated cup c, containing some cut-up zinc and mercury to keep it (the zinc) amalgamated. The top of each cell is furnished with a porcelain cover, through which the wires attached to the positive and negative poles of the cell project.

Due to the large quantity of liquid contained in the cell, the tendency to alter its internal resistance is retarded; also by the arrangement of the porcelain cup, above detailed, the consumption of zinc and mercury, which in an ordinary Voltaic battery is very considerable, is materially diminished.

Chromic Acid or Bichromate Battery.—This form of battery is very similar to Grove's, the difference being that, in the place of the nitric acid as the exciting liquid, either chromic acid, or a solution of bichromate of potash, sulphuric acid and water is substituted.

A form of this battery, as designed by Dr. Hertz, is used in connection with the German system of torpedo defence.

Leclanché Voltaic Battery.—This form of Voltaic battery was invented by M. Leclanché, some twelve years ago. At [Fig. 73] is shown a cell of this battery in its original form. The positive pole a consists of a plate of graphite in a porous pot b, and surrounded by a mixture of peroxide of manganese and graphite. The negative pole c is a rod or pencil of amalgamated zinc. The whole is enclosed in an outer vessel of glass d containing a solution of sal ammoniac.

A modified form of the Leclanché cell as used in a firing battery is shown at [Fig. 74]. It consists of an ebonite trough or outer vessel a about 16" long, 9" deep, and 2-3/4" wide. The negative pole or zinc plate b is of similar shape to the trough a, but with its base removed, and does not fit the trough exactly, the space between it and the trough being left to ensure the former being completely surrounded by the sal ammoniac solution; the positive pole, or carbon element, consists of four gas carbon plates c attached together at their head by means of lead, and enclosed in a flannel bag, in which they are firmly embedded in the peroxide of manganese mixture; the positive element is of such a shape that it fits loosely between the sides, and is nearly of the same height as the zinc plate.

The object of such a form of cell was to obtain an electric current of large quantity, with as few cells as possible, by which means the loss of power which might occur from the employment of a great number of small cells is avoided.

Advantages of a Leclanché Firing Battery.—The advantages of the Leclanché firing battery are:—

1.—The absence of chemical action when the battery circuit is not complete, and consequently there is no waste of material.

2.—Requires little or no looking after.

3.—It may be kept ready for action in store without in any way deteriorating.

4.—It is comparatively very cheap.

These advantages combine to make a Leclanché battery the most suitable of any other form of electrical battery for use as the exploding agent for electrical submarine mines, and it is now universally used for such purposes.

Signalling Battery.—The signalling battery should be so constituted as to be capable of working the electro magnet of the shutter apparatus effectually when the circuit is closed direct to earth, and yet not so powerful as by the continuous passage of the current generated by it to fire the fuze in the mine. In the case of a platinum wire fuze being in the circuit, plenty of power may be given to the battery without fear of a premature explosion from this cause, but in the case of a high tension fuze it is necessary to be very careful in order to guard against such a contingency.

As in the case of a signalling or shutter battery, the electric current will be continually flowing, it is necessary to employ a constant battery, or one that requires least trouble and expense to maintain it in working order, and it is for this reason that a modified form of Daniell battery has been adopted to work the shutter apparatus.

Daniell Signalling Battery.—At [Fig. 75] is shown the manner of arranging a Daniell cell. A glass or porcelain vessel a contains a saturated solution of sulphate of copper, in which is immersed a copper cylinder b open at both ends and perforated by holes; at the upper part of this cylinder there is an annular shelf d, also perforated by holes, and below the level of the liquid; this is for the purpose of supporting crystals of sulphate of copper for the replacing of that decomposed as the electrical action proceeds. Inside the cylinder b is a thin porous vessel c of unglazed earthenware; this contains either water, or a solution of common salt, or dilute sulphuric acid, in which is placed the cylinder of amalgamated zinc e. Two strips of copper p and n, fixed by binding screws to the copper and to the zinc, serve for connecting the elements in series, or otherwise.

For the purposes of testing, either the Leclanché or Daniell battery specially arranged, or the Menotti battery, which is really a modification of the Daniell, may be used.

FIRING BATTERIES, TESTING BATTERIES.

Description of a Menotti Cell.—A Menotti cell, shown at [Fig. 76], consists of a copper cup containing some crystals of sulphate of copper and covered with a fearnought diaphragm a, placed at the bottom of an ebonite cell b; over this cup is put some sawdust, and resting on top of this is a disc of zinc c on another piece of fearnought. The upper portion of the zinc and its connection with the insulated wire are carefully insulated. Fresh water poured on the sawdust renders the battery active.

Description of a Menotti Test Battery.—[Fig. 77] represents a plan of the top of such a test battery with a 20-ohm galvanometer attached thereto. The connections are made as follows:—

One of the wires w of the object to be tested is attached to the terminal f, which is also connected by an insulated wire to the copper cup a; the other main wire w1 is attached to the terminal g of the galvanometer; h, the other terminal of the galvanometer, is connected by a short piece of wire k to the terminal l of the contact key m; and the contact point n is in connection with the zinc plate c; thus the current from the battery flows along the wire w through the object to be tested, back along the wire w1, through the coils of the galvanometer, along the wire k to the contact key m, and if this is pressed down to the zinc plate c, so completing the circuit.

To steady the needle of the galvanometer a bar magnet is used, which is inserted in the space r. The whole of the apparatus is enclosed in a leathern case fitted with a cover and strap.

This is a very compact and simple form of test battery, and will be found extremely useful in boats, &c., when placing mines in position.

Telegraph Battery.—For the purposes of telegraphing between torpedo stations, &c., a form of Leclanché battery, known as No. 3 commercial pattern, is generally used.

Voltaic Batteries.—The following points in connection with the use of voltaic batteries, which are taken from Beechey's 'Electro Telegraphy,' should be carefully observed:—

1.—Each cell of a battery should be carefully insulated.

2.—The floors and tables in the battery room should be kept scrupulously clean and dry, so as to prevent the least leakage or escape of the current.

3.—The plates of a battery should be clean.

4.—Porous cells should be examined, and cracked ones replaced.

5.—No sulphate of zinc or dirt should be allowed to collect at the lips of the cells.

In the case of a Daniell battery—

1.—The solutions should be inspected daily, and crystals of sulphate of copper added as required.

2.—The zinc plate must not touch the porous cell, or copper will be deposited on it (the zinc).

3.—The battery should be charged with sulphate of zinc from the first.

4.—The copper solution must be watched and prevented from rising over the edge of the porous jar, the tendency of such solutions being to mix with each other by an action termed osmosis.

These being in addition to foregoing general directions for Voltaic batteries.

Defects in a Voltaic Battery on its Current becoming Deficient.—On the electric current of a Voltaic battery becoming deficient, the following defects should be looked for:—

1.—Solutions exhausted; for instance, sulphate of copper in a Daniell's entirely or nearly gone, leaving a colourless solution.

2.—Terminals or connections between the cells corroded, so that instead of metallic contact there are oxides of almost insulating resistance intervening in the circuit.

3.—Cells empty, or nearly so.

4.—Filaments of deposited metals stretching from electrode (pole) to electrode (pole).

Also intermittent currents are sometimes produced by loose wires or a broken electrode, which alternately makes and breaks contact when shaken. Inconstant currents are also sometimes produced when batteries are shaken. The motion shakes the gases off the electrodes, thus increasing temporarily the electro-motive force of the battery.

Firing Keys and Shutter Apparatus.—The following is a description of the various firing keys and shutter signalling apparatus, which is used in connection with a system of electrical submarine mines. By means of the former the firing or other batteries may be thrown into circuit at will, whilst by means of the latter the firing battery is thrown in circuit without the aid of an operator, and a signal at the same instant given, indicating that a certain mine of the system has been struck.

Description of a Series of Firing Keys.—At [Fig. 78] is shown a plan and section of a series of firing keys as arranged for firing several mines by observation.

It consists of a strong wooden frame a, of a convenient form for the purpose of attaching it to the firing table by screws through the holes b, b. On this frame a series of keys c, c, c are fixed at convenient intervals. These consist of a strong brass spring firmly screwed to a series of brass plates d, d, d on the front of the wooden box a. From these latter short copper wires pass through the woodwork, and of such a length that, when required, the mine wires may be easily attached by means of binding screws, as shown at f. The inner end of each key is fitted with an ebonite knob (which is shown at c in the section) to insulate the hand of the operator when using the key. On the frame, and directly under each of the ebonite knobs, are arranged a series of metallic points g, g, g, so placed that on either of the keys c being pressed down, a perfect contact is made between it and its respective metallic point; h, h, h are copper wires leading from the metallic points g, g, g through the box, and of such a length that binding screws f, f, f can be easily attached to them when necessary.

A single firing key of an improved form is shown at [Fig. 79]. It consists of a strong wooden box a a, weighted at the bottom with lead in order to steady the key on the table, &c., on which it may be placed; on the inside of the bottom of the box is fixed a piece of ebonite, by which means the metallic point b, and the terminal of the firing key c, are insulated from each other; d d' are two terminals at the end of the box, to which the circuit wires are attached, one of these terminals is connected in metallic circuit to the firing key at c, the other one to the metallic point b; a wooden cover h, fitted with a catch k, protects the connections of the wires; by means of a plate, and catch e e, the key can be rendered inactive, thus preventing the danger of a premature closing of the electric circuit; by means of a spring s a break is always established between the key and the metallic point. It is immaterial to which of the two terminals d d' either wire is connected.

The Morse Firing Key.—This form of key is so well known in connection with the Morse telegraph, that it is not necessary to describe it.

It is usually employed in torpedo work in connection with a testing and firing table.

The Shutter Apparatus.—The shutter signalling and firing apparatus was devised to enable the firing battery current to be thrown in circuit without the aid of a personal operator, the signalling current (which is always kept in circuit) at the same instant ringing a bell, by which is known the particular mine that has been struck.

At [Fig. 80] is represented a diagram of such an apparatus. a is an armature working on a pivot between the two horns of an electro magnet b b, and held in position by a spiral spring c; the latter is in connection with a regulating screw, by which more or less pressure may be brought to bear in an opposite direction to that of the attractive action of the electro magnet. A stud i regulates the distance to which the armature may be drawn back; d is a shutter on which a reference number for each mine should be indicated, attached to a lever pivoted at the point e, the inner arm of which is just long enough to catch under the point of the armature a; when a current of sufficient strength is passed through the coils b b of the electro magnet, the armature a is attracted, releasing the lever attached to the shutter d, which by its own weight falls into the position shown by the dotted lines. f and g are two mercury cups, the former being in connection with the signalling current, and the latter with the firing current. When the lever is horizontal and the shutter drawn up and ready for action, the circuit of the signalling battery s is completed through the mercury cup f, along an arm h of the lever to the pivot e, and thence to the mine by the line wire w. When the circuit closer is struck by a passing vessel, and consequently the shutter thrown into the position shown by the dotted lines, another arm k, a prolongation of the lever, falls into the mercury cup g, which latter is in connection with the firing battery F. The armature a is prevented from coming into actual contact with the horns of the electro magnet by two small studs. The object of this is to prevent any effect of residual magnetism which might otherwise interfere with the rapidity of action of the armature when released and drawn back by the spring c.

FIRING KEYS, SHUTTER APPARATUS.

The object of employing Mercury Cups.—Mercury cups were devised in the place of the springs used in connection with the original design of a shutter apparatus, for the reason that electrical circuits dependent on the pressure of springs are always liable to interruption from dirt or oxide intervening between the points of contact.

Shutter Apparatus used with a Circuit Breaker.—When the circuit breaking system is used with the shutter signalling apparatus, the action of the armature in releasing the lever must be reversed; that is to say, that when the current is passing and the armature a attracted to the electro magnet b b, the shutter d must be held up, and when the current ceases, and the armature a drawn back by the spring c, the lever must be released, and the shutter allowed to fall. This is effected by altering the end of the lever, so that it hooks into, instead of abutting against the armature a.

To each shutter apparatus an electric bell is fitted, by which notice is given when a circuit closer has been struck. For general service, a box containing seven such shutter signalling and firing apparatus has been adopted, a plan of which is represented at [Figs. 81], [82] and [83]. The connections of the different circuits are as follows:—

The insulated wire of the upper bobbin of the electro magnet is connected to the spring of the armature; the pivot of the lever is connected with the right-hand terminal B, or main line connection on the top of the box; the insulated wire from the lower bobbin is connected to the middle brass plate k in the front ledge of the apparatus, the circuit from B to k being thus completed. The front adjoining brass plate A, provided with a terminal, is connected with the negative pole of the signalling battery, the positive pole being put to earth.

On a brass plug being put in the hole l, the signalling current will flow to the plate k, thence through the lower and upper bobbin to the spring of the armature, along the latter to the shutter lever, and from the pivot through the main line wire to the mine. The innermost brass plates H H are all connected in the same metallic circuit, and to them are attached by means of the binding screw D the test battery and galvanometer. Thus on the brass plug being removed from l, and placed in m, the signalling battery is cut out of circuit, and the test battery thrown in. In this way the condition of each individual mine may be ascertained while the connections of the remaining mines are left undisturbed. The positive pole of the firing battery (the negative being to earth) is connected to the terminal S at the right-hand corner of the lower ledge of the box; the plate to which the terminal S is fixed is divided at G, the left-hand portion being connected to a bar which runs horizontally the whole length of the box, and in metallic connection with each mercury cup g, [Fig. 80]. A brass plug is placed in the hole G, and when from any cause the lever drops, the firing battery will be thrown into circuit, and the mine to which the lever that has fallen is attached will be exploded.

Shutter Instrument and Observing Telescope.—Each mine is given a number, which is put on the disc of the shutter instrument connected to it, and also on the corresponding tablet C. From the brass plate in connection with the spring c, [Fig. 80], a wire is taken to the terminal f, [Fig. 81], on top of the box. From this terminal a wire is led to the connections of the observing telescope, and thus the mines can be fired by judgment if required, without the aid of the circuit closer.

The signal battery current is always circulating, even when the system is in a state of rest, but in consequence of the resistance placed in this circuit, which may be either a resistance coil in the circuit, added to the resistance of the fuzes, when high tension fuzes are used, or only the former resistance in the case of low tension fuzes, this current is too feeble to form an electro magnet; directly, however, a circuit closer is struck, this resistance is cut out, and thus the signal battery current becomes sufficiently powerful to work the electro magnet of that particular mine.

The circuit of the signal battery, and that to the observing telescope, are broken the instant the lever commences to fall.

To enable the apparatus to be used on the circuit breaking system, a spare lever E is provided for that purpose with each box.

The object to be gained by a system of testing is to ascertain the condition of the electrical submarine mines placed in the defence of a harbour, &c., and should there exist any fault, not only to detect its exact position and cause, but also its magnitude, so that it may be at once determined whether it is necessary to remedy the fault, or whether the electrical apparatus is sufficiently powerful to overcome the defect.

Tests.—There are two distinct kinds of tests, viz.:—

[SHUTTER APPARATUS.]

Mechanical tests are applied to ascertain that the mechanical arrangements of the shutter apparatus, circuit closers, and all similar appliances work efficiently and easily; that the several parts of the mine case when put together for service are thoroughly watertight; that the chains, wire cables, and ropes in connection with the mooring apparatus are of sufficient strength to perform the work required of them; that the weights of the anchors, or sinkers, are such as to keep the mines in position after submersion; and that the case of the mine be sufficiently strong to enable it to bear the external pressure due to the depth at which it may be submerged for a considerable time without any leakage.

The foregoing tests of the mine case and moorings would of course be performed during the process of manufacture, but to prevent any chance of failure they should be repeated before being employed on actual service.

Electrical Tests.—Electrical tests are those which are applied to the several component parts of the system, to ascertain that the electrical conditions necessary to a successful result exist.

The importance of being able to carry out the above in its entirety is understood when it is remembered that a submarine mine becomes practically valueless unless it acts efficiently at the single instant of time that it would be required so to do.

List of Instruments used in Testing.—The following are some of the instruments that are employed in connection with a system of electrical tests:—

Electrometers indicate the presence of a statical charge of electricity, by showing the force of attraction or repulsion between two conducting bodies placed near together. This force depending in the first place on the quantity of electricity with which the conducting bodies are charged, ultimately depends on the difference of potential between them; an electrometer is therefore strictly an instrument for measuring difference of potential.[J]

Sir William Thomson's quadrant electrometer is the most perfect form of electrometer yet constructed, and the one usually employed in cable testing. It consists of a very thin flat aluminium needle spread out into two wings, and hung by a wire from an insulated stem inside a Leyden jar, which contains a cupful of strong sulphuric acid, the outer surface of which forms the inner coating of the Leyden jar. A wire stretched by a weight connects the aforesaid needle with this inner coating. A mirror, rigidly attached to this needle by a rod, serves to indicate the deflection of the needle by reflecting the image of a flame on to a scale. The needle hangs inside four quadrants, which are insulated by glass stems: each pair of opposite quadrants are in electrical connection. Above and below the quadrants two tubes, at the same potential as the needle, serve to screen it and the wires in connection with it from all induction except that produced by the four quadrants. Suppose the needle charged to a high negative potential (-), then if the quadrants are symmetrically placed, it will deflect neither to the right nor to the left, so long as the near quadrants are at the same potential. If one of these be positive relatively to the other, the end of the needle under them will be repelled from the negative quadrant to the positive one, and at the same time the other end of the needle will be repelled from in the opposite direction. This motion will be indicated by the motion of the spot of light reflected by the mirror, and the number of divisions which the spot of light traverses on the scale measures in an arbitrary unit the difference of potential between the + and - quadrants.

The reflecting electrometer being a very delicate instrument, requires careful handling, and should only be used by a practised electrician. Its use would therefore be restricted to important stations, and special tests of a delicate nature.

Thomson's Reflecting Galvanometer.—A galvanometer is an instrument intended to detect the presence of a current and measure its magnitude.

The most sensitive galvanometer as yet constructed is the reflecting galvanometer of Sir William Thomson, a diagram of which is shown at [Fig. 84].

A small piece of magnetised steel watch spring, 3/8ths of an inch long, is fastened with shellac on the back of a little round concave mirror, and of about the size of a fourpenny piece. This is suspended by a piece of unspun silk thread in the centre of a coil of many hundred turns of fine copper wire insulated with silk, and well protected between the turns with varnish. The two ends of the coils are soldered to terminal screws a, b, so that any conducting wire can be joined up to it as required. The little mirror hangs in the middle of its coil, with the magnet lying horizontally. By means of a lamp L placed behind the screen, the light of which passes through a slit M, and is thrown on the face of the mirror, a spot of light is reflected on the scale N.

When a current passes through the coil, the little magnet is deflected, and since the magnet is attached to the mirror, which is very light, both are deflected as forming one body, and the spot of light moves accordingly along the scale N.

A powerful steel magnet S is placed above the coil, and can be moved up or down, whereby the directive force of the earth may be increased or weakened. This magnet S is used to steady the spot of light, which otherwise would shake about, and there would be no certainty about the measurement. A second magnet T is placed perpendicular to the magnetic meridian, to adjust the zero of the instrument, i.e., to bring back the spot of light to a fiducial mark at the centre of the scale when no current is passing.

This instrument should only be used at important stations, and when special tests of a delicate nature are required to be applied.

Astatic Galvanometer.—An astatic galvanometer is that in connection with which an astatic needle is employed, by the use of which the sensitiveness of a galvanometer is greatly increased.

An astatic needle is a combination of magnetised needles with their poles turned opposite ways.

At [Fig. 85] a diagram of such an instrument is shown. Two magnets D and C are joined, with the north pole of one over the south pole of the other, forming one suspended system. In the ordinary form of astatic galvanometer the needles D and C are about two inches long, and are each covered by a coil, these latter being so joined that the current must circulate in opposite directions round the two so as to deflect both magnets similarly. The deflection of the needles D and C is observed by means of a pointer or glass needle A, B, rigidly connected with the astatic system by a prolongation of the brass rod connecting the needles D and C. The coils are flat and of the shape indicated in [Fig. 85], and are also made in two halves, placed side by side with just sufficient space between them to allow the rod to hang freely.

This form of galvanometer, though less delicate than the preceding one, is still a very sensitive one, and should only be applied in the case of fine and delicate tests.

Differential Galvanometer.—A differential galvanometer consists of a magnetic needle surrounded by two separate coils of equal length and material carefully insulated from each other and wound in opposite directions. In using it one circuit acts against the other. If a current of equal strength were passing through each there would be no deflection of the needle, because the influence in both directions is equal. If one current were stronger than the other, the needle would be deflected by the stronger.

This form of galvanometer will be found extremely useful in connection with a system of electrical tests.

Latimer Clark's double shunt differential galvanometer is the instrument best adapted for submarine mine tests.

Detector Galvanometer.—A detector galvanometer is usually made with a vertical needle, and is employed to detect and roughly estimate the strength of a current where no particular accuracy is required.

It consists of a magnetic needle pivoted in the centre of a coil of insulated wire, and having an index needle attached to move with it, the latter appearing on a dial, divided into 360 equal arcs or portions: a diagram of such an instrument is shown at [Fig. 86].

This instrument should be of small size and portable form, and as sensitive as it is possible to make it, under such conditions.

Three Coil Galvanometer.—The three coil galvanometer is provided with a vertical needle, and is in other respects very similar in appearance to the detector galvanometer before described. It is formed with three coils of 2, 10, and 1000 ohms resistance; each coil is connected with a brass plate on the top of the box which encloses the whole, and may be switched into circuit by means of a plug at will. The object of the three resistances is to suit the different resistances that may occur, with a perfect, or imperfect state of the electrical combination in connection with each mine. A diagram of this instrument is shown at [Fig. 87], the dotted portions are inside the case.

Thermo Galvanometer.—A thermo galvanometer is an instrument used to ascertain the power of a firing battery which is employed to ignite platinum wire or low tension fuzes.

The form of thermo galvanometer generally used in connection with a test table, is arranged as follows:—

Two ebonite studs, fitted with brass connecting screws, are fixed to the lid of a box containing some resistance coils, and placed in circuit with them; these studs, placed about ·3 of an inch apart, are arranged to receive a piece of platinum wire which is stretched from one stud to the other; the firing battery being placed in circuit with the platinum wire, and the resistance coils, its working power would then be tested by the fusion of the wire through a given electrical resistance, as indicated by the resistance coils put in circuit.

Another form of thermo galvanometer, which is very compact and portable, is shown at [Fig. 88]. It consists of a wooden box a, with a cover of ebonite b, within the box is placed a resistance coil c; d and e are two ebonite standards ·3" apart, the former of which is connected by a copper wire with the terminal f, the latter to the terminal g; the terminal h is similarly connected to the contact piece k, and the terminal l to the firing key m, at n; the resistance coil c is connected to the terminal g and to the copper wire n; the platinum wire (of which several lengths are used, according to the resistance of the coil c) is placed between the standards d and e. To test a battery, it is only necessary to connect it to the terminals f and h, when by pressing down the key m the power of the battery, according as to its fusing or not the platinum wires, will be ascertained; the use of the terminals g and l is to cut out the resistance, which is effected by connecting them by means of a copper wire.

Siemens's Universal Galvanometer.—Siemens's universal galvanometer is an instrument combining in itself all the arrangements necessary for the following operations:—

The instrument which is shown in elevation and plan at Pl. xxiii., [Fig. 1] and [2] respectively, consists of a sensitive galvanometer which can be turned in a horizontal plane, combined with a resistance bridge (the wire of which bridge instead of being straight is stretched round part of a circle). The galvanometer has an astatic needle, suspended by a cocoon fibre, and a flat bobbin frame wound with fine wire. The needle swings above a cardboard dial divided in degrees; as however, when using the instrument the deflection of the needle is never read off, but the needle instead always brought to zero, two ivory limiting pins are placed at about 20 degrees on each side of zero.

The galvanometer is fixed on a graduated slate disc, round which the platinum wire is stretched. Underneath the slate disc three resistance coils of the value of 10, 100, and 1000 Siemens' units are wound on a hollow wooden block, which protrudes at one side, and on the projection carries the terminals for the reception of the leading wires from the battery and unknown resistance. The adoption of three different resistance coils enables the measuring of large as well as small resistances with sufficient accuracy.

GALVANOMETERS FOR TESTING.

The whole instrument is mounted on a wooden disc, which is supported by three levelling screws, so that it may be turned round its axle. On the same axle a lever is placed which bears at its end an upright arm, carrying a contact roller. This roller is pressed against the platinum wire round the edge of the slate disc by means of a spring acting on the upright arm, and forms the junction between the A and B resistances of a Wheatstone's bridge, which resistances are formed by the platinum wire on either side of the contact roller, one of the three resistance coils forming the third resistance of the bridge. G is the galvanometer, k a milled head from which the needles are suspended, and by turning k they can be raised or lowered, m is the head of a screw which arrests or frees the needle when in motion. h1, h2, h3, h4, are the terminals of the respective ends of the three resistance coils, viz., 10, 100, and 1000 units, which are wound on the wooden block C; these terminals may be connected to each other by means of stoppers, and therefore one or more of the resistances may be brought into circuit as desired, and to the ends of these terminals the wires of the artificial resistances are connected as shown on diagrams Pl. xxiv., [Figs. 1], [2], [3a] and [3b]; f is the graduated slate disc, round which the platinum wire is stretched in a slight groove at the edge of the disc, and is inserted in such manner that about half its diameter protrudes beyond the slate. The ends of the platinum wire are soldered to two brass terminals l and l1, which are placed at the angles formed by the sides of the gap in the slate disc, and which form the junctures, as in the ordinary resistance bridge, between A, n, and the galvanometer on one side, and B, X, and the galvanometer on the other side, of the parallelogram. The terminal l is permanently connected by a thick copper wire or metal strip to terminal h_{1}, and the other terminal l1 is connected in a similar manner to terminal III.

Slate is adopted for the material of which to make the disc f, because it is found by experience to be the material which is the least sensitive to variations in the weather or temperature.

The slate disc is graduated on its upper edge through an arc of 300 degrees, zero being in the centre, and the graduations figured up to 150 on each side at the terminals l and l1 of the bridge wire.

In the centre of the circular plate E of polished wood, supported upon three levelling screws b, b, b, a metal boss is inserted, in which turns the vertical pin a which carries the instrument. This pin, being well fitted to the boss, supports the instrument firmly, but at the same time allows it to be turned freely round its vertical axis without losing its horizontal position when once obtained.

On the arm D D, which turns on the pin a, and somewhat behind the handle g, there is a small upright brass arm d turning between two screw points r, and carrying in a gap at its upper end a small platinum jockey pulley e turning on a vertical axis. This pulley forms the movable contact point along the bridge wire, against which it is kept firmly pressed by means of a spring acting on the arm d. The arm D D, which is insulated from the other parts of the apparatus, is permanently connected with the terminal I. On the top of d a pointer Z or a vernier is fixed, which laps over the upper edge of the slate disc and points to the graduations.

To the pin a is attached a circular disc of polished wood C, about one inch thick, and having a groove turned in its edge for the reception of the insulated wires composing the resistances. The disc C has a projection c, which carries the five insulated terminals marked I., II., III., IV., V., as shown on [Fig. 1] and [2], Pl. xxiii. Terminals III. and IV. can be connected by a plug, II. and V. by the contact key K. Terminal I. is in connection with the lever D D.

[Fig. 3] and [4], Pl. xxiii. show the shunt box supplied with the galvanometer if specially desired; the copper connecting arms a, a are screwed to the terminals II. and IV. By inserting a plug at c ([Fig. 4], Pl. xxiii.), the galvanometer is put out of circuit altogether, whilst by plugging either of the other holes shunts of the value of 1/9, 1/99, or 1/999, are introduced into the circuit, and the effect upon the galvanometer is reduced to 1/10, 1/100, 1/1000, respectively of what it would have been without the insertion of the shunt.

[Fig. 5] and [6], Pl. xxiii., show a battery commutator allowing to bring into the circuit four different amounts of battery power. It is placed in the battery circuit whenever consecutive tests with different batteries are desired to be made, it being only necessary to change the place of the stopper in the battery commutator, the terminal screw a of the battery commutator being connected to terminal V. of the galvanometer, and the screws b, b, b, b to various sections of the battery: see diagram of connections, [Fig. 4], Pl. xxiv.

The application of the universal galvanometer will be clear from the diagrams on Pl ii.; instructions, however, for its practical use are added further on, and also tables for use when measuring conducting resistances.

As will be seen from diagram, [Fig. 1], Pl. xxiv., the proportion between the unknown resistance X, and the artificial resistance n is, when the deflection is read off on the side of the slate disc marked A:

X : n = 150 + a : 150 - a
or, X = 150 + a · n.
150 - a

but if read off on the B side of the disc—

X = 150 - a · n.
150 + a

The values of these two fractions, for every half degree, will be found in the columns headed A and B of the table in the Appendix.

SIEMEN'S UNIVERSAL GALVANOMETER.

SIEMEN'S UNIVERSAL GALVANOMETER.

SIEMEN'S UNIVERSAL GALVANOMETER.

SIEMEN'S UNIVERSAL GALVANOMETER

Measuring Electrical Resistances.—For this purpose the instrument is arranged as a Wheatstone's balance. The connections are made as shown at Pl. xxiv., [Fig. 1] and [5], where X is the unknown resistance.

a.—The needle i is to be brought to the zero point of the small cardboard scale by turning the galvanometer G round its vertical axis, taking care that the needle moves with perfect freedom.

b.—The pointer or vernier Z is to be brought, by means of the handle g, to the zero point of the large scale on the slate disc.

c.—A plug is to be inserted between the terminals marked III. and IV.

d.—The holes 10, 100, and 1000 are, two of them, to be plugged, and one left open, according to the extent of the unknown resistance to be measured; either 10 or 100 must be left open if the resistance is small, and 1000 if it is large.

e.—The two ends of the unknown resistance are to be connected to terminals II. and IV.

f.—The two poles of some galvanic battery are to be connected to terminals I. and V.

When the above-mentioned connections have been made, and on depressing the key K, the battery current is sent into the combination and deflects the needle, say, to the right-hand or B side of the instrument, the pointer or vernier Z must then be pushed, by means of the handle g, to the B side of the instrument. If this is found to increase the deflection of the needle i, the pointer Z should be pushed to the other or A side of the instrument beyond the zero point of the large scale until the needle remains stationary when the key K is depressed.

The number indicated by the vernier Z should be read off carefully, and notice taken whether it is on the A or B side of the large scale. This number must then be referred to the galvanometer table,[K] when the figure opposite to the number, multiplied by the resistance unplugged, is the resistance of X. The value of the resistance to be determined will be thus found by a single operation.

Supposing the reading to be 50 on the A side of the large scale, the resistance n unplugged having been 100 units, we get according to the before-mentioned law of resistance bridge the following proportion (see [Fig. 5], Pl. xxiiiA.):—

X : 100 = 150 + 50: 150 - 50
X 150 + 50 × 100
150 - 50
X = 200 units.

For measuring very small resistances a single cell will be found sufficient; but for large resistances more should be used, say, 15 to 20. If very accurate measurements of small resistances are to be taken, the screw at the end of the moving arm D D should receive one battery wire, terminal V. receiving the other.

Comparing Electromotive Forces.—For this purpose Professor E. du Bois-Reymond's modification of Poggendorff's compensation method is used.

The connections are made as shown at Pl. xxiiiA., [Fig. 2] and [6].

For comparing two electromotive forces E1 and E2, a third electromotor of higher electromotive force E0 is used, and two separate tests taken.

The manipulations a and b are to be the same as before.

c.—The hole between III. and IV. to be left unplugged.

d.—Plugs to be inserted in 10, 100 and 1000.

e.—The two poles of the electromotor of an electromotive force E0 are to be connected to the terminals III. and V.

f.—The poles of the battery whose electromotive force E1 is to be compared are connected to terminals I. and IV. in such a manner that the similar poles of the two electromotors are joined to terminals I. and III., and to IV. and V. respectively.

When depressing the key K the galvanometer needle will be deflected and can be brought back to zero by turning the pointer Z either to the right or to the left. Should for instance the pointer have to be brought to 30° on the A side we have the following equation—

E1 = E 0 150 - 30. . . . . . . . (1),
300 + n

where n is the resistance of the battery E0.

The electromotor E2 is now to be inserted in the place of E1, and the galvanometer needle, when it deflects, again brought back to zero by moving the pointer Z. If for instance the pointer has to be pushed to 40° on the B side to obtain equilibrium we have—

E2 = E0 150 - 40. . . . . . . . (2).
300 + n

By eliminating n from equations 1 and 2 we have

E1 : E2 = (150 - 30) : (150 + 40) = 12 : 19 . . . . . . . . (3).

The two electromotive forces are in the same proportion as the two observed distances of the pointer Z from 150° on the A side of the instrument.

For measuring the Intensity of a Current.—For this purpose the instrument is simply used as a sine galvanometer. The connectionsare made as shown at Pl. xxiv., [Figs. 3a] and [7].

The manipulations a, b, c, and d same as in the second case.

e.—Connect one pole of a battery to terminal II. and put the other pole to earth.

f.—Connect the line to terminal IV.

The galvanometer is then to be turned in the same direction as the needle is deflected until the needle coincides with the zero point. Whilst this is being done the large scale on the slate disc will move under the pointer Z, which must be left stationary; the sine of the angle indicated by Z will thus give the value proportionate to the strength of the current. Should the shunt box be required, it has to be connected with terminals II. and IV.

[Fig. 4] shows the same connections as [Fig. 7], but without the shunt box, and with the battery commutator. Fig. 3a shows diagram of the same connections but with the key K, and [Fig. 3b] the same without the key.

A Shunt.—A "Shunt" is a second path offered to a current traversing a given circuit, or portion of a circuit, so as to diminish the amount of the current flowing through that portion of the circuit. In the diagram shown at [Fig. 89] the shunt diminishes the amount of the current flowing along the circuit between A and B.

If only 1/Nth of the current is to pass along the circuit between A and B (of resistance R) then the resistance of the shunt must equal R/(N - 1).

By the aid of shunts it is quite possible to make use of very sensitive instruments to measure powerful currents.

Commutators or Switch Plates.—A commutator or switch plate is an apparatus by which the direction of currents may be changed at will, or by which they may be opened or closed. Bertin's commutator, which is represented at [Fig. 90], consists of a small base of hard wood on which is an ebonite plate, this by means of the handle m is turned about a central axis between two stops c and c'. On the disc are fixed two copper plates, one of which o is always positive, being connected by the axis and by a plate (+) with the binding screw P, which receives the positive electrode of the battery; the other copper plate i, e, bent in the form of a horse-shoe, is connected by friction below the disc with a plate (-), which plate is connected with the negative electrode N. On the opposite side of the board are two binding screws b, and b', to which are attached two elastic metal plates r, and r'.

On the disc being turned as shown in the figure, the current coming by the binding screw P passes into the piece o, the plate r, and finally the binding screw b, which by means of a copper wire leads the current to the apparatus in connection with b; then returning to the binding screw b', the current reaches the plate r', the piece i, e, and so to the battery by the binding screw N.

If the disc is turned so that the handle m is half way between c and c', the pieces o and i, e, being no longer in contact with the plates r and r', the current will not pass. If m is turned as far as c, the plate o will then touch r', and the current pass to b', and return by b, thus reversing its direction.

"Peg" switches are also often used; they are arranged so that the removal or insertion of a brass peg or plug cuts out, or completes a circuit.

Rheostat.—A rheostat is an instrument used for the comparison of resistances.

SHUNT, COMMUTATOR, RHEOSTAT.

Wheatstone's rheostat, which is shown in elevation at [Fig. 91], consists of two cylinders A and B, one of brass and the other of non-conducting material, so arranged that a copper wire can be wound off the one on to the other by turning a handle C. The surface of the non-conducting cylinder B has a screw thread cut in it for its whole length, in which the turns of the copper wire lie, so that its successive convolutions are well insulated from each other. Two binding screws D, D' connected with the ends of the copper wire are provided, to which the circuit wires are connected. A scale is attached at E, by means of which the number of convolutions on B can be read off; and parts of a revolution are indicated on a circle at one end. The handle C can be shifted from one cylinder to the other.

Supposing the rheostat introduced into a circuit, and the whole of the copper wire wrapped on the metal cylinder A, then, on account of the large section of this metal cylinder, its resistance may be entirely neglected, but for every convolution of the wire on the non-conducting cylinder B, a specific resistance is introduced into the circuit. The amount of resistance can thus be varied as gradually as desired by winding on and off the cylinder B. This instrument is often used in connection with the thermo galvanometer.

Resistance Box.—The general arrangement of a resistance box is shown in the diagram [Fig. 92].

Between two terminal binding screws T and T1 secured on a vulcanite slab are fixed a series of brass junction pieces a, b, c, d; each of these is connected by a resistance coil to its neighbour, as shown at 1, 2, 3, and 4. A number of brass conical plugs with insulating handles of vulcanite are provided, which can be inserted between any two successive junction pieces, as between T and a, or a and b.

With all the plugs inserted, the electrical current will flow direct from T to T1, the large metallic junction pieces directly connected by the plugs would offer no sensible resistance; but if all the plugs were removed, then the current would flow through each of the coils 1, 2, 3, and 4, and the resistance in the circuit would be the sum of the resistances of those four coils. With the plugs arranged as in the figure, the current would flow through coil 4 only, and the resistance in the circuit would be equal to the resistance of that coil.

Wheatstone's Balance.—The electrical conductivity of a body is determined by ascertaining the ratio between the resistance of a certain length of the conductor in question, having a given section, to that of a known length of a known section of some substance taken as a standard.

For this purpose Wheatstone's bridge in connection with a box of resistance coils is the most convenient method.

At [Fig. 94] is shown Wheatstone's balance (Post-office pattern), and at [Fig. 93] the apparatus is reduced into the form of a parallelogram, which is the usual diagram of Wheatstone's bridge. The theory of the bridge is as follows:

Four conductors A B, B C, A D, and D C are joined at A and C to the poles of a battery Z; the resistance between A and B is R; that between A and D is r; that between D and C is R1; and that between B and C is x, the unknown resistance to be measured. A convenient constant ratio is chosen for R1 and r, such as equality 1 to 10, 1 to 100, or 1 to 1000; and then R1 is adjusted until no current flows through the galvanometer G; when this is the case we have R : r=R1 : x, or x = (r/R) × R1; so that if r = R/100, x will be equal to R1/100.

Two keys a and b are inserted; the current is wholly cut off the four conductors until contact is made at a; and then after the currents in the four conductors have come to their permanent condition, contact is made at b to test whether any current flows through the galvanometer. The three resistances R, R1 and r and the resistance of the galvanometer should be small if x is small, and great if x is great.

The conductors A B and A D of the bridge are each formed of three resistance coils having a resistance of 10, 100, and 1000 ohms respectively, inserted between the terminals B and D of the balance, [Fig. 94].

The conductor D C is formed of a set of resistance coils from 1 up to 4000 ohms, amounting altogether to 11,110 ohms, inserted between the terminals D and C of the balance; in the balance, a brass plug being inserted between the terminals D and D1, they may be considered as one terminal D. The conductor B C is the wire to be tested, and is connected to the terminals B and C of the balance.

Measurement of Resistances.—When a resistance is to be measured that is within the range of the coils in R1, R and r are made equal. The needle of the galvanometer will move in a different direction, either to the right or to the left, according as the resistance in R1 is greater or less than the line wire x. The needle remains at zero only when the resistance in R1 is equal to that in x. For r : R :: R1 : x.

WHEATSTONE'S BRIDGE.

When the resistance of x is greater than that of R1, as in an insulation test, the resistance in r is made less than that in R, in order that r and R may have such a proportion one to the other as will enable the coils in R1 to balance a resistance in x, greater than their own, that is to say, greater than 11,100 ohms; thus r : R :: R1 : x, or 10 : 1000 :: 10,000 : 1,000,000, the resistance in the line to be tested would be 1,000,000 ohms, supposing the values of r, R and R1 to be respectively 10, 1000, and 10,000 ohms.

When the resistance to be tested is less than that of the least coil in R1 (1 ohm), then the resistance in r is made greater than in R. Thus r : R :: R1 : x, or 100 : 10 :: 2 : 0·2; the resistance of the line to be tested would in this case be 1/20 of an ohm.

Manipulation.—In all cases the key in connection with the battery should first be depressed, then the galvanometer key, making very short contacts by the latter, just sufficient to show the direction of the deflection, until the coils in R1 are nearly adjusted, otherwise considerable time will be lost in making a series of tests, owing to the swing given to the needle, which will take some little time before it again remains steady at zero. When once the coils in R1 are adjusted, and a balance obtained, it should be ascertained whether the needle will remain steady when contact is made and broken.

Test Tables.—In connection with a system of testing electrical submarine mines, for the sake of convenience and simplicity it is necessary to use a table (termed a "Test Table"), on which all the apparatus used for the purpose of testing are fixed. Several forms of tables have been designed for such a purpose. At [Fig. 95] is shown the method of arranging such a table.[L]

A is an astatic galvanometer placed between two switch plates, B and C; ten other similar switch plates, 1, 2, 3, 4, D, 5, 6, 7, E, and 8, are arranged in front of the galvanometer A; F, G, and H are three terminal plates; K is a box of resistance coils used in connection with the thermo galvanometer M; L is a firing key, and N a battery commutator; O is a three-coil galvanometer; R is a Wheatstone balance (Post-office pattern).

The ten switch plates, 1, 2, 3, 4, D, &c., are used for the connection of any particular line to be tested, as well as for the earth connections and instruments employed in that operation.

"Sea Cell" Tests.—The arrangement shown in the figure is that required in connection with the sea cell test, and Mr. Brown's method of keeping certain earth plates in a bucket instead of in the sea.

If two plates of suitable metal to form a Voltaic battery are placed in salt water and connected by a metallic conductor, a battery is at once formed capable of producing considerable deflection on a moderately delicate galvanometer. Testing by this arrangement has been termed the "sea cell" test.

Arranging Earth Plates.—Mr. Brown's, Assistant-Chemist to the War Department, method of arranging the earth plates is as follows:—

A series of earth plates, such as copper, carbon, tin, zinc, &c., are placed in a bucket filled with sea water, and which is placed in the testing room. The water in the bucket is put in connection with the water of the sea by means of a conducting wire, terminating at one end with a zinc plate in the bucket, and at the other with a zinc plate in the sea. By this means the tests made with the different earth plates in the bucket are identical with those made with corresponding earths placed absolutely in the sea, and therefore these latter may be done away with, the sea cell tests being entirely carried out by means of the bucket earth plates.

In addition to the bucket earth plates there will be several other earth plates in connection with the testing room, these being placed in the sea, such as the zinc earth for the firing battery, the zinc earth for the signalling battery, &c.

Connections of Switch Plates.—The switch plate D is used for the connection of any particular mine cable which it may be required to test. The switch plate E is connected with a zinc earth plate used for testing the firing battery. This must always be in the sea. The switch plate 1 is in connection with a zinc earth in the bucket; 2 is attached to a copper earth plate in the bucket; 3 is attached to a carbon earth plate in the bucket; 4 to a tin earth plate in the bucket; 5 is used for connection with the zinc signalling earth connection in the sea; 6 is attached to a copper earth plate used for the sea cell test, or any other purpose required, in the sea; 7 is attached to a zinc earth plate in the sea; and 8 is a common zinc earth in the sea.

The terminal plates G and H are used for the connection, for testing purposes of the negative and positive poles, of the firing battery, and F is connected with a zinc earth in the sea, for a similar purpose. These plates are in connection with the resistance coils K and the thermo galvanometer M, employed for testing the firing battery, the circuit being closed by the firing key L. Other ways of using these plates may of course be adopted if desired. The resistance coils K range from 0·5 to 100 ohms, and are composed of wire adapted for the passage of a quantity current. A reversing key is generally used in connection with a testing battery and the three-coil galvanometer O. This reversing key would consist of two bridges completely insulated from each other, the upper one attached to the negative, the lower one to the positive pole of the test battery. In their normal position both keys press against the upper bridge, and until one or other of the keys is pressed down no current will pass, the direction of the current being altered by pressing down a different key. The point of each key is provided with a terminal and connected, the one to a zinc earth through the switch plate 8, the other to one terminal of the three-coil galvanometer when the tests are to be applied.

The Wheatstone balance R is used in finding the resistances of electrical cables, balancing fuzes, &c. By means of a commutator, N, the necessary number of cells for any particular test may be thrown in circuit when required.

Test of Platinum Wire Fuze for Conductivity.—The platinum wire fuze may be tested electrically as follows:—

If placed in circuit with a few cells of a Daniell or Leclanché battery and a detector galvanometer, before the platinum wire bridge of the fuze is fixed, there should be no deflection of the needle, for no metallic circuit exists; if it did, such would be fatal to the efficiency of the fuze. If similarly placed in circuit after the bridge has been fixed, a considerable deflection of the needle should result, such deflection being due to the current passing through the metallic bridge, which to be efficient ought to be the sole medium through which the circuit is completed.

Test of Resistance of Platinum Wire Fuze.—The electrical resistance of a platinum wire fuze is ascertained by means of the Wheatstone's balance R and galvanometer A, [Fig. 95]. The terminals of the fuze are connected to the binding screws of the balance, the commutator N and galvanometer A being connected up in circuit. The resistance of the coils is then adjusted by taking out plugs until the needle of the galvanometer A is brought to zero, when the sum of the resistances indicated by the unplugged coils will be equal to that of the fuze. The resistance of a platinum wire fuze might also be ascertained by means of a differential galvanometer instead of a Wheatstone balance.

The electrical resistance of 3/10" of fine platinum wire, weighing 1·9 grains to the yard, is 3/10 of an ohm nearly (Schaw).

Testing High Tension Fuzes.—High tension fuzes require very delicate and careful management in testing them, due to the high electrical resistance of such fuzes, which ranges from 1500 to 2000 ohms, combined with the danger of premature explosion when testing even with a small number of battery cells. Very sensitive galvanometers, such as the reflecting galvanometer, should if possible be used, otherwise the mode of making the tests for conductivity and resistance of a high-tension fuze is similar to that already given for a platinum wire fuze.

Detonating fuzes should always be placed in an iron case during the process of testing.

Insulation Test for Electrical Cables.—To test an electrical cable for insulation, it should first be put in a tank of water, or in the sea, and allowed to soak for at least forty-eight hours. The object of this is to allow the water to penetrate the outer protection of hemp and iron wires, &c., and to search out and get into any weak places there may be in the insulation under the armouring. At [Fig. 96] is shown the method of performing this test. A is a tank holding the electrical cable, which has been in soak for forty-eight hours; B is an astatic galvanometer; C, Z a Leclanché or Daniell battery of great power; and C is an ordinary firing key. One end of the electric cable D is connected to the galvanometer B through the firing key C; the other end of the cable is very carefully insulated; one pole of the battery is connected to the galvanometer B, the other is put to earth in the tank at F; should the insulation be perfect, no deflection of the needle should follow on the key being pressed down. A very slight deflection might be observed on a moderately sensitive galvanometer, due to the current passing through the insulation; its whole length being immersed, the surface through which such a current would pass would be large, and the sum of the infinitesimally small quantities escaping over the whole length, would in the aggregate be sufficient to deflect the needle to a small extent in completing the circuit of the battery. Should any considerable deflection occur, it would indicate a defect or leak in the insulation of the cable, the extent of which would be roughly measured by the amount of such deflection.

By using a reflecting galvanometer a very much more delicate test would be obtained, but for the comparatively short lengths of electric cables used in connection with submarine mines, such accuracy is hardly necessary.

To test an electric cable for conductivity, it would be only necessary to expose the metallic conductor G, and put it in the water of the tank. If the conductivity were good, then the whole of the current would pass through the cable and the needle of the galvanometer would be violently deflected. If the continuity were broken, no deflection would be observed.

Defects observed in the Conductivity of the Cable.—To ascertain the position of a defect in the insulation of a cable, as indicated by the tests above described, it would be only necessary to keep a continuous current flowing through the cable, and gradually take it out of the tank. If the fault existed at a single point, the deflection of the needle would be suddenly reduced at the moment of that point of the cable being lifted out of the water, and therefore its position would be determined with considerable accuracy. Should several defects exist as each was lifted out, a sudden reduction of the deflection would occur.

Discharge Test.—The conductor of an electrical cable may be broken without destroying the insulation, and on applying the foregoing tests, good insulation would be indicated, but no conductivity, and no information would be given as to the position of the fault. Under such circumstances the following test must be applied:—

Put one pole of a very powerful battery to earth, and charge one end of the defective cable, then immediately discharge it through a reflecting galvanometer, and note the extreme limit of the swing of the needle, then, charge the other end of the cable in a similar manner, and discharge it through the same galvanometer, noting as before the swing of the needle. This should be done three or four times, and the average of the deflections taken. Then the position of the fault would be indicated by the proportion between the average deflections in each case, and the cable might safely be cut at that point. Should the precise position of the fault not be discovered in thus cutting the cable, each section should be tested again for conductivity, and that in which a fault was still found to exist should be again tested by the discharge as before.

Test of Electrical Resistance of Cable.—This is effected by balancing it against the Wheatstone balance, in a similar manner to that explained for a fuze. The electrical resistance of the conductor of a cable affords a very correct indication of the quality of the metal of which it is composed. For a very delicate test the reflecting galvanometer should be used.

Electrical Test of Insulated Joints.—Insulated joints and connections, whether of a permanent or temporary nature, should be tested electrically, in a precisely similar manner to that explained for electric cables.

They should be soaked for forty-eight hours, and then tested for insulation, conductivity, and electrical resistance.

In testing permanent joints special tests are carried out, which are described by Mr. Culley in his 'Handbook of Practical Telegraphy.'

Voltaic batteries should be subjected to the following tests:—

For the purpose of testing the potential of a battery, one pole should be put to earth, and with the other one pair of the quadrants of a Thomson's reflecting galvanometer should be charged; when this is done, a certain deflection of the spot of light will occur, and the amount of such deflection, as compared with that produced by a standard cell applied to the instrument in a similar manner, would give the relative value of the potential of the battery.

The following method of determining the internal resistance of a battery is that recommended by Mr. Latimer Clark in his book on electrical measurements.

The instrument employed is a double shunt differential galvanometer, a diagram of which is shown at [Fig. 97]. Connect the battery and a set of resistance coils in circuit between the terminals A and D, and insert plugs in the resistance coils so that they give no resistance; insert plugs at A and C, and also both the shunt plugs at A and D. The current will now flow through one half of the galvanometer circuit only, being, however, reduced to 1/100 of its amount by the shunt D; the deflection of the needle must be carefully read. The plug A must now be removed to B, which causes the battery current to flow through both halves of the galvanometer (each being shunted). The circuit will now be as shown in the figure, and the needle will of course be deflected somewhat more than before. Now unplug the resistance coils which are in circuit with the battery until the deflection of the needle is reduced to its original amount, and the resistances unplugged will be equal to the internal resistance of the battery.

The following is another method of ascertaining the internal resistance of a battery cell.

A circuit is formed, consisting of the battery cell, a rheostat, and a galvanometer, and the strength C is noted on the galvanometer. A second cell is then joined with the first, so as to form one of double the size, and therefore half the resistance, and then by adding a length l of the rheostat, the strength is brought to what it originally was, C.

Then if E is the electromotive force, and R the resistance of cell, r the resistance of the galvanometer, and other parts of the circuit, the strength C in the one case is C = E / (R + r), and in the other = E / ((1/2)R + r + l), and since the strength in both cases is the same, R = 2l, i.e., the internal resistance of the cell is equal to twice the resistance corresponding to the length l of the rheostat wire.

The comparative electromotive force of a battery may be determined by means of a double shunt differential galvanometer in the following method, as recommended by Mr. Latimer Clark.

"This can only be done relatively in terms of some other standard battery. First determine the resistance of the standard and of the other cells to be measured; then insert the shunt plugs at A and D, [Fig. 97], and also at C and B, and join up the standard cell in circuit with a resistance coil to the terminals A and D, and unplug the resistance coils until a convenient deflection is obtained, say 15°; note the sum of the resistances in circuit, including that of the battery galvanometer, resistance coil and connecting wires; now change the battery for another, and by unplugging the resistance coils bring the needle again to the same deflection, 15°; having again found the total resistance in the circuit, the relative electromotive force will be directly proportional to these resistances."

The electromotive force of a battery may also be measured statically by means of Thomson's quadrant electrometer, the poles of the battery being connected with the two chief electrodes of the instrument, in which arrangement no current will pass, and the electromotive force will be directly indicated by the difference of potential observed.

In the case of a quantity battery, that is, a battery capable of fusing a fine platinum wire, its electromotive force and internal resistance may be determined by means of the resistance coils K, and thermo galvanometer M, shown at [Fig. 95].

Tests after Submersion.—After an electrical submarine mine has been placed in position, it should be immediately tested to ascertain that all is right, and similar tests should be applied at intervals to ascertain that the charge remains dry; that the insulation and conductivity of the electric cable remains the same; and that its electrical resistance indicates a state of efficiency.

The nature of the tests applied to determine these points will depend upon the nature of the combination in which the mine is arranged.

The manner of applying the "sea cell" test, by which is ascertained the condition of a system of electrical submarine mines, will be readily understood from the following examples.

The arrangements for testing to ascertain whether a charge is dry, or wet, is shown at [Fig. 98].

z is a plate of zinc introduced in the circuit within the charge, and between the fuze and the shore; another earth plate of carbon x is connected with the electric cable beyond the fuze, forming the ordinary earth connection of the system at that point; and at home a copper earth plate c is used.

First, in the case of a dry charge with the insulation and conductivity of the cable, good; under these circumstances there would be formed a sea cell between the earth plates x, and c, which would produce a certain deflection of the needle of a galvanometer g, which is placed in the circuit, and in a certain direction.

Secondly, in the case of a charge becoming wet, through leakage, with the insulation and conductivity of the cable, good; under these circumstances, a sea cell would be formed between the plates c and z, causing a different deflection of the needle in amount and in direction, by which it would be at once indicated that the charge had become wet.

TEST TABLE, DIFFERENTIAL GALVANOMETER.

"Sea cell" Test for Insulation.—Again, in the case of the insulation of the electric cable being damaged to such an extent as to expose the copper conductor. Under these circumstances there would be formed a sea cell between the copper earth plate c, and the exposed copper conductor of the cable, by which a certain definite deflection of the galvanometer would be observed, which deflection would differ in character from that produced by the copper carbon sea cell, when the insulation of the cable was good, and the system in working order, and therefore it would indicate that some change in the electrical conditions of the system had occurred. The fact that a leak existed in the insulation would be proved by changing the earth plate at home from copper to zinc, carbon, tin, &c.

In the case of no deflection being produced on the galvanometer, on applying the sea cell test, a want of continuity, or inefficient connections would be indicated.

The foregoing afford examples of the vast utility of the "sea cell" in connection with a system of electrical tests for submarine mines, numerous variations of which may be effected by employing a series of earth plates, of different metals, at the home end of the circuit, in connection with a carbon and zinc earth plate at the other end. And the mode of manipulating these tests may, by means of numerous switch plates, as shown at [Fig. 95], be made extremely simple and efficient.

Armstrong's System of Electrical Testing.—A very simple method of testing electrical submarine mines, with which low tension fuzes are used, has been devised by Captain Armstrong, R.E., and is shown at [Fig. 99]. a is the electric cable leading from the shore; b the cable attached to a polarised relay c, and connecting the charge through the fuze f to the earth; b' the cable, attached to another polarised relay c', and connecting the mine with the circuit closer; the polarised relay c, in the mine, is arranged to be worked by a positive current, that is to say, the wire surrounding the core is so wound as to increase the polarity of the electro magnet, near the armature d, when a positive current is passed through it, and to diminish the polarity when a negative current is passed through the wire surrounding the core; the polarised relay c' within the circuit closer is arranged to be worked by a negative current, the coil being so wound as to produce an influence exactly the reverse of c.

Then, a positive current passing along the line wire a, the armature d in the charge will be attracted, while d' will remain unaffected; again, if a negative current be circulated, the armature d' within the circuit closer will be attracted, while the armature d will remain unaffected. Two insulated wires forked together are wound round each electro magnet, one a thin wire (g and g') having a considerable resistance, about 1000 ohms, being connected direct to the earth plates e and e', and the other a thick wire (h and h') offering a very small resistance, and so arranged that when the armature is attracted, they may be in contact with and complete the circuit through the armature to earth.

The thin wire coils are so arranged that a certain number of Leclanché cells (ten or twelve, as may be desired) will make the electro magnets act, while with fewer cells the current would be too weak, and would therefore pass through them to earth without affecting the armature.

By means of the three-coil galvanometer, a table of the deflections, obtained by the foregoing system of testing, should be carefully recorded, when the circuit is known to be in good working order, so that any defect in the circuit would be at once indicated on the application of the various tests, by the results so obtained differing from those originally recorded. When a system of submarine mines is placed in position for the purposes of practice and experiment, every trouble should be taken to endeavour to fix the exact position of any defect that may exist, also to ascertain its magnitude, &c., but in time of war, should a defect exist in the system, no time must be lost in such operations, but the mine at once lifted, and the fault repaired, or a fresh one laid in its place, unless the presence of an enemy or other imperative cause should prevent such work being done.

Austrian Testing Table.—The following is a description of the Austrian testing table, and their mode of making electrical tests with it, in connection with their system of self-acting electrical submarine mines.

METHODS OF TESTING.—ARMSTRONG,—AUSTRIAN.

Its design is shown at Fig. 100; c z represents the battery with one pole to earth at e, and the other in connection with an intensity coil a, through which the current passes to the contact plate b. When it is desired to put the system of mines in connection with the table, in a state of preparation to be fired by the contact of a vessel, a plug is inserted between the contact plates b and f, and the current passes through the galvanometer g, and electrically charges the conducting wires connecting the mines with the battery, through the several binding screws on the contact plates, numbering 1, 2, 3, &c. The fact that the charge has been fired is also at once indicated on the galvanometer g.

Test to discover an Exploded Charge.—It then becomes necessary to ascertain which particular mine of the system has been exploded; for this purpose a separate circuit in connection with a single cell d is employed. This cell is in connection through a galvanometer g' (a more sensitive instrument than the galvanometer g) with the pivot of the key h, and rheotome R, which latter is connected, as shown by the dotted lines, with each individual mine of the system attached to the contact plates numbered 1, 2, 3, &c. The handle of the rheotome is moved round, to each number in succession and directly it is placed in contact with that corresponding to the exploding mine, the electrical circuit is completed through the exposed end of the fractured wire, and this is indicated by the galvanometer g'. During the testing process the firing battery c z must be disconnected; this is done by raising one of the bridges i i with which each group of ten mines is provided.

Insulation Test.—The rheotome and testing galvanometer g' are also used to test the insulation of the electric cables connecting the mines to the testing table. This is done in precisely the same manner as testing for an exploded mine: the handle of the rheotome is turned round, and each cable connected in succession with the testing circuit as before; should the galvanometer g' remain stationary, the insulation is good; but should a defect of insulation exist, the current passing through it would act on and deflect the galvanometer, indicating the particular line in which it exists, and, roughly, its extent in proportion to the deflection shown; should the fault be considerable, the defective cable should be at once detached, as the current lost through it might so diminish the working power of the firing battery, as to prevent it exploding any of the fuzes attached to the group in connection with it. By the above arrangement, the insulation of each line can be tested at any moment required.

In making the delicate test for insulation, which should invariably be done at leisure, and, if possible, when an enemy's vessels are not in the vicinity of the mines, a large number of Daniell's or other cells of suitable form should always be used. To do this, it would only be necessary to connect such a battery in place of a single cell permanently arranged, as described, in the testing circuit, and to proceed with the details of the operation as before. As the cable would, in actual work, always be charged with the full power of a firing battery, the value of its insulation to resist an electrical charge at such a high potential would be an important point to determine. The fuzes being entirely out of the circuit till the moment of the action arrives, no danger of a premature explosion need be apprehended; if a fuze were in such a position as to be fired prematurely, it would be exploded, in connection with the firing circuit, independently of the operation of testing the insulation of the cables.

To render a Channel Safe.—In order to render the channel safe for a friendly vessel, it is only necessary to remove the plug from between the contact plates b and f; this disconnects the firing battery from the circuit.

Defence of Harbours by Booms, &c.—Booms or cables supported by rafts may also be employed in the defence of harbours, or rivers, either by themselves, or in combination with submarine mines; in the latter case, the booms, &c., may be moored either in advance of the mines, or in rear of the front row, this last method of mooring them being the most effective one.

There are a great variety of forms in which a boom may be constructed. The qualities essential for a good and practicable boom are:—

Construction of a Boom.—The general construction of a boom consists of a main cable, buoyed up at intervals by floats. The main cable may be either wire, chain, or rope, the former being very much superior for this purpose to chain or rope. The floats consist of balks of timber built round the main cable and bound together by means of iron hoops &c. A space is left between each float, by which a certain amount of flexibility in the boom is obtained, without which it would be of comparatively little use, as it might be easily overrun.

It must be borne in mind, in constructing all such booms, that the smaller the proportion of timber used in forming the floats to the cable, consistent with buoyancy, the stronger will be the structure.

A very important feature in connection with such a mode of defence is the manner of mooring it; for if it be moored so as to be unyielding, then its sole power of resisting a vessel charging it is the actual strength of the materials composing the structure, but if it be moored so that it is capable of yielding to a sudden blow, this force will be to some extent absorbed, and resistance of the defence greatly increased.

The raft employed to support the main cable should be moored by means of very heavy chains (without anchors) in the direction of the attack, and with ordinary anchors and cables on the other side.

As a rule, the booms should be moored obliquely to the direction of the current, where there is any, as the tendency of the current to overrun the boom when so placed will be less, and also a ship ramming it must place herself athwart the current to attack the boom at right angles.

Clearing a Passage through the Torpedo Defences of an Enemy.—The subject of clearing a passage through the torpedo defences of an enemy is one fraught with innumerable difficulties, on account of the varied nature and impracticability of obtaining accurate and certain information of such defences, and thus it is impossible to lay down any fixed rule or plan for carrying out such an operation.

In fact, it will be only under the most favourable circumstances that such a service will be successfully accomplished, that is to say, in the case of a harbour or river defended by submarine mines but unsupported by guns, or guard boats, or where the electric light is used.

Numerous methods have been devised from time to time to effect the destruction of an enemy's submarine defences, among which are the following:—

Projecting Frames, &c., from the Bows of a Vessel.—This method was adopted by the Federals during the American civil war of 1861-5, and in many instances it was the means of saving their ships when proceeding up rivers which had been torpedoed by the Confederates, though notwithstanding this precaution several vessels were sunk. The submarine mines against which this mode of defence was used, were in nine cases out of ten mechanical ones, and therefore the framework defence afforded a better means of protection then, than would be the case now that electrical ground mines and circuit closers are used, as the framework would catch the circuit closer only, and the vessel would probably be over the mine when the explosion took place. The Americans moor their circuit closers in rear of their mines, so that a vessel fitted with a bow frame or not, coming in contact with the former must be right over the charge at the instant of explosion.

Against ground electrical mines fired at will, the bow net, &c., is no protection whatever, still under certain circumstances it would be found extremely useful.

Sweeping for Submarine Mines.—This method of clearing a channel of submarine mines could not possibly be carried out under artillery fire, but in waters not so defended it would prove of some value.

Where only buoyant mines, or ground mines with circuit closers are to be cleared away, two or more boats dragging a hawser between them would be sufficient to discover them, and so lead to their destruction; but where dummy mines and inverted creepers are moored in addition, another method of sweeping must be resorted to, viz., that of bringing an explosive charge of gun-cotton to act on the obstruction grappled, and thus destroy it. This is effected by lashing a charge to each end of the sweep, so that whatever is grappled may slide along it, until caught by hooks, which are attached for this purpose to the centre of the charge. On grappling an obstruction, the two boats drop their anchors, one hauling in, the other veering out the sweep, until the charge is hooked by the obstruction; this being effected, the boats move out of range, and the charge is fired.

Creeping for Electrical Cables, &c.—Creeping is the method employed for picking up the electric cables of the enemy's submarine mines, and is effected by boats towing an ordinary grappling iron, or specially prepared creeper on the ground.

In both sweeping and creeping it would be found necessary to employ a diver, who would ascertain the nature of the grappled obstructions which could not be easily raised by the boats.

The Lay torpedo boat, which is fully described in the chapter on offensive torpedoes, is capable of being used for the foregoing purposes.

Countermining.—Countermining, that is, the destruction of submarine mines by the explosion of other mines dropped close to them, will under certain conditions prove of great use in clearing harbours of mines. This method could not be operated in waters properly guarded and swept by artillery fire.

There are two distinct methods of laying out countermines, viz.:—

1.—In a boat, which may be either towed, or hauled out to its destination, or may be steered, and controlled by electricity.

2.—By attaching them to buoys so that they are suspended at the proper depths, and then hauled out by means of a warp to an anchor which has been previously placed in position.

Both of the foregoing methods have been successfully manipulated in practice, the first method, where the boat carrying the countermines is towed either by a pulling or steam boat being the most practicable one. A large amount of material would be required for clearing a channel by means of countermines: for example, if the mines to be attacked require 500-lb. gun-cotton charges to be used, 7-1/2 tons of the explosive, besides cables, buoys, &c., would be required to clear a passage about one mile in length and 200 feet in width.

A ship's launch will carry about twelve of these 500-lb. countermines, with all the gear attached thereto.

Experiments to ascertain the effect of countermining have been carried out in England and Europe for the last five years, some of which are given at length in the chapter on "Torpedo Experiments." During the Turco-Russian war, a portion of the Danube was swept in the ordinary and most simple manner by the Turks, and five Russian electro contact buoyant mines were picked up; one other exploded during the process of dragging it to the surface, but no injury occurred to those at work.

Destruction of Passive Obstructions.—To clear away booms, or other passive obstructions, if not possible to cut them away, they may be destroyed by outrigger boats exploding their torpedoes underneath, and in contact, or by attaching charges of gun-cotton at intervals, and then exploding them simultaneously. When a chain is horizontal, and therefore somewhat taut, a charge of 3-1/2 lbs. of gun-cotton (this explosive, being the most effective and convenient for such purposes, should always be used) will be found sufficient to destroy it, no matter what size, and whether the chain is in or out of the water, the charge being of course placed in contact with it. Great uncertainty must always attend the supposed clearance of a channel, or passage of submarine mines, as was exemplified during the American civil war, when most of the Northerners' vessels were destroyed while moving over ground which had been previously carefully dragged, and buoyed, and this fact, coupled with the tediousness and danger of performing such a service, proves the enormous value of a system of defence by submarine mines.