LIST OF ILLUSTRATIONS.
Frontispiece, The Queen 45″ Spark Coil.
| FIG. | PAGE | |
| [1]. | Section of Coil | 4 |
| [2]. | Insulating Tube Ends | 10 |
| [3]. | Sectional Winding | 11 |
| [4]. | Section " First Method | 12 |
| [5]. | "" Second Method | 13 |
| [6]. | Proportional Diagram of Coil | 15 |
| [7]. | Section Winder, End View | 17 |
| [8]. | "" Face View | 17 |
| [9]. | Assembly of Coils | 18 |
| [10]. | Polechanging Switch | 31 |
| [11]. | Disruptive Tesla Coil | 35 |
| [12]. | Spark Coil for Gas Engine | 38 |
| [13]. | Reproduction of a 32-inch Spark | 47 |
| [14]. | Simple Medical Coil | 53 |
| [15]. | Connections for Simple Medical Coil | 55 |
| [16]. | Interchangeable Medical Coil | 56 |
| [17]. | Vibrator for Medical Coil | 60 |
| [18]. | Simple Contact Breaker | 65 |
| [19]. | Imperfect Form of Contact Breaker | 67 |
| [20]. | Superior Form of Contact Breaker | 67 |
| [21]. | Spotteswoode Contact Breaker | 69 |
| [22]. | Polechanging Contact Breaker | 74 |
| [23]. | Wehnelt Interrupter | 78 |
| [24]. | Ribbon Vibrator | 81 |
| [25]. | Queen Contact Breaker | 82 |
| [26]. | Adjustable Contact Breaker | 86 |
| [27]. | Cone Contact Breaker | 88 |
| [28]. | Coil Head Contact Breaker | 89 |
| [29]. | Leyden Jar | 101 |
| [30]. | Plate Condenser | 102 |
| [31]. | Arrangement of Condenser Plates | 104 |
| [32]. | Condenser Charging, First Method | 110 |
| [33]. | Condenser Charging, Second Method | 112 |
| [34]. | Adjustable Condenser | 118 |
| [35]. | Spark between Balls | 125 |
| [36]. | Short Spark between Balls | 125 |
| [37]. | Sparkling Pane | 125 |
| [38]. | Luminous Design | 128 |
| [39]. | Electric Brush | 128 |
| [40]. | Spectrum—Solar | 132 |
| [41]. | Spectroscope and Coil | 133 |
| [42]. | Simple Air Pump | 141 |
| [43]. | Geissler Air Pump | 144 |
| [44]. | Sprengel Air Pump | 144 |
| [45]. | Solution Tube | 150 |
| [46]. | Fluorescent Bulbs | 150 |
| [47]. | Ruby Tube—Crookes | 150 |
| [48]. | Iridio-platinum Tube—Crookes | 151 |
| [49]. | Revolving Wheel | 154 |
| [50]. | Tube Holder | 157 |
| [51]. | Side View of Wheel | 157 |
| [52]. | Geissler Tubes | 160 |
| [53]. | Triangle on Disc | 161 |
| [54]. | Maltese Cross on Disc | 161 |
| [55]. | Gas Lighting Circuit | 165 |
| [56]. | Connections for Gas Burners | 169 |
| [57]. | Bartholdi Automatic Burner | 172 |
| [58]. | Connections for Automatic Burner | 174 |
| [59]. | The Grenet Cell | 180 |
| [60]. | The Fuller Cell | 184 |
| [61]. | The Gethins Cell | 193 |
| [62]. | Lead Plate for Storage Cell | 201 |
| [63]. | Wooden Separator | 201 |
| [64]. | Charging with Rheostat | 207 |
| [65]. | Charging with Lamps | 207 |
| [66]. | Harrison Electrodes | 211 |
| [67]. | Hydrometer | 221 |
| [68]. | Hertz Resonator | 227 |
| [69]. | Tesla Circuit | 229 |
| [70]. | Tesla Cut Out | 231 |
| [71]. | Tesla Cut Out, Top Plan | 232 |
| [72]. | Circuit for X Ray Apparatus | 237 |
| [73]. | Queen's Self-Regulating X Ray Tube | 240 |
| [74]. | Transmitter for Wireless Telegraphy | 250 |
| [75]. | Receiver for Wireless Telegraphy | 252 |
| [76]. | The Branley Coherer | 254 |
| [77]. | Clarke's Oscillator | 259 |
| [78]. | Triple Oscillator | 259 |
| [79]. | Air Wire Insulators | 263 |
CHAPTER I.
COIL CONSTRUCTION.
In commencing a description of the Ruhmkorff coil and its uses, a brief mention of the fundamental laws of induction directly bearing on its action will assist in obtaining an intelligent conception of the proper manner in which it should be constructed and handled.
Any variation or cessation of a current of electricity flowing in one conductor will induce a momentary current in an adjacent conductor; and if the second conductor be an insulated wire coiled around the first conductor, also a coil of insulated wire, the effect is heightened. The intensity of the secondary or induced current increases with the number of turns of its conductor, the abruptness and completeness of the variation of current in the first or primary coil, and the proximity of the coils. And the insertion of a mass of soft iron within the primary coil by its consequent magnetization and demagnetization augments still further the inductive effect. There are other contributing causes which cannot be treated of here, but are of not so much importance as the foregoing.
In the Ruhmkorff coil, which is an application of the above laws, the primary coil is of large wire and the secondary coil of extremely fine wire, of a length many thousand times greater than the wire of the primary coil. The current is abruptly broken in the primary circuit by a suitable device—the contact breaker or rheotome. The current induced in the secondary at the make of the circuit is in the opposite direction to that of the primary coil and battery, but the current at the break of the circuit is in the same direction as that of the primary. The effect of the current at the break of the circuit is more powerful than that at the make, which latter is also somewhat neutralized by the opposing battery current. A condenser or Leyden jar is connected across the contact breaker to absorb an extra current induced in the primary coil by the break of the circuit, which would tend to prolong the magnetization of the core beyond the desired limit.
The whole apparatus is mounted on a wood base, having the condenser in a false bottom for the sake of compactness.
It is not herein intended to describe all the minor operations in the construction of a Ruhmkorff coil. A sufficient description and review of the main points to be considered, however, will be given to enable a person fairly proficient in the use of simple tools to construct a serviceable instrument.
The parts and their arrangement in relation to one another are shown in Fig. 1, but are not drawn strictly to scale, although very nearly so.
Fig. 1.
C is the core, consisting of a bundle of soft iron wires as fine as can be obtained. The greater the subdivision of the core the quicker will it respond to the magnetizing current in the primary coil, and lose its magnetism when the current ceases. It has another advantage, in that the disadvantageous eddy, or Foucault currents, are lessened, which fact, however, is of not enough importance to need extended consideration.
Many coil-makers saturate the core with paraffin or shellac, which is of slight benefit. This core is wrapped in an insulating layer of paraffined paper or enclosed in a rubber shell, there not being any great necessity to use more than ordinary insulation between the core and the primary coil.
In the majority of induction coils or "transformers" used in the alternating current system of electric lighting, the iron cores form a closed magnetic circuit. A closed magnetic circuit in a Ruhmkorff coil could be obtained by extending the iron core at each end and then bending and securing the ends together, forming, as it were, a ring partly inside and partly outside the coil. But although the inductive effects would be heightened and less battery power required, the slowness of the circuit to demagnetize would alone be detrimental to rapid oscillations of current.
There would also be a loss from a greater hysteresis (energy lost in the magnetization and demagnetization of iron). A core magnetizes quicker than it demagnetizes, and the latter is rarely complete; a certain amount of residual magnetism remains, hysteresis being strictly due to this retention of energy (Sprague). Hysteresis shows itself in heat, but must not be confounded with Foucault or eddy currents. The latter are corrected by subdividing the metal, but the former depends upon the quality of the metal, and increases with its length.
Moreover, a coil with a closed magnetic circuit requires an independent contact breaker.
In most of the alternating currents used in lighting their rapidity of alternation is but one hundred and twenty-five periods per second. As in the simple electromagnet, the proportions of diameter and length of the primary coil and core will determine its rapidity of action. A short fat coil and core will act much quicker than a long thin one. But on a short fat coil the outside turns would be too far removed from the intensest part of the primary field. A good proportion of core length is given in the following table:
| Spark Length of Coil. | Iron Core. |
|---|---|
| ¼ | 4″ × ½″ |
| ½ | 5″ × 10∕16″ |
| 1 | 7″ × ¾″ |
| 2 | 9″ × 1″ |
| 6 | 12″ × 1⅛″ |
| 12 | 19″ × 1½″ |
The primary coil P consists of two or not more than three layers of insulated copper wire of large diameter, being required to carry a heavy current in a 2-inch spark coil, probably from 8 to 10 amperes. In designing the primary coil a great advantage arises from using comparatively few turns but of large wire. Each turn of wire in the primary has a choking effect upon its neighbor by what is termed self-induction.
As the primary coil and core may be considered as an electro magnet, it may not be out of place to notice the rule governing such. Magnetization of an iron core is mainly dependent upon the ampere turns of the coil surrounding it—that is, one ampere carried around the core for one hundred turns (100 ampere-turns) would equal in effect ten amperes flowing through ten turns. Practically speaking, there would be certain variations to the rule, for one difficulty would arise in that the smaller wire used in conveying the smaller current would fit more compactly and allow more turns to be nearer the core, the active effect of the turns always decreasing with their distance from the core. And although a large current and few turns would not have so much self-induction, there would be trouble at the contact breaker, owing to the large current it would have to control.
The most suitable sizes of wire for the primary coil are: No. 16 B. & S. for coils up to 1 inch spark; No. 14 B. & S. up to 4 inches of spark, and No. 12 B. & S. for a 6-inch spark coil. The coil should be, say, one-twelfth of the core length shorter than the core.
I is the insulating tube between the primary coil and the secondary coil S. Here great precaution is necessary to prevent any liability of short circuiting or breaking through of sparks from the secondary coil. This danger cannot be underestimated, and the tube should be either of glass or hard rubber, free from flaws, varying in thickness with the dimensions of the coil. It should extend at least one-tenth of the total length of the primary coil beyond it at each end. The end of this tube can be turned down so as to allow of the hard rubber reel ends being slipped on and held in position by outside hard rubber rings (Fig. 2).
Fig. 2.
The secondary coil consists of many turns of fine insulated copper wire separated from the primary coil by the insulating tube and a liberal amount of insulating compound at each end. In coils giving under 1 inch of spark this coil may be wound in two or more sections.
Fig. 3.
The usual manner of constructing these sections is to divide up the space on the insulating tube by means of hard rubber rings placed at equal distances apart, in number according to the number of sections desired (Fig. 3). The space between each set of rings, or between the coil end and a ring, is wound with the wire selected, the filled sections constituting a number of complete coils, which are finally connected in series. The sectional method of winding prevents the liability of the spark jumping through a short circuit, but heightens its tendency to pass into the primary coil at the ends, where it must be therefore specially insulated from it.
Fig. 4.
In winding these sections there is a method now generally adopted which has many good points, although at first it may seem complicated. The old way of filling two sections was to wind both in the same direction as full as desired, then join the outside end of the left-hand coil to the inside end of the right-hand coil. This necessitated bringing the outside end down between two disks, or in a vertical hole in the sectional divider, and thereby rendered it liable to spark through into its own coil. This is shown in Fig. 4, A and C inside ends, B and D outside ends, the disk being between B and C.
Fig. 5.
Reference to Fig. 3 shows the new method, and Fig. 5 shows an enlarged diagram of sections 2 and 3 of Fig. 3.
Sections 1 and 3, Fig. 3, are filled with as many turns as desired; the spool is then turned end for end, and sections 2 and 4 are wound, being thus in the opposite direction of winding to sections 1 and 3.
The inside ends of 1 and 2 and 3 and 4 are soldered together, and the outside ends of 2 and 3 are also soldered together.
The outside ends of 1 and 4 serve as terminals for the coil.
This method of connection leaves all the turns so joined that the current circulates in the same direction through them all, as will be seen by an examination of the enlarged diagram, Fig. 5.
Sprague, in his "Electricity: Its Theory, Sources, and Application," recommends that the turns of wire in the secondary coil shall gradually increase in number until the middle of the spool is reached, and then decrease to the spool end, in order that the greatest number of turns be in the strongest part of the magnetic field (see Fig. 6). D D D are section dividers, S secondary windings, P primary coil. The selection of the size of wire to be used depends on the requirements as to the spark. If a short thick spark be desired, use a thick wire, say No. 34 B. & S.; if a long thin one, use No. 36 to No. 40 B. & S.
Fig. 6.
Although it is impossible to lay down rules for determining the exact amount of wire to be used to obtain a certain sized spark, yet a fair average is to allow 1¼ pounds No. 36 B. & S. per inch spark for small coils and slightly less for large ones.
The most satisfactory and perhaps the easiest way for large coils is to wind the secondary in separate coils, made in a manner similar to that employed in winding coils for the Thompson reflecting galvanometer. This method, first described by Mr. F. C. Alsop in his treatise on "Induction Coils," is somewhat as follows:
A special piece of apparatus (Figs. 7 and 8) is necessary, but presents no great difficulty in manufacture. A metal disk, D, one-sixth of an inch thick and 7 inches in diameter, is mounted on the shaft S. A second disk is provided with a collar and set screw, A, in order that it may be adjusted on the shaft at any desired distance from the stationary one. When the diameter of the coil to be wound has been decided upon, a wooden collar, W, with a bevelled surface is slipped on the shaft, it corresponding in diameter with the desired diameter of the hole through the centre of the secondary coil. As these coils are going to be made as flat rings and slipped on over the insulating tube, a remark here becomes necessary on this diameter. Reference to Fig. 9 will show that it is intended that the coils near the reel ends shall fit very loosely on the tube T (Fig. 1)—in fact, that there shall be a clearance of possibly one-half inch in the extreme end, diminishing gradually to a fifteenth of an inch in the centre coils. Therefore it becomes necessary to provide a number of wooden rings equal to the desired diameter of the central hole in the coil. The thickness of the wood determining the width of the individual coil depends on the selection of the operator; but the rule may be laid down that the narrower the coils the better the insulation of the complete coil will be on completion.
Fig. 7. Fig. 8.
Fig. 9.
One-sixteenth of an inch is a very fair average, and has been generally adopted by the writer.
A quantity of paper rings are now cut out of stout writing paper which has been soaked in melted paraffin. If a block or pad of letter paper be soaked in paraffin and allowed to become cold under pressure, the ring may be scratched on the surface of it and the block cut through on a jig saw. The central apertures of course will vary in size with their position on the tube T (Fig. 9).
The coil winder is now either mounted in a lathe or fixed in a hand magnet winder in such manner that it can be steadily and rapidly rotated. The wire to be wound comes on spools, which can be so threaded on a piece of metal rod that they turn readily. A metal dish containing melted paraffin is provided with a round rod, preferably of glass, fixed under the paraffin surface, so that it can rotate freely when the wire passes under it through the paraffin. Two paper rings are slipped on the winder that they may form, as it were, reel ends for the coil, and if the metal disks have been warmed it is an easy matter to lay them flat.
The end of the wire is then passed through the paraffin under the glass rod and through the hole H in the metal disk for a distance of, say, 6 inches, and held to the disk outside with a dab of paraffin or beeswax. Then the winder is rotated and the space between the paper disks is filled with wire. The paraffin, being hot, will adhere to the wire, and cooling as the wire lays down on the winder, hold the turns together and at the same time insulate them from each other. It will not be possible to lay the wire in even layers, as would be necessary in winding a wider coil, but the spaces can be filled up, using ordinary care that no radical irregularity occurs—that is, that only adjacent layers are likely to commingle.
When the space is filled up to the level of the paper disks and the paraffin is hard, loosen the set screw, and removing the outside disk, the coil can be slipped off, or a slight warming will loosen it. Any number of these coils can be made, and there are the advantages in this mode of construction that a bad coil will not spoil the whole secondary, and that the wire can be obtained in comparatively small quantities.
As each coil will not be of very high resistance, the continuity of the wire can be readily tested by means of a few cells of battery, connecting one end of the coil to one pole of the battery, and the other pole of the battery and coil end touched to the tongue. If a burning sensation is experienced, the connection is not broken. Where possible the coils should be measured as to their resistance on a Wheatstone bridge.
When the requisite number of coils has been prepared, they are assembled in the following manner (Fig. 9): The coils, having their aperture diameter graded, are placed in order, and starting with the one having the largest hole, it is slipped over the primary protection tube T, one end being brought out through a hole in the reel end drilled vertically or between the reel end and the coil. A couple of paper rings are then slipped on the tube, and another coil placed over them, having its ends connected as in Fig. 3. This process is continued until all the coils are in place. The annular space between the coils and the tube T (Fig. 9) is filled in with melted paraffin and the coils gently pressed together, so as to form a compact mass, paraffin being poured over the outside of the whole combination. Before winding any wire used in this work it must be perfectly dry, which end can be accomplished by subjecting the whole spool to a short period of baking in a moderately warm oven.
The accompanying table gives the length of No. 36 silk-covered wire that will fill a linear space equal to one thickness of the wire in different-sized rings. This size wire wound tight will give 125 turns per linear inch. Therefore on a ring having a middle aperture of 1½ inches and an outside diameter of 4 inches, there will be 156 turns, or a total length of 1347 inches. This is obtained as follows: 1½ inches × 3.1416 = 4.7124 (or 4.712); 4 inches × 3.1416 = 12.5664 (or 12.56); (4.712 + 12.56)∕2 = mean circumference—viz., 8.635 inches.
This mean × number of turns in thickness of ring between the two circumferences—viz., 156 = 1347 inches.
Table of Secondary Windings.
| No. 36 Silk-Covered Wire.125 Turns per Linear inch. 13,306 Feet per Pound | 1½″ Aperture Diameter, 4.712″ Aperture Circumference. | 2″ Aperture Diameter, 6.283″ Aperture Circumference. | 2½″ Aperture Diameter, 7.854″ Aperture Circumference. | ||||||
|---|---|---|---|---|---|---|---|---|---|
| Outside diameter | 4″ | 5″ | 6″ | 4″ | 5″ | 6″ | 5″ | 6″ | 7″ |
| Outside circumference | 12.56 | 15.70 | 18.84 | 12.56 | 15.70 | 18.84 | 15.70 | 18.84 | 21.99 |
| Mean circumference | 8.635 | 10.20 | 11.78 | 9.421 | 10.99 | 12.56 | 11.78 | 13.35 | 14.92 |
| Turns between circumferences | 156 | 219 | 282 | 125 | 188 | 250 | 156 | 219 | 282 |
| Distance between aperture and outside, in inches | 1.25 | 1.75 | 2.25 | 1 | 1.50 | 2 | 1.25 | 1.75 | 2.25 |
| Length of wire, in inches | 1347 | 2234 | 2650 | 1178 | 2066 | 3140 | 1838 | 2924 | 4207 |
To obtain the length of wire necessary for a ring occupying more than the space of one turn on the primary insulating tube, multiply the length before obtained by the number of turns in the space it occupies. Thus a flat ring one-tenth of an inch thick would equal 1347 inches × 12.5.
This rule is necessarily only approximate, owing to the way the wires bed on each other from their cylindrical section. In actual practice, when the wire is run through the paraffin bath not more than 50 per cent of the calculated wire will occupy the space. And the thickness of the paper rings must also be added when figuring the total length of the coil. In the iron-clad transformers or induction coils of highest efficiency used in the alternating current electric light system, the rule for determining the windings of the coils is based on the ratio of the turns of wire in the primary to the turns in the secondary, the electromotive force in the primary, and the lines of force cut by the windings.
The secondary ends can be attached to binding posts mounted on the reel ends. Unless these reel ends be very high and clear the outside of the coil considerably, it is better to mount the binding posts on the top of the hard rubber pillars. A neat plan is to mount on the top of the coil a hard rubber plate reaching from reel end to reel end, and place the binding posts on that.
A discharger consists of two sliding metal rods with insulated handles passing through pillars attached to the secondary coil. The inside ends of these rods is provided with screw threads for the ready attachment of the balls, points, etc., which are to be used. The substance to be acted upon is laid on a rubber or glass table midway between the rod pillars and slightly below the level of the rods.
By hinging the rod pillars, or using a ball and socket joint, the discharger can be inclined so as to be better brought near the substance on the table.
The next important part of the coil is the contact breaker.
The armature R is a piece of soft iron carried at the end of a stiff spring, in about the middle of which, at B, is riveted a small platinum disk or stud. The adjusting screw A has its point also furnished with a piece of platinum, which is intended to touch the platinum on the spring when the latter is in its normal position. The core C of the coil serves as an electro-magnet. When the current flows from the battery (represented by the figure at L) through the primary coil and armature spring to the adjusting screw, it causes the armature to be drawn to the magnetized core, but thereby draws the platinum disk away from the adjusting screw. In so doing it breaks the circuit, the magnet loses its power, and the elasticity of the spring reasserting itself, carries the armature back, thereby reclosing the circuit. This is repeated many times in a second, the result being a continual vibration of the spring, and a consequent interruption to the current.
The condenser or Leyden jar J, connected as in the diagram to the base of the vibrating spring at K and to the adjusting screw wire M, is constructed as follows: On a sheet of insulated paper is laid a smaller sheet of tinfoil, one edge of which projects an inch or so over one end of the paper. Another sheet of paper covering this carries a second sheet of tinfoil, one end of which projects as in the first sheet, but at the opposite end of the paper. Tinfoil and paper sheets are laid in this manner alternately until a sufficient number is attained. The projecting ends are then clamped together and the whole pile immersed in melted paraffin, as will be described in a subsequent chapter. Wires are affixed to these clamped ends which serve to connect the condenser with the contact breaker. The conventional sign for a condenser is that used at J, showing the two series of plates, the insulation or dielectric, as it is called, being understood.
The size of condenser to use with different-sized coils varies according to the winding of the primary and the battery used. A primary coil of few turns would not necessitate as large a condenser as one of a large number of turns. At the same time, a condenser may be made of too great a capacity, and thereby weaken the action of the coil.
The base upon which the coil and its parts are mounted may be of dried polished wood. But where the coil is designed to give large sparks—over 2 inches—it is an advantage to use hard rubber one quarter of an inch and upward in thickness. Glass, were it not for the difficulty of drilling it and its brittleness, would be a desirable material for a coil base in a dry atmosphere. Hard red or black fibre coated with shellac varnish is also serviceable, and, moreover, is extremely easy to work. Slate must never be used; there is too much liability of iron veins being found in it, which in such high tension experiments as will be described would seriously impair the usefulness of the apparatus. The material selected for the base must be one that will not absorb moisture. A paraffined surface collects moisture up to a certain point in isolated drops, whereas a glass and even a hard rubber surface condenses the moisture as a film, which latter is extremely undesirable. But unfortunately the fact that a paraffined surface does not present a pleasing appearance would probably result in its rejection. And lastly, by mounting the coil on hard rubber blocks, or extending the reel ends to raise the coil body, a high insulation can be obtained at the sacrifice perhaps of appearance or height. From the care taken to insulate the secondary coil, it may be considered a superfluous precaution to so carefully select a base, but practical work with the instrument at some important crisis will demonstrate the necessity of extreme care in the smallest details relating to insulation. It may be well to note here that hard rubber is acted upon by ozone, and is thereby impaired as an insulator.
Fig. 10.
The base forms the top of a flat box in which the condenser lies; but there are a few points worth considering right here. As the connections of the coil will probably be under the base, a sufficient space must intervene between the base and the top of the condenser. It is a good plan to lay the condenser at least one half inch below the top of this box, and fill up to, say, one eighth of an inch with melted paraffin, leaving the condenser wires projecting for attachment. The connections of the primary coil and contact breaker should by all means be soldered, not simply wires held under screw nuts. And, moreover, all wires under the base should be so run that they do not cross one another, which precaution only requires a little planning. Then, when the connections are all made and the base laid on top of the box, it can be pressed down if the paraffin be warm, so that the screw heads and wires mark out their own channels and cavities in which to lie.
A commutator or pole-changing switch is often added to change the polarity of the battery current. The diagram of connection is shown in Fig. 10. When the levers are as in the figure, the circuit is broken and no current flows through the coil.