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.