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.]