In the instrument, at the firing point _a_ is a handle, by the turning of which a step by step motion is given to the pointer of a dial _b_ and a simultaneous movement to the pointer _F_ of the instrument _C_ in the submerged box. When the handle _a_ has made a half turn it couples one pole of the battery to the cable and the other to the earth connection, and when it has made a complete turn the connections are reversed. The pointer of the dial _b_ then moves forward from one division of the dial to the next, and simultaneously the pointer _F_ is turned in unison with it. The operator at the firing point can therefore always see which of the torpedoes is in electrical connection with the wire of the cable, and he can test each torpedo in succession by moving a handle, say at _h_, to cause the current passing back from the torpedo to pass through a galvanometer at _e_, and by the movement of the needle of the galvanometer it can be seen whether the resistance of the circuit through this torpedo is in its normal and proper working state. When the pointer of the dial _b_ is brought to zero, or as it is marked in the drawing to "signal," then the pointer _F_ of the apparatus _C_ is in electrical communication with a contact point which is coupled to all of the branch wires _D_, and usually the apparatus is left in this condition, the handle _a_ being then locked and prevented from turning by a bolt actuated by a handle at _G_. The current from the battery at the firing point then passes to earth through the resistances in all of the torpedoes. If now any one or other of the torpedoes is struck by a passing vessel and the wire from its fuze put directly to earth, so that the current passes freely to earth instead of having first to pass through the resistance, the fact of the current passing freely to earth is notified at the firing point by the movement of the needle of a galvanometer _d_; the movement of the needle of this galvanometer effects an electrical connection by which a small battery is caused to sound a bell at _c_. The operator at the firing point can then if he pleases at once fire the torpedo that has been struck by moving a handle at _f_ and coupling up to the wire of the cable a battery of greater strength; the strong firing current will pass to earth through the fuze of the torpedo that has been struck, and will ignite this fuze, but will not affect the fuzes of the other torpedoes, as to pass through these fuzes it has also to pass through resistances which impede its passage and reduce its strength, so that the portion of the current which passes to earth through them is not of sufficient strength to ignite the fuzes.