Again, if it be desired that the torpedoes should be so arranged that when any of them are struck by a passing vessel, the fact of its having been struck should be instantly signalled to the firing station. The dial apparatus in the junction box is arranged so that at one point of its revolution, termed the "zero point," all the torpedo branch wires are in circuit with the main cable, and that then a constant current is passing from the firing station through all the circuit closers, and out through resistance coils to "earth." In this case, if one of the circuit closers be struck, and therefore short circuit formed, the current passes direct to earth without going through the aforesaid resistance, and the fact of its having done so is at once indicated by a galvanometer at the firing point, by the movement of which a bell is rung at the station. The operator can then explode such torpedo at once by merely switching in the firing battery. At the same time the passage of the strong firing current may fuze a connection in the junction apparatus, by which the exploded torpedo is detached, i.e. the direct "earth" connection of such a torpedo is cut off, and the remaining submarine mines are left in proper working order; this effect may also be arrived at by other means. _General Description of Apparatus._--The following is a general description of this exceedingly clever and useful invention:-- At Fig. 168 is shown a diagram view of the apparatus. _A_ is the instrument at the firing point on the shore or vessel; _B_ is the cable wire led to a submerged box situated near the spot where the several torpedoes are grouped; _C_ is the instrument enclosed in the submerged box; _D_, _D_ are insulated wires led away from the box to the several torpedoes, there being a separate wire for each torpedo. Each of the wires _D_ is coupled to one or other of a series of metallic contact pieces _E_ ranged in a circle round the axis of a metallic pointer _F_, which can be turned with a step by step motion and successively brought into electrical contact with the several contact pieces _E_. The axis of the pointer is in electrical communication with the wire of the cable. The wire from the cable is first led to the coils of an electro magnet _G_, and thence passes to the axis of the pointer. _H_ is a magnetic armature in front of the electro magnet _G_; when a positive current of sufficient strength is sent through the cable the armature is rocked in one direction, and when a negative current is sent, it is rocked in the opposite direction. From the armature motion is transmitted to a pawl which works into the teeth of a ratchet wheel on the axis of the pointer _F_, so that by sending a succession of reversed currents of sufficient strength through the cable, the pointer _F_ is turned with a step by step motion and is successively brought into electrical contact with the several contact pieces _E_. [Illustration: M^{c.}EVOY'S SINGLE MAIN SYSTEM PLATE LIV.]