_Closer._--On the circuit closer being struck, the weight of the lead ball _t_ causes the steel rod _p_ to be deflected and brings the brass ring 6 in contact with one of the springs 5; the signalling current which up to this moment has been passing through the 1000 ohms coil to earth, then passes to the contact ring 6 (avoiding the resistance coil) thence to the spring which is in contact with it, and from there by means of the wire connections to the set screws _s_ and _r_, and so to earth through the metal body of the apparatus; the effect of the resistance coil being thus eliminated, is to strengthen the signalling current, and thus enable it to work the shutter apparatus, by which means the firing current is thrown into circuit and the mine exploded. [Illustration: MATHIESON'S CIRCUIT CLOSER. PLATE XIII] _Circuit Breaker._--By altering the mode of connecting the wires, the above apparatus may be used as a circuit breaker, that is to say, the signal may be given, and the mine exploded by the cessation of a passing current, instead of by the closing of the electric circuit. This system was specially designed for use with platinum wire fuzes, but is rarely used. _Circuit Closer of Electro Contact Mines._--When the inertia circuit closer is employed in connection with electro contact mines, the circular ebonite piece _m_ is replaced by a similar shaped piece of brass, and which is in metallic connection through the brass pillars _l_, _l_, _l_ with the mass of the metal of the apparatus which forms the earth plate. The insulated wire of the base plug is connected to one pole of a platinum wire fuze, the other pole of which is connected by another wire to the outer metal rim of the disc of the spindle. As long as the circuit closer remains undisturbed, a break will remain in the circuit, which is due to the ebonite insulation between the spindle and the outer metal rim of the disc; but the moment the apparatus is struck, which causes the spindle to vibrate, the outer metal rim will come in contact with one of the springs completing the circuit, through the circular metal portion and the pillars of the circuit closer to earth. _Adjustment of Circuit Closer._--The sensitiveness of Mathieson's inertia circuit closer is determined by the distance between the disc 4 and the springs 5, 5, 5, which is regulated by means of the adjusting screws 7, 7, 7, which press against the inner faces of the springs. Owing to the great weight of the leaden ball, when by any cause the circuit closer is inclined for a length of time, a permanent set is given to the spindle, thereby destroying the adjustment of the instrument. _Improvements in the Inertia Circuit Closer._--To remedy this very serious defect, a cylinder of india rubber is substituted for the leaden ball; a circuit closer so fitted is also less affected by the action of counter mines, which is a very important advantage.