_Testing Current._--For testing purposes the current from the test battery arrives by the line wire _L_, and passes thence through the resistance coil to earth by means of the wire _E_, which is attached to a zinc earth plate placed in a recess in the jacket of the circuit closer.
_Action of the Circuit._--The action of the apparatus is as follows:--
_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.
_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.