The springs are connected together by means of the wires 9, [Fig. 55], one end of which is secured to the connecting piece by the screw 10, and the other passes through to the top of the ebonite piece, and is attached to the top of the spring next in succession to that to which it is fixed below.
One terminal of a coil of 1000 ohms resistance (which is used for testing purposes) is attached to the line L, terminal of the ebonite base plug, which latter is also connected to the screw 8, on the circumference of the contact ring 6; the other terminal of the resistance coil is connected to the earth, E terminal of the base plug.
A bare copper wire of No. 16 B. W. G. connects the top of the last contact spring with the set screw s; a piece of similar wire jointed to it is passed round one of the brass collars and connected to the screw r. As a precaution against bad contact, the contact springs are connected together by bare wires A, B, C. This completes the connections for the signalling circuit, the earth being formed by the body of the instrument; D is a hole left in the metal base for the passage of the insulating wire which connects the earth plate to the earth E terminal of the base plug.
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.
MATHIESON'S CIRCUIT CLOSER.
