The induction coil produces a rapid succession of sparks, which may
spring across an interval of forty inches. The secondary generally ends
in special terminals or electrodes between which the sparking takes
place. A plate of glass, two inches in thickness, can be pierced by
them. In the great Spottiswoode coil there are 280 miles of wire in the
secondary, and the wire is about No. 36 A.W.G.
Fig. 100. VERTICAL SECTION OF INDUCTION COIL.
Fig. l01. PLAN OF INDUCTION COIL CONNECTIONS.
Induction coils have quite extended use in electrical work. They are
used in telephone transmitters, their primary being in circuit with the
microphone, and their secondary with the line and receiving telephone.
In electric welding, and in the alternating current system they have
extended application. In all these cases they have no automatic circuit
breaker, the actuating current being of intermittent or alternating
type.