104. Construction. Fig. 102 shows one form of home-made induction coil, given here merely to explain the action and connections. Nearly all induction coils have some form of automatic current interrupter, placed in the primary circuit, to rapidly turn the current off and on.
Fig. 103.
Details of Figs. 102 and 103. Wires 5 and 6 are the ends of the primary coil, while wires 7 and 8 are the terminals of the secondary coil. The primary coil is wound on a bolt which serves as the core, and on this coil is wound the secondary which consists of many turns of fine wire. The wires from a battery should be joined to binding-posts W and X, and the handles, from which the shock is felt, to Y and Z. Fig. 103 shows the details of the interrupter.
If the current from a cell enters at W, it will pass through the primary coil and out at X, after going through 5, R, F, S I, B, E and C. The instant the current passes, the bolt becomes magnetized; this attracts A, which pulls B away from the end of S I, thus automatically opening the circuit. B at once springs back to its former position against SI, as A is no longer attracted; the circuit being closed, the operation is rapidly repeated.
A condenser is usually connected to commercial forms. It is placed under the wood-work and decreases sparking at the interrupter. (See "Apparatus Book," Chapter XI., for home-made induction coils.)
Fig. 104.
Fig. 104 shows one form of coil. The battery wires are joined to the binding-posts at the left. The secondary coil ends in two rods, and the spark jumps from one to the other. The interrupter and a switch are shown at the left.
Fig. 105 shows a small coil for medical purposes. A dry cell is placed under the coil and all is included in a neat box. The handles form the terminals of the secondary coil.