(B) Wet the ends of two fingers of the left hand, press one upon L and the other on R, thus making a shunt with your hand. With the right hand work the key rapidly. If the current is strong enough you should feel a slight shock in the fingers each time the circuit is broken. The extra current (§ 444) causes the shock as it shoots through the fingers. (C) If you have electric bells or telegraph sounders use them for this experiment. _=473. Induction Coils=_ are instruments for producing induced currents of high E. M. F. The apparatus shown in Fig. 141 forms a simple induction coil. The _primary_ coil is made of coarser wire and has less turns of wire than the _secondary_ coil. The current in the primary circuit is usually interrupted by an _automatic interrupter_ (Exp. 195), thus producing an alternating current in the secondary coil, the voltage of which depends upon the relative number of turns in the two coils. Induction coils are used in telephone work, for medical purposes, for X-ray work, etc., etc. (For Home-made Induction Coils see Apparatus Book, Chapter XI.) [Illustration: Fig. 159.] =474. Action of Induction Coils.= Fig. 159 shows a top view of one of the home-made induction coils described, in full, in the Apparatus Book. Wires 5 and 6 are the ends of the primary coil, while wires 7 and 8 are the terminals of the secondary coil. The battery wires should be joined to binding-posts W and X, and the handles to Y and Z. Fig. 160 shows the details of the automatic interrupter which is placed in the primary circuit. [Illustration: Fig. 160.] If the current 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 S I, as A is no longer attracted; the circuit being closed, the operation is rapidly repeated. (For commercial forms and uses of induction coils see "Things A Boy Should Know About Electricity.")