472. Directions. (A) Use the two electromagnets joined to the connecting plates ([Fig. 132]), to generate a self-induced or extra current. Connect R of [Fig. 132] with the zinc of a dry cell, and between L and the carbon of the cell place a key; in other words, join the electromagnets, cell, and key in series. Two good cells in series can be used to advantage.
(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[200] 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.)
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.
