The coil through which we send the battery current is called the “primary coil,” and the one in which a current is induced is called the “secondary coil.” The two momentary currents in the secondary coil do not both flow in the same direction. The current induced on making the circuit flows in a direction opposite to that of the current in the primary coil; and the current induced on breaking the circuit flows in the same direction as that in the primary coil. If the two coils are exactly alike, the induced current will have the same voltage as the primary current; but if the secondary coil has twice as many turns of wire as the primary coil, the induced current will have twice the voltage of the primary current. In this way, by multiplying the turns of wire in the secondary coil, we can go on increasing the voltage of the induced current, and this is the principle upon which the induction coil works. We may now describe the construction of such a coil. The primary coil is made of a few turns of thick copper wire carefully insulated, and inside it is placed a core consisting of a bundle of separate wires of soft iron. Upon this coil, but carefully insulated from it, is wound the secondary coil, consisting of a great number of turns of very fine wire. In large induction coils the secondary coil has thousands of times as many turns as the primary, and the wire forming it may be more than a hundred miles in length. The ends of the secondary coil are brought to terminals so that they can be connected up to any apparatus as desired. [Illustration: FIG. 17.--Diagram showing working of Contact-Breaker for Induction Coil.] In order that the induced currents shall follow each other in quick succession, some means of rapidly making and breaking the circuit is required, and this is provided by an automatic contact breaker. It consists of a small piece of soft iron, A, Fig. 17, fixed to a spring, B, having a platinum tip at C. The adjustable screw, D, also has a platinum tip, E. Normally the two platinum tips are just touching one another, and matters are arranged so that their contact completes the circuit. When the apparatus is connected to a suitable battery a current flows through the primary coil, and the iron core, F, becomes an electro-magnet, which draws A towards it. The platinum tips are thus no longer in contact and the circuit is broken. Immediately this occurs the iron core loses its magnetism and ceases to attract A, which is then moved back again by the spring B, so that the platinum tips touch, the circuit is once more completed, and the process begins over again. All this takes place with the utmost rapidity, and the speed at which the contact-breaker works is so great as to produce a musical note. There are many other types of contact-breakers, but in every case the purpose is the same, namely, to make and break the primary circuit as rapidly as possible.