LESSON SEVEN. THE INDUCTION COIL.
The Induction Coil is an apparatus for producing currents of a very high electromotive force. It consists of a helix of large, insulated wire surrounding an iron core, and this again surrounded by a second coil consisting of many thousand turns of very fine wire carefully insulated. The inner or primary coil is connected in series with a battery, the circuit also including a device called an interrupter. The object of the interrupter is to make and break the primary circuit in rapid succession. Every time the current is turned on in the primary circuit, the primary coil creates a magnetic field which induces a current in the secondary in accordance with the laws of induction.
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Likewise at every "break" in the circuit caused by the interrupter, the lines of force disappear and a second current impulse is induced in the secondary coil. As the number of lines of magnetic force created and destroyed at each make and break is the same, the two electromotive impulses in the secondary are equal. By adding a condenser, however, the current at "make" is caused to take a considerable fraction of time to grow, while at "break" the cessation is instantaneous in comparison. The rate of "cutting" of the lines of force is very much more rapid at "break" than at "make" therefore. The currents at "break" manifest themselves as a brilliant torrent of sparks between the ends of the secondary wires when they are brought near enough together.
The central iron core around which the coils are wound is for the purpose of increasing or concentrating the number of lines of force that pass through the coils. Magnetic lines flow more easily through iron than through air and so prefer that path. It is made up of a bundle of fine iron wires in order to avoid induced currents which would be set up in the iron were it a solid mass and so retard its rapidity of magnetization or demagnetization as to hamper the efficiency of the coil.