The induction coil is a device for getting a high voltage from a low one. It was at one time used for X-ray tubes, and is nowadays used in car radios. It consist of a core of iron wires, around which is wrapped a coil of about a hundred turns of thick insulated wire, called the primary. Around the primary is wound the secondary coil, which has many thousands of turns of fine insulated wire. The primary is connected to a battery of accumulators, via a make-and-break M, which works in the same way as the contact-breaker of an electric bell: it switches the current on and off many times a second, thus varying the magnetic flux.

When the primary current I is switched on, the rise of its magnetic field induces an e.m.f. Es in the secondary. A similar e.m.f., but in the opposite sense, is induced in the secondary when the primary current is switched off, by the collapse of the magnetic field. The secondary e.m.f.s are determined by the number of turns in the secondary coil, and by the rate of change of the magnetic flux through the iron core. Because of the great number of secondary turns, the secondary e.m.f. may be high and of the order of thousand of volt.

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In practice, an induction coil such as we have described consisting simply of primary, secondary, and contact – breaker – would not give high secondary e.m.f.s. For, at the make of the primary current, the current would rise slowly, because of the self- inductance of the primary winding. The rate of change of flux linked with the secondary would therefore be small, and the secondary e.m.f. low. And at the break of the primary current a spark would pass between the contacts of the make-and-break. The spark would allow primary current to continue to flow, and the primary current would fall slowly. At the instant of break, before the spark began, the primary current would be falling rapidly and the secondary e.m.f. would be high; but the e.m.f. would remain high for only a very short time: as soon as the spark passed the secondary e.m.f. would fall to a value about as low as at make.

Nothing can be done about the low secondary e.m.f. at make. But the secondary e.m.f. at break can be made high, by preventing sparking at the contact-breaker. To prevent sparking, a capacitor is connected across the contact.