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coil, in·duc·tion

A coil in which by mutual induction the electromotive force of a portion of a circuit is made to produce higher or lower electro-motive force, in an adjoining circuit, or in a circuit, part of which adjoins the original circuit, or adjoins part of it.

An induction coil comprises three principal parts, the core, the primary coil and the secondary coil. If it is to be operated by a steady current, means must be provided for varying it or opening and closing the primary circuit. A typical coil will be described.

The core is a mass of soft iron preferably divided to prevent extensive Foucault currents. A cylindrical bundle of soft iron wires is generally used. Upon this the primary coil of reasonably heavy wire, and of one or two layers in depth, is wrapped, all being carefully insulated with shellac and paper where necessary. The secondary coil is wrapped upon or over the primary. It consists of very fine wire; No. 30 to 36 is about the ordinary range. A great many turns of this are made. In general terms the electro-motive force developed by the secondary stands to that of the primary terminals in the ratio of the windings. This is only approximate.

The greatest care is required in the insulating. The secondary is sometimes wound in sections so as to keep those parts differing greatly in potential far from each other. This prevents sparking, which would destroy the insulation.

A make and break, often of the hammer and anvil type, is operated by the coil. (See Circuit Breaker, Automatic.) As the current passes through the primary it magnetizes the core. This attracts a little hammer which normally resting on an anvil completes the circuit. The hammer as attracted is lifted from the anvil and breaks the circuit. The soft iron core at once parts with its magnetism and the hammer falls upon the anvil again completing the circuit. This operation goes on rapidly, the circuit being opened and closed in quick succession.

Every closing of the primary circuit tends to produce a reverse current in the secondary, and every opening of the primary circuit tends to produce a direct current in the secondary. Both are of extremely short duration, and the potential difference of the two terminals of the secondary may be very high if there are many times more turns in the secondary than in the primary.

The extra currents interfere with the action of an induction coil. To avoid their interference a condenser is used. This consists of two series of sheets of tin foil. Leaves of paper alternate with the sheets of tin-foil, the whole being built up into a little book. Each sheet of tin-foil connects electrically with the sheet next but one to it. Thus each leaf of a set is in connection with all others of the same set, but is insulated from the others. One set of leaves of tin-foil connects with the hammer, the other with the anvil. In large coils there may be 75 square feet of tin-foil in the condenser.

The action of the condenser is to dispose of the direct extra current. When the primary circuit is opened this current passes into the condenser, which at once discharges itself in the other direction through the coil. This demagnetizes the core, and the action intensifies and shortens the induced current. The condenser prevents sparking, and in general improves the action of the coil.

Many details enter into the construction of coils, and many variations in their construction obtain. Thus a mercury cup into which a plunger dips often replaces the anvil and hammer.

The induction coil produces a rapid succession of sparks, which may spring across an interval of forty inches. The secondary generally ends in special terminals or electrodes between which the sparking takes place. A plate of glass, two inches in thickness, can be pierced by them. In the great Spottiswoode coil there are 280 miles of wire in the secondary, and the wire is about No. 36

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