Induction Coil, a device for converting low-voltage direct current (DC) into high-voltage alternating current (AC). The coils are used chiefly in the electrical systems of automobiles and to operate X-ray tubes.

A typical induction coil has a core of soft iron, a primary coil, and a secondary coil. The primary coil consists of a few turns of fairly heavy wire around the core; the secondary consists of many turns of fine wire around the primary. The primary coil forms part of a circuit called the primary circuit that includes a direct current source and a circuit breaker, or interrupter.

When the primary circuit is closed, direct current flows through the primary coil, producing a magnetic field. As the magnetic field builds up, it induces an electric current in the secondary coil. At the same time, the iron core becomes magnetized. The magnetized core draws the interrupter away from a metal contact, breaking the primary circuit. The direct current in the primary coil ceases and the coil's magnetic field collapses, again inducing an electric current in the secondary coil, only in the opposite direction. Simultaneously, the core loses its magnetism and releases the interrupter, which is pulled back against the contact by a spring. The cycle continues to repeat rapidly, supplying an alternating current at the terminals of the secondary coil. The voltage in the secondary coil is higher than in the primary coil because of the greater number of turns in the secondary coil.

A capacitor, or condenser, is often used with an induction coil. The capacitor prevents sparking between the interrupter and contact by briefly storing the electric charge that would otherwise jump the gap between them.