The induction coil: (left) schematic diagram; (right) circuit diagram. In each case the primary circuit is in blue, the secondary in red. The capacitor prevents sparking between the terminals of a make-and-brake relay.
An induction coil is a type of transformer that provides very high voltage alternating current from lower voltage current. A common example is the induction coil used for sparking such as in a car ignition system.
In an induction coil, a large secondary and small primary coil are wound together on a ferromagnetic core. When a low current in the primary is interrupted by a switch, the rapid change induces (see electromagnetic induction) a large voltage in the secondary. In a variant producing high-frequency AC (alternating current) voltage pulses, the primary current switches itself on and off through a relay, much as in an electric bell mechanism.