An induction coil or "spark coil" is a type of electrical transformer. It is used to produce high-voltage pulses from a low-voltage direct current (DC) supply. To create the flux changes necessary to induce voltage in the secondary coil, the direct current in the primary coil is repeatedly interrupted by a vibrating mechanical contact called an interrupter.
The induction coil was the first type of transformer. It was widely used in X-ray machines, spark-gap radio transmitters, arc lighting and quack medical devices from the 1880s to the 1920s. Today its only common use is for ignition coils in internal combustion engines and in physics education to demonstrate induction.