English: An induction coil used for ignition in early automobiles such as the Model T in the early 20th century, and also in high voltage devices such as x-ray machines and spark-gap radio transmitters.

It consists of a step-up transformer with a primary with relatively few turns, powered by the car's battery, and a secondary with many turns of fine wire which produces a high voltage in the range of 10,000 - 20,000 V at the terminals on top, for the car's spark plug. To operate the transformer on DC, a vibrating contact called an "interrupter" or "trembler" (visible at left) is used to repeatedly break the current from the battery, creating the flux changes needed for induction. It is located at the end of the transformer's iron core. The current in the primary creates a magnetic field in the core which attracts the iron armature, opening the contacts and interrupting the primary current. The magnetic field then collapses, creating a pulse of high voltage in the secondary coil, and the contact springs closed again. This cycle is repeated 20-40 times per second.