A device in the ignition system of a reciprocating engine that produces the spark which ignites the fuel-air mixture inside the cylinder. A spark plug has a steel shell and an insulated center electrode. Accurately spaced gaps are formed between the ground electrodes on the shell and the insulated center electrode.

The high-voltage lead from the ignition distributor is connected to the insulated terminal of the spark plug and, at the correct time, a pulse of high-voltage electrical energy is sent to the spark plug. As this electrical energy passes to ground, it produces an intense spark between the two electrodes.