A spark plug is used in internal combustion engines (except diesel engines) to ignite the fuel air mixture in the cylinder. It consists of an outer steel casing terminating in a bent tip that makes up one of the electrodes and a ceramic insulator sealed into the casing, through which a central electrode passes. The electrodes are separated by a gap that varies from 25 thousandths of an inch to 40 thousandths of an inch. When the engine is running, a pulse of electrical energy at more than 10,000 volts is delivered to the terminal of the plug at the correct moment. The high voltage is supplied by a small generator or a chargeable battery in combination with an induction or ignition coil.
This causes a spark to jump between two electrodes. This spark provides the energy needed to ignite the compressed fuel air mixture in the cylinder to drive the piston. The precise moment at which the spark is produced is pre-set depending on the position of the piston and the compression cycle.