A spark plug is an ignition system fitted in the cylinder head of an internal-combustion engine. It consists essentially of two electrodes separated by an air gap. When the fuel/air mixture is fully compressed, the distributor connects a voltage of 20 kilovots across the electrodes, producing a spark that ignites the mixture. The width of the gap between the electrodes must be correctly set.

The accompanying diagram shows an automobile spark plug: (1) terminal; (2) central conductor; (3) insulator (capable of resisting 30,000 volts at 900°C); (4) antiflashover ribbing; (5) body with hexagon nut; (6) conducting link between central conductor and center electrode; (7) gasket; (8) gas sealing washer; (9) screw thread; (10) center electrode; (11) ground (earth) electrode.