A spark plug (sometimes in British English, a sparking plug) is an electrical device that fits into the cylinder head of some internal combustion engines and ignites compressed aerosol gasoline by means of an electric spark. Spark plugs have an insulated center electrode which is connected by a heavily insulated wire to an ignition coil or magneto circuit on the outside, forming, with a grounded terminal on the base of the plug, a spark gap inside the cylinder.
These are spaced to provide a gap for firing an electrical spark to ignite air-fuel mixtures. The high-voltage burst from the coil via the distributor is received at the spark plug's terminal and conducted down a center electrode protected by a porcelain insulator. At the bottom of the plug, which projects into the cylinder, the voltage must be powerful enough to jump a gap between the center and side electrodes through a thick atmosphere of fuel mixture. Fuel is ignited in the cylinder when the spark bridges the gap.