The Spark Plug is the device which generates the spark for combustion of fuel in an internal combustion engine. The spark is generated by the high voltage which is supplied to the spark plug by the ignition coil.
The Spark Plug contains a central electrode which is enclosed in a porcelain body. The metal shell of the spark plug is connected to the engine block or the cylinder head and is thus electrically grounded.
The spark gap is the location where the spark appears. The gap is formed by the central electrode and the electrode from the grounded body. The gap between the two electrodes is very small.
When the high voltage from the Automobile ignition coils is applied to the the central electrode, the high voltage causes an arc to form across the two electrodes.
This spark ignites the fuel/air mixture in the engine.
The spark gap experiences very high temperature during the spark. Hence, special materials and fabrication techniques are adopted in its manufacture.