A spark plug functions in the combustion chamber of an engine under severe condition. For example, the temperature of the combustion chamber reaches about 2,500˚C and the pressure reaches 5MPa. Additionally , a spark plug is exposed to sudden changes temperature and pressure , from the high temperature of burned gases to the relatively low temperatures of the air/fuel mixture. Moreover it must endure high voltages , mechanical vibrations and the chemical corrosion of the combustion gases.