A Spark Plugs is a device for delivering electric currentfrom an ignitionsystem to the combustionchamber of a spark-ignitionengine to ignite the compressed fuel/air mixture by an electric spark, while containing combustion pressure within the engine. Aspark plug has a metal threaded shell, electrically isolated from a central electrode by a porcelain insulator. The central electrode, which may contain a resistor, is connected by a heavily insulated wire to the output terminal of an ignition coil or magneto. The spark plug's metal shell is screwed into the engine's cylinder head and thus electrically grounded. The central electrode protrudes through the porcelaininsulator into the combustionchamber, forming one or more spark gaps between the inner end of the central electrode and usuallyone or more protuberances or structures attached to the inner end of thethreaded shell and designated the "side", "earth", or"ground" electrode(s).