A
spark plug (sometimes, in
British English, a
sparking plug, and, colloquially, a
plug) is a device for delivering electric current from an
ignition system to the
combustion chamber of a
spark-ignition engine to ignite the compressed fuel/air mixture by an
electric spark, while containing combustion pressure within the engine. A spark 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 porcelain insulator into the
combustion chamber, forming one or more
spark gaps between the inner end of the central electrode and usually one or more protuberances or structures attached to the inner end of the threaded shell and designated the
side,
earth, or
ground electrode(s).