A spark plug (whiles, in Breetish Inglis, a sparking plug, an, colloquially, a plug) is a device for delivering electric current frae an ignition seestem tae the combustion chamber o a spark-ignition ingine tae ignite the compressed fuel/air mixture bi an electric spark, while containing combustion pressure within the ingine. A spark plug haes a metal threaded shell, electrically isolated frae a central electrode bi a porcelain insulator. The central electrode, which mey contain a resistor, is connected bi a heavily insulated wire tae the output terminal o an ignition coil or magneto. The spark plug's metal shell is screwed intae the ingine's cylinder heid an sicweys electrically grounded. The central electrode protrudes throu the porcelain insulator intae the combustion chamber, forming ane or mair spark gaps atween the inner end o the central electrode an uisually ane or mair protuberances or structures attached tae the inner end o the threaded shell an designated the side, earth, or ground electrode(s).
- The Bosch book of the Motor Car, Its evolution and engineering development, St. Martin's Press, copyright 1975, Library of Congress # 75-39516, pp 206–207.