A variety of different metals can be used in spark plugs. Almost all spark plugs feature a copper cored inner central electrode because copper is an excellent electrical conductor. Copper, however, is a very soft metal with low resistance which means that it is not suitable as an outer centre or ground electrode. NGK SPARK PLUG’s standard range of plugs feature a nickel alloy outer centre electrode (fused to an inner copper core) and a nickel ground (side) electrode. The company’s premium range features extremely hard metals like iridium and platinum which guarantee longer replacement intervals.