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Various metal types are used by spark plug manufacturers for ground electrodes and center electrodes.
Copper is a great electrical conductor with low resistance.
Almost all spark plugs use a copper cored center electrode to conduct the electricity, jump the gap, and promote heat dissipation. Due to its soft nature and low melting point, using copper as an outer electrode material would result in a plug that lasted only minutes.
"Copper spark plugs" is a term mistakenly used for a standard or traditional spark plug. A standard spark plug typically uses a nickel-alloy outer material fused to a copper core. Some plugs include a coating or pad of precious metals to extend plug life. But as a general rule of thumb, all spark plugs utilize a copper core.
Nickel is a hard, inexpensive metal used in most all spark plug designs. Nickel is not a good conductor and has a high level of resistance, so most center electrodes feature a copper core, covered in a nickel-alloy. This creates a balanced, team effort where the copper core conducts electricity and promotes heat dissipation, and the nickel-alloy outer material prolongs the life and durability of the center electrode.
While nickel is a choice material used in the majority of spark plug designs, it has its limitations. Other materials are harder, stronger, have a lower electrical resistance and a higher melting point, and are therefore used to enhance nickel electrodes and prolong a plugs life in high performance designs.