Racing spark plugs
Spark plugs used for racing must withstand especially hard stresses. The speeds during a race are frequently in excess of 15,000 rpm. Temperature, pressure, vibrations and currents in the combustion chamber are so immense that conventional earth electrodes could break or melt.
For this reason, racing spark plugs are designed with a ring-shaped earth electrode. The spark creeps from the middle electrode to the earth electrode instead of jumping.