Different manufacturers use different metals in the center and side electrodes of a spark plug. The metal is meant to channel the high voltage from the spark plug wire through the rest of the plug. This enables it to spark across the small gap between the central electrode and the side electrode, which starts off the combustion process. Hard metals like platinum and iridium are used to reduce the wear caused by high voltage sparks. Also, they help to decrease the rate of misfire, offering a more reliable engine performance.