A solenoid is an electromagnetic relay used in a direct current wiring system between a low current capacity switch and a high current load requirement.

In an engine starting system, the ignition switch in the start position may have a rating of 5 amps whilst the starter motor may have a cranking requirement of 200 to 350 amps.

Once energized by the ignition switch, the solenoid coil moves the high current capacity contact to a closed position, thereby energizing the starter motor.