A solenoid is an electromechanical switch. A small amount of current from the ignition switch energizes a coil in the solenoid creating a magnetic field. The magnetic field closes the circuit for a higher current to go from the battery to the starter. A solenoid that mounts directly on the starter mechanically engages the starter pinion gear with the engine’s flywheel gear. Some starting systems utilize a starter relay. A relay, like a solenoid, needs a small amount of current to open and close a high current circuit.