Relays are devices that utilize a central iron core fitted closely to the inside of a coil of wire. When the wire is energized the iron core will be drawn down the length of the coil, the direction dependent upon the direction of current flow. If the relays iron core is fitted with large, high current-carrying contacts it can be used as a high-current switch. Relays are used throughout cars for horns, electric fans, air conditioning clutches, etc. and the most important one is the starter solenoid.
STARTER SOLENOID "Relay" — All cars are wired so that the batterys main cable connects to the starter motor windings "the thick cable is needed for large current flow, right?". This wire must be switched on and off, of course, and it would be costly and inefficient to route it through the ignition switch "not to mention the size of the switchs components required to carry such current!". Consequently, a relay is necessary.
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