There is one final use that will be discussed here, for electromagnets, and that is in "relay" switches.


One application for the "relay" switch is in the starter motor of a car. A terrific amount of current is needed in the starter motor, to turn the starter motor to start the engine. It would not be safe for this sort of power to be controlled with the ignition key that the user holds, so the ignition system itself (the bit that the key operates) is considerably lower in its current rating. The relay allows a low current circuit to use electromagnetism to operate a high current circuit taking into account the safety of the user.


How it works is quite straightforward, take a look at the diagram:




  • The "on switch" on the left-hand side of the diagram is in the low current circuit. 
  • When this is closed electrical current flows through the coil producing a magnetic field which induces temporary electromagnetism into the iron electromagnet. 
  • Induced magnetic fields like this always attract non-magnetised magnetic materials so the induced electromagnetic field will attract the small iron contact at the top of the left-hand side of the diagram. 
  • The iron contact is fixed to a pivoted rocker which pushes together two contacts in the high current section of the circuit on the right. 
  • When these contacts close the starter motor operates.