Electricity and Magnetism

Studying a commercial relay

Practical Activity for 14-16 PRACTICAL PHYISCS


If students have made a model relay, this demonstration will reinforce their learning.

Apparatus and Materials

  • Cell, 1.5 V
  • Commercial relay, small
  • Power supply, low voltage, DC
  • Switch
  • Iron yoke
  • Electric motor, fractional horsepower

  • Magnadur (ceramic) slab magnets, 2
  • Copper wire, stiff, bare, SWG 32 and SWG 26
  • Clamp, or wooden support blocks
  • Crocodile clips, 2
  • Leads, 4 mm, 2

Health & Safety and Technical Notes

Read our standard health & safety guidance

Choose a relay which will control a 12 V circuit. It is desirable to choose one whose internal workings can be seen, or which can be revealed by later dissection.


  1. Connect up the control circuit: cell, switch and relay.
  2. Connect up the circuit to be controlled: battery and motor.
  3. Show the small-current control circuit switching the large current motor.

Teaching Notes

  • If students have made a model relay, this demonstration will reinforce the class experiment rather than spoil it. A commercial relay operates quickly and surely and can control a large current.
  • You could insert ammeters in both circuits. Both instruments should have the same range, so that the different sizes of the two currents will be graphically evident.

This experiment was safety-tested in April 2006

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