Faraday's induction ring, 1831.
© Science Museum / Science & Society Picture Library
This iron ring with two coils wound around it is a replica of the one used by Michael Faraday (1791-1867) in 1831, in one of his most famous experiments, when he discovered the principles of electromagnetic induction. He found that although the coils were not actually connected to each other, electricity was produced in one coil every time the current through the other one changed. He realised that the 'induction' of electricity in the second coil could be explained only by the presence of such a magnetic effect in the iron core.