Faraday's experiments. About the year 1832 Michael Faraday was making experiments at the Royal Institution which led to the discovery of the law of electromagnetic induction, which made possible all the modern applications of electric power. He made two coils of insulated wire, each about two hundred feet in length, winding them on the same wooden block, attaching the ends of one coil to the terminals of a battery and the ends of the other to a galvanometer. Calling the circuit which contains the battery the ‘primary’ circuit and the other the ‘secondary’, he observed that whenever the primary circuit was completed the galvanometer indicated the passage of a transient current in the secondary circuit in the opposite direction to that in the primary; also that whenever the primary circuit was broken a transient current passed through the secondary in the same direction as the current in the primary. It was further observed that so long as the current in the primary was steady there was no current through the secondary. The currents in the secondary circuit are called induced currents.

Other experiments were performed with primary and secondary circuits separated from one another and situated in parallel planes. It was found that, when there was a steady current in the primary, relative motion of the circuits resulted in an induced current in the secondary; and that the two currents were in the same sense when the motion was causing increased separation of the circuits, and in opposite senses when the circuits were approaching one another.