In 1820 Oersted discovered the connexion between the polarity of the Voltaic pile and that of the magnet, and he also proved the existence of a magnetic field. In 1821 Michael Faraday discovered electromagnetic rotation by causing a wire conveying a voltaic current to rotate continuously round the pole of a permanent magnet. W. Sturgeon in 1824 wound a copper wire round a horseshoe-shaped iron bar and showed that the bar was powerfully magnetized when the voltaic current was passed through the wire: this was the discovery of the electromagnet. Soon similar electromagnets of great power were being built, both by Sturgeon himself and, in the United States, by Joseph Henry, as well as by Faraday and others. An immediate consequence of this development was the invention of the galvanometer.
Arago showed that a copper plate rotating under a suspended magnetic needle tended to drag the needle after it and vice versa. Finally, Ampère found that a magnet could be made to rotate on its own axis when a current was passed through it. Various other scientists then showed that a copper disc could be made to rotate between the poles of a horseshoe magnet, when a current was passed through the disc from the centre to the circumference. This was the first electric motor.
All these factors led Faraday to his clear conception that there existed "lines of magnetic force".