File:Faraday ring transformer.jpg

English: British scientist Michael Faraday's ring transformer, with which he discovered the principle of electromagnetic induction on 29 August 1831, preserved in a museum, from a 1922 technical magazine. It was a soft iron ring, 7/8 in. thick and 6 in. diameter. He wound one side with 3 lengths of insulated wire each about 24 feet long so he could try different combinations, and the other side with a single winding of 60 feet. When he suspended the wire from one of the windings over a compass needle, and connected the other to a battery, the compass needle jerked when the battery was connected and disconnected. Description from p. 380-381 of the source. Although this is the first example of a transformer, Faraday is not considered the inventor of the transformer. He never applied alternating current to the device, only pulses of current, and never discovered the principle that the voltage produced is proportional to the number of turns.