English: Drawing of the first transformer, built by British scientist Michael Faraday in the 1830s. It consisted of an iron ring with two windings of insulated wire around it. Faraday attached one winding to a sensitive galvanometer. When he touched the other winding to a single cell battery, the winding created a changing magnetic field in the ring which induced a momentary current in the second winding due to electromagnetic induction, which was registered by the galvanometer.

Although this device looks remarkably like a modern transformer, Faraday is not considered the inventor of the transformer. He only applied individual pulses of current to his device, not the alternating current that allows modern transformers to work continuously. More importantly the device had the same number of turns on both windings. Faraday did not discover the principle that makes transformers useful - that it can be used to transform the voltage up or down by using different numbers of turns in the seconday than in the primary winding.