BTH Type RK (Rice-Kellogg) "mains energised" loudspeaker, 1926-1929.

A number of loudspeaker manufacturers produced "mains energised" speakers during the 1920s and 1930s. These used an electromagnet instead of a permanent magnet. The higher magnetic field produced made the speaker more sensitive and doubled as a high-tension smoothing choke for the radio's power supply. The moving coil principle was patented in 1924 by two Americans, Chester W. Rice and Edward W. Kellogg. There is some controversy in that an application was made earlier by the Briton Paul Voigt but not granted until later. Voigt produced the first effective full range unit in 1928, although using electromagnets rather than permanent magnets, and he also developed what may have been the first system designed for the home.