Early Voigt mains energised moving coil loudspeaker - possibly an example of his original 1928 design. Bristol number: J 2233.

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.