Carbon transmitters, invented by Thomas Edison in 1886, converted sound into an electrical signal that could travel through wires and be converted back into sound. The first hearing aids used this technology until the 1920s, when vacuum tubes were introduced to hearing aids and, it goes without saying, were worn outside the ear. Transistors were used in 1952, two years before the technology was put to work in radios. In 1956, Zenith introduced the first behind-the-ear model called the Diplomat. Since then a variety of hearing aids have been made available.
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