De Forest thermionic triode valve, c 1907.
© Science Museum / Science & Society Picture Library
Lee De Forest (1873-1961) was an American electrical engineer and inventor. In 1907, while working for the Western Electric Company, he patented the thermionic triode, which he called the 'audion' valve. Esentially a development of the diode patented by John Ambrose Fleming, but with an additional electrode between the cathode and the anode, the triode was able to amplify weak electrical signals, making it a crucial invention for the development of radio. For 50 years, valves were a fundamental component in all radio equipment, until being generally replaced by the transistor. In the United States, De Forest is known as 'the father of radio'.