Radio valve of Clearton valve type.
© 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, or radio valve. Esentially a diode 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, radio receivers relied on valves, until being generally replaced by the transistor from the 1950s.