John Ambrose Fleming

Fleming valve

The Fleming valve is a two-electrode thermionic diode invented in 1904 to detect high-frequency radio signals in wireless telegraphy. It rectifies AC by permitting electron flow only from heated filament to positive anode — exploiting the Edison effect. It replaced the coherer in Marconi’s transatlantic receiver. It does not amplify. It requires continuous filament heating. It was used in radio and radar for over 50 years before being superseded by solid-state devices.