Group of Fleming valves preserved at the Science Museum, London. These examples showed the various forms of anode that Fleming tried. The Fleming valve or thermionic diode, invented by John Ambrose Fleming around 1904, was the first vacuum tube, used as a detector in early radio receivers. It consisted of an evacuated glass envelope containing a cathode in the form of a filament, and an a metal plate anode next to it. When the filament was heated by a sep