File:Group of Fleming valves 1904.jpg

English: Group of the first experimental Fleming valves made by John Ambrose Fleming 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 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 a metal plate anode next to it. When the filament was heated by a separate current it released electrons into the bulb. When the anode had a positive voltage with respect to the cathode the electrons would be attracted to it, and a current of electrons would flow from cathode to anode, but when the anode was negative, no current would flow.