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The first prototype Fleming valves. The Fleming valve, invented in 1904 by John Ambrose Fleming, as a detector for radio waves in early radio receivers, was the first thermionic diode and the first vacuum tube. The basic configuration consisted of an evacuated glass bulb containing two electrodes a cathode in the form of a wire filament heated white-hot by a current through it, and a metal anode usually consisting of a flat plate. Since only the filament produces electrons, the current of electrons through the tube can only pass in one direction, from filament to anode. In these early tubes, the filament is the vertical loop of wire, while the anode has various shapes, such as the coil of wire in the lefthand tube. The caption says "Photograph of the oscillation valves first employed by Dr. J. A. Fleming, FRS, in October 1904, for the rectification of high-frequency electric oscillations as used in wireless telegraphy".

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