File:Fleming valve.jpg

English: Early Fleming valve or thermionic diode, the first vacuum tube, invented by John Ambrose Fleming in 1904. It consists of an evacuated glass envelope containing two electrodes: a cathode in the form of a filament heated by a separate current, which emitted electrons, and a metal plate anode, which in this tube is a sheet metal cylinder surrounding the filament. The Fleming valve was used as a detector in early radio receivers, to extract the audio modulation (sound) from the radio frequency carrier wave. The incoming radio signal was applied across the tube. On the positive half cycles of the wave when the anode was positive with respect to the cathode, the negative electrons are attracted to the anode, causing a current through the tube, while when the anode is at a negative potential with respect to the cathode the electrons are repelled and no current flows. Thus the Fleming valve was able to rectify the alternating radio current to a pulsing direct current (DC). It was used in receivers built by the Marconi Co. until about 1920, when the Audion (triode) replaced it.