The Fleming Valve
While at University College London in the 1880s, British scientist and Professor John Ambrose Fleming investigated the "Edison effect," which, discovered in 1883, is electrical conduction within an evacuated glass bulb, from an incandescent filament, to a metal plate. In 1904 his prototype rectified high-frequency oscillations and thus detected wireless signals. Fleming patented the device, ultimately named the "Fleming valve," "oscillation valve," or "Fleming diode."
While experimenting with his newly discovered incandescent lamp, Thomas Edison introduced an extra electrode and realized it carried a current when it was of (only) a positive potential, relative to the filament. The Edison Effect was only later interpreted to be a flow of electrons from the hot filament to the extra electrode (and not in the other direction, from electrode to filament). Fleming used this effect to rectify and thus measure a weak wireless signal. Unrectified, the signal's oscillations self-cancel and do not show on a galvanometer.
With immediate practical ability to detect Morse coded messages, Fleming's valve was precursor to a new tube: After reading Fleming’s 1905 paper on his oscillation valve, American engineer Lee DeForest created in 1906 a three-element tube ("triode") which functioned as an amplifier and oscillator as well as detector by "biasing" the extra element negative with respect to the heated, electron-emitting filament and using it to modulate or control the electron current flow. With immediate and future applications, the Fleming valve partly laid the foundation for electronics.
adapted from http://www.ieee.org/organizations/history_center/fleming.html
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Last modified 29 Sep 2020