This electronic diode was one of Sir John Ambrose Fleming's (1849-1945) own valves or vacuum tubes of the type used with the pioneering Marconi-Fleming radio receiver. The diode rectifies high frequency oscillations, that is, it converts them from alternating to direct current. The glas bulb has a high vacuum inside and contains a carbon filament surrounded by an aluminium tube open at both ends. Fleming invented the thermionic valve and pioneered the use of electricity for light and heat on a large scale.
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