thermionic valves

In electronics, a vacuum tube, electron tube (inside North America), thermionic valve, or just valve (elsewhere); is a device used to amplify, switch, otherwise modify, or create an electrical signal by controlling the movement of electrons in a low-pressure space, often not tubular in form. Many devices called vacuum tubes are filled with low-pressure gas: these are so-called soft valves (or tubes); as distinct from the hard vacuum type, which have the internal gas pressure reduced as far as possible. Almost all depend on the thermal emission of electrons, hence thermionic.
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