This article is about the electronic device. For experiments in an evacuated pipe, see free-fall. For the transport system, see pneumatic tube.
In electronics, a vacuum tube, electron tube (in North America), thermionic valve (elsewhere, especially in Britain) 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. Some special function vacuum tubes are filled with low-pressure gas: these are so-called soft tubes as distinct from the hard vacuum type which have the internal gas pressure reduced as far as possible. Almost all tubes depend on the thermionic emission of electrons.
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