In electronics, a vacuum tube, electron tube (in North America), thermionic valve, or just valve (elsewhere, especially in Britain) is a (usually tubular) evice 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 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.