Vacuum tube

[technoguy]

In electronics, a vacuum tube (U.S. and Canadian English) or (thermionic) valve (outside North America) is a device generally used to amplify, or otherwise modify, a signal by controlling the movement of electrons in an evacuated space. For most purposes, the vacuum tube has been replaced by the much smaller and less expensive transistor, either as a discrete device or in an integrated circuit. However, tubes are still used in several specialised applications such as audio systems and high power RF transmitters, as the display device in cathode ray tube television sets, and to generate microwaves in microwave ovens.