Vacuum tube

In electronics, a vacuum tube, electron tube (in North America), or thermionic valve (elsewhere, especially in Britain), reduced to simply "tube" or "valve" in everyday parlance, is a device that relies on the flow of electric current through a vacuum. Vacuum tubes may be used for rectification, amplification, switching, or similar processing or creation of electrical signals. Vacuum tubes rely on thermionic emission of electrons from a hotfilament or hot cathode, that then travel through a vacuum toward the anode (commonly called the plate), which is held at a positive voltage relative to the cathode. Additional electrodes interposed between the cathode and anode can alter the current, giving the tube the ability to amplify and switch.

Vacuum tubes were critical to the development of electronic technology, which drove the expansion and commercialization of radio communication and broadcasting, television, radar, sound reproduction, largetelephone networks, analog and digital computers, and industrial process control. Although some of these applications had counterparts using earlier technologies, such as the spark gap transmitter or mechanical computers, it was the invention of the triode vacuum tube and its capability of electronic amplification that made these technologies widespread and practical.

In most applications, vacuum tubes have been replaced by solid-state devices such as transistors and other semiconductor devices. Solid-state devices last much longer, and are smaller, more efficient, more reliable, and cheaper than equivalent vacuum tube devices. However, tubes still find particular uses where solid-state devices have not been developed or are not practical, or where the tube device is regarded as having superior performance over the solid-state equivalent, as can be the case with some devices used in professional audio. Tubes are still produced for such applications and to replace those used in existing equipment such as high-power radio transmitters.