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A vacuum tube is a device used to control the flow of electric current using a vacuum in a sealed container, which usually takes the form of a glass tube, hence the name. The vacuum tube is the predecessor of the modern transistor and was used in similar ways as electronically controlled switches, rectifiers, amplifiers, oscillators and in other creative ways that transistors may be used today. The cathode ray tube (CRT) used extensively in early television sets and computer monitors as screens is a kind of vacuum tube.
The vacuum tube was invented by English physicist John Ambrose Fleming in 1904 as a basic component for electronic devices and used throughout the first half of the twentieth century. It brought about great innovations in television, radio, radar, sound recording and reproduction, telephone networks, industrial automation and, most importantly, the development of analog and digital computers. It was essentially the predecessor of the modern transistor, which brought about a revolution in technology and paved the way for the development of the personal computer.
The vacuum tube was composed of a cathode that produces electrons and an anode that collects the electrons, at least the very basic ones called a diode; however, other types of vacuum tubes existed that were classified according to the number of electrodes present. These electrodes are then enclosed in a casing, usually glass, with all the air removed since air can act as a conductor when energized enough, becoming a pathway for electrons in the same way that lightning travels through the air. So, because of the shape of the vacuum, it became commonly known as the vacuum tube.
Because the vacuum tube needed a heating filament to produce electrons, it usually required massive amounts of power and thus produced a lot of heat, leading to the quick degradation of the components, so it burned out rather quickly. It also required three different power sources of varying capacities and ratings in order to work. But as vacuum tube technology progressed, the size and power consumption also became smaller, up to the point where the tubes were as small as large Christmas lights.
Vacuum tubes, however, are not completely obsolete as they are still being used in big radio stations and high-power UHF TV stations, especially those utilizing power levels above 10,000 W and frequencies above 50 MHz. The reason is cost-efficiency since transistors are really just good for low frequencies; at high frequencies, a hundred transistors in parallel and wired together in a cascade will be needed, which creates massive heat, so heat sinks are required. An equivalent transmitter will only use a single tube, which requires considerably less power and can be cooled with forced air or water cooling. Vacuum tubes are also very popular in sound amplifiers because solid-state amplifiers cannot replicate the peculiar distortion and speaker damping effect of vacuum tubes.