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 * A vacuum tube, also known as an electron tube, is a device used to amplify, switch, modify, or create an electrical signal by controlling the movement of electrons in a low-pressure space.
 * A vacuum tube consists of electrodes in a vacuum in a (usually tubular) insulating heat-resistant envelope.
 * Vacuum tubes were critical to the development of electronic technology, which drove the expansion and commercialization of radio broadcasting, television, radar, sound reproduction, large telephone networks, analog and digital computers, and industrial process control. Some of these applications pre-dated electronics, but it was the vacuum tube that made them widespread and practical.

Prewar history.--While the vacuum tube was known in various forms prior to the war (WWI, 1914) the most significant was for the transmission of speech by radio. In spite of their impressive usefullness, vacuum tubes had significant drawbacks; they were expensive, had to be replaced often, and required heavy batteries in order to operate.

In 1924, oscillating crystal circuits replaced the vacuum tube.

http://earlyradiohistory.us/sec014.htm, http://en.wikipedia.org/wiki/Vacuum_tube