vacuum tube: see electron tube electron tube, device consisting of a sealed enclosure in which electrons flow between electrodes separated either by a vacuum (in a vacuum tube) or by an ionized gas at low pressure (in a gas tube).
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vacuum tube

Electron tube consisting of a sealed glass or metal enclosure from which the air has been withdrawn. It was used in early electronic circuitry to control a flow of electrons. In the first half of the 20th century, vacuum tubes allowed the development of radio broadcasting, long-distance telephone service, television, and the first electronic digital computers, which were the largest vacuum-tube systems ever built. Transistors have replaced them in virtually all applications, but they are still occasionally used in display devices for television sets and computers (cathode-ray tubes), in microwave ovens, and as high-frequency transmitters on space satellites.

An electronic device that controls the flow of electrons in a vacuum. It is used as a switch, amplifier or display screen. Used as on/off switches, they allowed the first computers to perform digital computations. Although vacuum tubes have made a comeback in high-end stereo components, most vacuum tubes today are the picture tubes (CRTs) in monitors and TVs. See vacuum tube types.

Early Vacuum Tube
Early vacuum tubes were used to amplify signals for radio and other audio devices. This one was made in 1915. Tubes were not used as switches in calculating machines until 1939. (Image courtesy of AT&T.)

Tubes in the 21st Century
Vacuum tubes are still used and not just for CRTs. Many audiophiles swear that tubes are the only electronics that can faithfully reproduce musical sounds, and they would be delighted with these Model One amplifiers from Red Rose Music (www.redrosemusic.com). Designed by legendary audio engineer Mark Levinson, for a mere $25,000, you can own all 212 pounds of this top-of-the-line equipment.

vacuum tube - electron tube