Electron tube consisting of a sealed glass or metal enclosure from which the air has been withdrawn.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.