Vacuum tube

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The predecessor of semiconductor electronic devices.

Vacuum tubes (also known as thermionic valves) are arrangements of electrodes surrounded by vacuum within a glass envelope, superficially similar to incandescent light bulbs. Like light bulbs, tubes have a filament through which current is passed, heating the filament. In its heated state it is ready to release electrons into the vacuum. These electrons are electrostatically drawn to a positively charged outer metal plate called the anode, or more commonly just the plate. Electrons do not flow from the plate back toward the filament, even if the charge on the plate is made negative, because the plate is not heated. The arrangement of a filament and plate is called a diode and invented in 1904 by John Ambrose Fleming scientific adviser to the Marconi company based on an observation by Thomas Edison.

The next innovation, due to Lee de Forest in 1907, was to place another electrode, the grid, between the filament and plate. The grid is a bent wire or screen, leaving plenty of room for electrons to get past it to complete their journey toward the plate. De Forest discovered that the current flow from filament to plate was highly dependent on the voltage of the grid, and that the current drawn by the grid was very low. The resulting three-electrode device, the triode, was therefore an excellent amplifying device.

Many further innovations followed. It became common to use the filament to heat a separate electrode called the cathode, and to use the cathode as the source of electron flow in the tube rather than the filament itself. It was discovered that additional grids could be used to lower the current draw of the triode's control grid, and to improve the linear response of the tube. The triode gave way to the tetrode and later the pentode.

Other vacuum tube electronic devices include the magnetron, klystron and cathode ray tube.

See also: Irving Langmuir

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