Vacuum tube

In electronics, a vacuum tube, electron tube (in North America), thermionic valve, tube, or valve is a device controlling electric current through a vacuum in a sealed container. The container is often thin transparent glass in a roughly cylindrical shape. The simplest vacuum tube, the diode, is essentially an incandescent light bulb with an added electrode inside. When the bulb's filament is heated white-hot, electrons are "boiled" off its surface and into the vacuum inside the bulb. If the electrode -- called a "plate" or "anode" -- is made more positive than the hot filament, a direct current flows through the vacuum to the electrode (a demonstration of the Edison effect). As the current only flows in one direction, it makes it possible to convert an alternating current applied to the filament to direct current.

The introduction of a third electrode, a grid between the filament and the plate, yields another function. A voltage applied to the grid controls the current flowing from the filament to the plate. Thus, it allows the device to be used as an electronic amplifier.

Vacuum tubes are thus used for rectification, amplification, switching, or similar processing or creation of electrical signals.

The vast majority of modern day tubes consist of a sealed container with a vacuum inside, and essentially rely on thermionic emission of electrons from a hot filament or a cathode heated by the filament. Some exceptions to this are dealt with in the section about gas-filled tubes below.

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