
In electronics, a vacuum tube, electron tube (in North America), tube, or thermionic valve or valve (in British English) 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 similar to an incandescent light bulb with an added electrode inside. When the bulb's filament is heated red-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.
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