Under the Fifth Sun
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.
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, and could be used for all applications that semiconductors (and thus computer ships) could be used for, just at a very large (and much less efficient) scale.