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Vacuum tubes, or thermionic valves, are arrangements of electrodes in a vacuum within an insulating, temperature-resistant envelope.
Although the envelope was classically glass, power tubes often use ceramic and metal.
In it's simplest form a vacuum tube can be created to operate as a diode: a device that conducts current only in one direction.
A third element called a "control grid" can be added to the design which provides the ability to amplify a signal.