Valve

In electronics, a valve is a device consisting of two or more metal plates enclosed in an evacuated glass bulb. One of the metal plates is heated, causing electrons to be emitted. If a positive charge is applied to the other plate, the electrons will move towards it and the valve will conduct electricity. Valves have largely been superseded by transistors which are smaller.

See also: Acorn Valve, Heptode, Triode.