"In electronics, a vacuum tube, an electron tube, or valve (British usage) or, colloquially, a tube (North America), is a device that controls electric current flow ...
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in a high vacuum between electrodes to which an electric potential difference has been applied.
The type known as a thermionic tube or thermionic valve uses the phenomenon of thermionic emission of electrons from a heated cathode and is used for a number of fundamental electronic functions such as signal amplification and current rectification.
Non-thermionic types, such as a vacuum phototube however, achieve electron emission through the photoelectric effect, and are used for such as the detection of light levels. In both types, the electrons are accelerated from the cathode to the anode by the electric field in the tube." - (en.wikipedia.org 21.09.2019)
