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A vacuum tube is a hollow glass cylinder from which as much air as possible has been removed. The cylinder also contains two metal electrodes: the cathode, or negative electrode, and the anode, or positive electrode. Current flows within a vacuum tube from the cathode, which has an excess of electrons, to the anode, which has a deficiency of electrons.

Vacuum tubes were a subject of great interest among both scientists and inventors at the end of the nineteenth century. Among scientists, vacuum tubes were used to study the basic nature of matter. Among inventors, vacuum tubes were used as a means of controlling the flow of electric current within an electrical system.

One of the first practical vacuum tubes was invented by English electrical engineer John Ambrose Fleming (1849–1945). Fleming's device permitted the...