thermionic emission, emission of electrons or ions by substances that are highly heated, the charged particles being called thermions. The number of thermions emitted increases rapidly as the temperature of the substance rises. The heated material may be in the form of a metal filament or of some compound that coats and is heated by the filament. If the heated body carries a positive or negative charge, the thermions will be of the same charge. At temperatures below red heat (see black body), thermionic emission from uncharged bodies is chiefly positive; at higher temperatures it is negative. The effect was discovered by Thomas A. Edison in 1883 when he was working on filaments for the electric light. Thermionic emission's most important practical application in electronics is in the electron tube, since it is the mechanism by which electrons are emitted from the cathode.