thermionic adjective. [Ɵǝ:mɪ'ɒnɪk] E20.
[from THERMION + -IC.]
Physics. Of, pertaining to, or employing electrons emitted from a hot, esp. an incandescent, surface.
thermionic emission: of electrons from a heated surface. thermionic valve an electronic device consisting of an evacuated envelope containing two or more electrodes, such that a current can flow only in one direction as a result of thermionic emission from one electrode.

■ thermionically adverb E20.

■ thermionics noun the branch of science that deals with thermionic emission E20.

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