Thermionic Electron Emission

^ T ^ H E R E have appeared in the Reviews of Modern Physics two excellent summaries on thermionic emission, one by Compton and Langmuir 1 and one by Dushman. 2 Compton and Langmuir, while dealing primarily with discharge in gases, also discussed m a n y phases of thermionic emission. Dushman's article is a comprehensive review on thermionics. He faithfully reflects whatever viewpoints and experiments appear in the literature. Besides reviewing the work that has been performed since 1930, the present article will be an a t t e m p t to review in a critical manner some of the matters which in the preceding reviews were left undecided. On the other hand, no attempt will be made to give a complete presentation of all the views appearing in the literature. As to the close connection between thermionic and adsorption phenomena, this will be dealt with in an article now in preparation. Recently there have been published two comprehensive books on thermionics. One is in English by A. L. Reimann. 3 The other is Vol. IV of Miiller-Pouillets Lehrbuch der Physik 4 edited by A. Eucken, with contributions by A. Eucken, R. Suhrmann, L. Nordheim and others. The topics which are fully covered in these two books and in the book 5 by W. Schottky and H. Rothe, Physik der Gluhelektroden will not be covered in detail in the present article. Since photoelectric phenomena are closely associated with thermionics, it is well to refer also to Linford's 6 review on the external photoelectric effect and the book by Hughes and DuBridge 7 on photoelectric phenomena.