Thermionic Electron Emission * 



By J. A. BECKER 



Introduction 



THERE have appeared in the Reviews of Modern Physics two ex- 

 cellent summaries on thermionic emission, one by Compton and 

 Langmuir ^ and one by Dushman.^ Compton and Langmuir, while 

 dealing primarily with discharge in gases, also discussed many phases of 

 thermionic emission. Dushman's article is a comprehensive review on 

 thermionics. He faithfully reflects whatever viewpoints and experi- 

 ments appear in the literature. Besides reviewing the work that has 

 been performed since 1930, the present article will be an attempt 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.^ The other is 

 Vol. IV of Miiller-Pouillets Lehrbuch der Physik ^ 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 ^ by W. Schottky and H. Rothe, Physik der Glilhelektroden 

 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 ^ review on the external photoelectric effect and the 

 book by Hughes and DuBridge ^ on photoelectric phenomena. 



Empirical and Theoretical Richardson Formulae 

 One topic on which considerable confusion has existed goes to the very 

 root of thermionic emission. It is the interpretation that is to be put 

 on the slope and intercept of a Richardson line and how the slope and 

 intercept are related to certain quantities in theoretical formulre. 

 Empirically it is found that the thermionic emission current density, 

 i, is related to the temperature, T, by the Richardson formula 



i = AnT'^expi-bJT), (la) 



* Published in Reviews of Modern Physics, April, 1935. 



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