788 THE BELL SYSTEM TECHNICAL JOURNAL, OCTOBER 1951 



unwilling to publish his observations until he felt sure that he understood 

 all that was taking place. It is in an article by another — the late H. D. 

 Arnold, first to hold the post of Director of Research in Bell Telephone 

 Laboratories and its antecedent organization — that we find a description of 

 Davisson's ''power-emission chart," now standard in the art. In Arnold's 

 words: "Dr. Davisson has devised a form of coordinate-paper in which the 

 coordinates are power supplied to the filament (abscissae) and thermionic 

 emission (ordinates). The coordinate lines are so disposed and numbered 

 that if the emission from a filament satisfies Richardson's relation, and the 

 thermal radiation satisfies the Stefan-Boltzmann relation, then points on 

 the chart coordinating power and emission for such a filament will fall on a 

 straight line." 



In a paper presented at a meeting toward the end of 1920 (it was a joint 

 paper of himself and J. R. Weeks) Davisson gives the theory of the emis- 

 sion of light from metals, deduces a deviation from Lambert's law and 

 verifies this by experiment. A connection between this and the study of 

 thermionics may be inferred from the words which I quoted earlier from 

 Arnold's description of Davisson's power-emission chart. This work was 

 published in full, some three years later, in the Journal of the Optical Society 

 of America. 



We turn now to Davisson's investigations of thermionic emission from 

 metals. 



Those whose memories go back far enough will recall that two laws have 

 been proposed for the dependence of thermionic emission on temperature. 

 Both were propounded by O. W. Richardson, and each, somewhat confus- 

 ingly, has at times been called ''Richardson's law." The earlier prescribed 

 that the thermionic current i should vary as r^exp(— ^/T), T standing for 

 the absolute temperature; the later prescribes that t should vary as 

 T^tx\i{—b/T). The former is derived from the assumption that the velocities 

 and energies of the electrons inside the metal are distributed according to 

 the classical Maxwell-Boltzmann law. The latter follows from the assump- 

 tion that these velocities and energies are distributed according to the 

 quantum-theory or Fermi-Dirac law: it was, however, derived from thermo- 

 dynamic arguments some thirteen years before the Fermi-Dirac theory was 

 developed, and the experiments about to be related were performed during 

 this thirteen-year period. 



In the interpretation of either law, b is correlated with the work of egress 

 which an electron must do (at the expense of its kinetic energy) in order to 

 go from the inside to the outside of the metal. I will leave to a later page the 

 phrasing of this correlation, and say for the moment that h multiplied by 

 Boltzmann's constant k represents what used to be called and is still some- 



