216 ANNUAL REPORT SMITHSONIAN INSTITUTION, 1937 



liglit of various frequencies, it should be possible to find out if the 

 "work-functions" w of different metals are indeed related to their 

 contact differences of potential in the manner predicted bj^ Richard- 

 son's theory. 



In two papers, by me in 1911 and jointly with Richardson in 1912, 

 it was concluded first that the contact difterences of potential are 

 related to the "work-functions" as Richardson had predicted, 

 and secondly that Einstein's photoelectric equation, rather than a 

 rival theory then under discussion, properly described the facts. 

 Practically simultaneously with this second paper, there appeared 

 the report of a shnilar verification of Einstein's equation by A. L. 

 Hughes, then in England, though lacking the quantitative connection 

 with contact differences of potential. 



Tliis earlj'- work was not veiy accurate, partly because of lack of 

 good vacuum technique for maintaining imtamished surfaces in a 

 vacuum, partly tlu-ough lack of constant sources of ultraviolet light 

 and partly because the ultraviolet spectrographs used to isolate the 

 various wave lengtlis of light gave a certain spectral impurity of scat- 

 tered light of other wave lengths. These sources of error were recog- 

 nized but not overcome when IMillikan, in 191G, made a strildng ad- 

 vance by using doubly purified light or other^viso correcting for the 

 effects of impurity, and secured a verification of Einstein's equation 

 wliich was far more accurate than the earlier work as regards the 

 value of Planck's constant h. In fact, Millikan's work remains to 

 this day as one of the best determinations of this important constant. 

 In regard to the "work-function," however, tliis work of Millikan's 

 was not so successful, for, after ha\dng apparently discovered facts at 

 variance with Richardson's interpretation of the equation and its 

 relation to contact potentials, these differences were ultimately found 

 to reside in faults of experimental procedure or interpretation, so that 

 Richardson's interpretation of Einstein's equation still holds. 



In both thermionic and photoelectric effects, theoretical refinements 

 have been introduced by the recent quantum mechanics, and great 

 advances made in experimental technique. However, it is fair to 

 say that their interpretations on the electron theory have been among 

 the major achievements of this theory. 



CONDUCTION OF METALS 



"VMiile we are on the subject of electricit}' in metals, what constitutes 

 the phenomenon of easy flow of electricity that is the distinguishing 

 feature of metals? J. J. Thomson at once suggested that tliis must be 

 due to the existence in metals of electrons free from their parent atoms, 

 moving freely, except for collisions, wiicnever an electric field was 

 applied in the metal. The theory thus worked out was attractive. 



