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BELL SYSTEM TECHNICAL JOURNAL 



that of the complete photoelectric emission. The surface to be inves- 

 tigated is illuminated by light from a source whose temperature is 

 varied. It is found that the resulting photo-current obeys a Richard- 

 son law and the slope of the Richardson line is taken as the work 



-2 



8-^ 



-6 



-8 



-10 



123456789 10 



4" X 10"* 



Fig. 2 — Typical Richardson plot. 



function. The temperature of the cathode is now altered and a new 

 Richardson line is obtained. It is found that the slope has changed. 

 It may be worth while to analyze critically the theory of this experi- 

 ment to ascertain whether the slope is related to the work function or 

 to the heat function. 



On True and Apparent Surface Areas 



One other point in the correlation between experiment and theory 

 is to be noted. In the empirical equation, i is the current per cm.^ of 

 apparent surface; while in the theoretical equation, i is the current 

 per cm. 2 of an ideal or true surface. The real surface in thermionic 

 experiments is not smooth ; it usually consists of a large number of 



