THE ELECTRON— COMPTON 215 



That was a question the study of which has led to most important 

 theoretical and practical consequences. 



Richardson, first as a pupil of Thomson and then as a professor at 

 Princeton in the early 1900's, developed the theory of thermionic 

 emission of electrons, according to which the electrons are evaporated 

 from the surface of a metal at high temperatures by a process very 

 analogous to evaporation of molecules. The electrons are assumed to 

 have the same distribution of kinetic energies that molecules possess 

 at the same temperature in accordance with the principles of kinetic 

 theory. They escape from the surface, if they reach it, with enough 

 energy to take them away in spite of the attraction tending to pull the 

 electron back into the metal. This attraction is expressed in terms of 

 the now famous "work-function," a sort of latent heat of evaporation 

 of electrons, which is the work that must be done to get an electron 

 clear of the surface. With these simple assumptions, an equation was 

 derived for the rate of emission of electricity as a function of tempera- 

 ture which has stood the test of perhaps as wide a range of experimen- 

 tation as any other equation of physics, a range of values of more than 

 a million-million fold in current without any detectable departure 

 from the theory, if this is properly applied. 



Richardson's measurements of the "work-functions" of various 

 metals showed that these values run closely parallel with one of the 

 longest known but least understood properties of metals, namely, their 

 contact potential properties. By contact difference of potential is 

 meant the voltage difference between the surfaces of two metals when 

 they are placed in contact. Richardson found that the difference 

 between the "work-function" of two metals was, within the limits of 

 accuracy of the data, the same as their contact difference of potential. 

 He therefore proposed the theory that the contact potential property 

 of a metal is determined simply by the work necessary to remove an 

 electron from its surface. 



As a beginning graduate student under Richardson in 1910, I was 

 given the job of undertaking a test of tliis theory through experiments 

 on the other electron-emitting phenomenon, the photoelectric effect. 

 Einstein a few years before had proposed his famous photoelectric 

 equation, which was a contribution to physical theory certainly com- 

 parable in importance and thus far more useful in its applications 

 than his more impressive and wider publicized general theory of rela- 

 tivity. According to it, an electron in a metal may receive from the 

 incident light an amount of energy proportional to the frequency of 

 the Hght — to be exact, an energy equal to Planck's constant h times 

 the frequency v. If it escapes from the metal, it must do an amount 

 of work V) to get away, so that its kinetic energy after escape from 

 the metal would be the difference hv—w. Obviously, by measuring 

 these kinetic energies of electrons liberated from various metals by 



