﻿Electron Iheory of Contact Electromotive Force. 2C>';) 



an absolute insulator again after a few hours. There is 

 possibly some connexion between this alteration of resistance 

 of thin films and the photo-electric fatigue mentioned by 

 Allen, and some experiments are being made at present to 

 test it. 



I have great pleasure in recording my best thanks to Pro- 

 fessor Stroud, of the Armstrong College, Newcastle-on-Tyne, 

 and to Professor W. M. Hicks, of the University of Sheffield, 

 for the use of the apparatus necessary for the above 

 experiments. 



XXIII. The Electron Theor// of Contact Electromotive Force 

 and Thermoelectricity. By 0. W. Richardson, Professor 

 of Physics, Princeton University *. 



ri^HE following method of deducing certain formula? winch 

 JL connect some of the physical properties of conductors, 

 particularly in the domain of thermoelectricity, with the 

 number and state of the free electrons present in them, is a 

 natural development of considerations which the writer put 

 forward some time ago in order to account for the laws 

 which regulate the emission of negative electrons from hot 

 bodies f- The method is based on two fundamental assump- 

 tions. The first, which is common to all forms of the electron 

 theory of metallic conduction, is that the electric current in 

 meials is carried by electrons which move about freely, exert 

 a pressure, and are otherwise dynamically equivalent to the 

 molecules of a gas. The second is that the potential energy 

 of an e)ectron is much less when it is inside an uncharged 

 conductor than when it is extracted and removed to a con- 

 siderable distance. It follows that a definite amount of 

 work w has to be done by an electron before it can eseape 

 from a conductor, and it is the force at the interface, which 

 corresponds to this amount of work, which retains the 

 electrons in the conductor at ordinary temperatures. 



This position has recently been strengthened by an accu- 

 mulation of experimental evidence. It has been found to 

 give an adequate quantitative explanation of the number of 

 electrons emitted by hot bodies under different conditions. 

 The emitted electrons have been found to have a distribution 

 of velocity in accordance with the requirements of Maxwell's 

 law, and to have a mean translatory kinetic energy identical 



* Communicated bv the Author. 



t Camb. Phil. Proc. vol. xi. p. 286* (1901); Phil. Trans. A. vol. 

 p. 497 (1903). 



