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XXI. The Complete Photoelectric Emission* . By Professor 

 0. W. Richardson, F.R.S., Wheat stone Professor of 

 Physics, University of London, King's College^. 



IT is well known that when light o£ sufficiently short 

 wave-length is allowed to fall on metals, an emission 

 of electrons takes place under its influence. Since all 

 substances emit light when they are raised to a high tern- 

 perature, an emission of electrons from hot bodies owing to 

 the action of light will occur even when they are not 

 illuminated from an external source. The emission of 

 electrons which arises in this way may conveniently be 

 termed the "complete photoelectric emission," to indicate 

 that it is excited by the complete (black body) radiation 

 with which the material is in equilibrium at the temperature 

 under consideration. It is of interest to enquire whether 

 this emission will resemble in its behaviour the thermionic 

 effects which have been investigated experimentally, and to 

 seek to determine how its magnitude compares with that of 

 the observed thermionic emission. 



The most striking property of the thermionic emission of 

 electrons is that expressed by the current temperature relation 



i = AT\r J ' T (1) 



(£*= maximum current from unit area, T = absolute tempe- 

 rature, A, A, and b constants). The writer % has shown that 

 it follows from simple thermodynamic considerations that 

 the complete photoelectric emission is also governed by 

 an equation of type (1)§. The thermodynamical argument 

 does not determine the numerical value of the constants A 

 and b (X is unimportant) which enter into the formula for 

 the complete photoelectric emission. It does, however,, 

 determine the meaning of b, which is closely related to the 

 work done by an electron in escaping from the metal. The 

 values of the minimum frequency of the light which is able 

 to excite any photoelectric emission from metals show that 

 b is not very different in the case of photoelectric and 

 thermionic emissions from a given material. It is not 



* Part of an address delivered by the Author in opening the dis- 

 cussion on Thermionic Emission at the Manchester Meeting of the British 

 Association, 10th September, 1915. 



t Communicated by the Author. 



X Phil. Mag. vol. xxiii. p. 619 (1912). 



§ The same conclusion has been reached by W. Wilson by a direct 

 application of the quantum hypothesis (Ann. der Physik, vol. xlii. p. 1154 



