from Hot Platinum. 287 



therefore small compared with that in which they move freely, 

 so that to determine their equilibrium state we can apply the 

 methods of the kinetic theory of gases. In this way we find 

 the corpuscles have a distribution of velocity which follows the 

 Boltzmann-Maxwell Law, and their average energy is the same 

 as that of a molecule of gas at the same temperature. If we 

 consider what happens at the surface of the metal we must 

 suppose there is here a discontinuity in the potential which 

 prevents the negative ions escaping. It is conceivable that as the 

 temperature is raised, some of the corpuscles will acquire sufficient 

 velocity to enable them to overcome this discontinuity in the 

 potential. Since the number of corpuscles with velocity com- 

 ponents between u, v, w and u + du, v + dv, w + dw in unit 

 volume is 



n {^y e -km(u*+vw } du dv dw> 



where n is the total number of corpuscles in unit volume, 3/4& is 

 the energy of a corpuscle and m is its mass ; the number having 

 these velocity components which strike unit surface perpendicular 

 to u per second is 



n (k^M-hnw+vW) du dv dWm 



If <J> is the work done by a corpuscle in passing through the 

 surface layer, the number which escape from unit area of the 

 metal surface per second is given by 



N= I / n ( — j ue- km <« 2 +» 2 +™ 2 > du dv dw 



m 



= | {kmir)-i e~'**= n tsj ^ e-*l™, 



since k is connected with the absolute temperature by the 

 relation k = (2R6)~~ 1 , R being the gas constant for a single 

 corpuscle. If then the negative radiation is due to the corpuscles 

 coming out of the metal, the saturation current (s) should obey 



6 



the law s= A'd&e'o. This law is fully confirmed by the experi- 

 ments to be described. 



In the experiments the current was measured which passed 

 from a thin platinum wire to an aluminium cylinder surrounding 

 it. To measure the current the cylinder could be put to earth 

 through a sensitive Thomson galvanometer but was, otherwise, 

 insulated. The wire was heated by a steady current of not more 



