﻿Potential and the Velocities of Electrons, 593 



induction-coil discharge during evacuation) the shift of the 

 curves with time indicates neither a change in tbe velocities 

 of the electrons nor in the distribution of velocities, but is 

 accounted for entirely by the change in the contact difference 

 of potential between the plates. Tt is significant that the 

 metals, like platinum and silver, which show the least fatigue 

 also show the least variation in contact difference of potential 

 and the least shift of the curves with time. It is possible 

 that there is an intimate relation between the cause of photo- 

 electric fatigue and the cause of the time-changes in the 

 contact difference of potential between metals. 



9. Summary. — The results of this investigation may be: 

 summarized as follows : — 



(1) The electrons moving between the emitting and re- 

 ceiving plates are acted on by the force due to the contact 

 difference of potential as well as by the externally applied 

 field. This effect must be allowed for when making measure- 

 ments on the velocities of the electrons. (Section 5.) 



(2) When this contact difference of potential is corrected 

 for, it is found that different metals, under the action of 

 ultra-violet light, give off electrons with practically equal 

 velocities. The ci distribution of velocity" curves are also 

 nearly identical. (Section 6.) 



(3) The evidence of reflected electrons remains unimpaired . 

 (Section 7.) When a perforated screen and an auxiliary 

 field are used to prevent this reflexion, there is a stray field 

 through the screen which may be calculated and corrected 

 for. (Section 4.) 



(4) The shift of the " distribution of velocity r curves 

 with time (when the metal has received no abnormal treat- 

 ment) is accounted for by the time-changes in the contact 

 difference of potential between the emitting and receiving; 

 plates. (Section 8.) 



In conclusion I wish to express my thanks to Professor 

 0. W. Richardson for suggesting this problem and my 

 appreciation of his continual interest and help during the 

 investigation. 



Palmer Physical Laboratory, 

 Princeton, N. J. 

 Aug. 3, 1911. 



Phil Mag. S. 6. Vol. 23. No. 136. April 1912 . 2 R 



