﻿Potential and the Velocities of Electrons. 591 



field and that due to the contact difference of potential are 

 corrected for, the curve should cut the current axis at *39. 

 It was thought that possibly the shift, heretofore attributed 

 to the prevention of electron reflexion, might be entirely 

 explained by the stray field. In view of the importance of 

 the existence or non-existence of electron reflexion, this 

 possibility was investigated, with the results shown in fig. 8 

 (PI. IX.). 



The perforated box G and the auxiliary field were removed 

 and the electrons received directly on B. Curve 1 (PL IX. 

 fig. 8) shows the result obtained in this manner. It has 

 been corrected for the contact difference of potential between 

 D and B. For comparison, curve (a), PI. IX. fig. 4, after 

 being corrected for the stray field and contact difference of 

 potential shifts, is shown as curve 2. If the shift heretofore 

 attributed to the prevention of electron reflexion were really 

 due to the stray field, curve 1 would coincide with curve 2. 



The following conclusions may now be drawn : — (1) The 

 evidence in favour of electron reflexion remains unimpaired. 

 In the present case it caused a shift of nearly two volts in 

 the "distribution of velocity" curve. (2) By comparing 

 fig. 4 and fig. 8 (PI. IX.) it is seen that an auxiliary field 

 of less than 8 volts is sufficient to prevent reflexion. 

 (3) When the auxiliary field method is used, besides pre- 

 venting reflexion, it causes a certain shift due to stray field 

 which may be calculated and corrected for. (4) Of the 

 curves in fig. 4 (PL IX.) curve (a) is most nearly the 

 shape that is obtained without the disturbing influence of 

 the perforated screen. This adds confirmation to the views 

 taken in the discussion of fig. 4. 



8. The Shift of the Curves accompanying Photo-electric 

 Fatigue. — Photo-electric fatigue is the name given to the 

 decrease in the photo-electric activity of a plate as the time 

 since polishing increases. It was frequently noticed during 

 the preceding experiments that there is usually a shift in 

 the " distribution of velocity " curve, sometimes to the 

 left and sometimes to the right, increasing with time. It 

 has been frequently mentioned in this paper that metals, 

 and especially the more electropositive metals, tend to 

 become more electronegative the longer the time which has 

 elapsed since polishing. Certainly this change in the contact 

 difference of potential between the plates will cause a shift 

 in the curves, either toward the right or the left accordingly 

 as the electropositive character of the receiving or the 

 emitting plate is changing the more rapidly. Is this change 

 in the contact difference of potential sufficient to account 



