﻿Ionization of Metals by Cathode Rays. 381 



such an apparatus as was used. The electrons emitted in 

 the process o£ ionization have practically all speeds less 

 than 20 volts ; the reflected electrons, when V is greater 

 than 40, have almost all speeds greater than 20 volts. 

 Accordingly, it might be thought that the current carried by 

 the reflected electrons would be measured by the change in 

 the current flowing to P, produced by changing the potential 

 difference between B and P from 20 to V volts. But it 

 must be remembered that when the reflected electrons fall 

 on B they cause ionization there, and the electrons so 

 emitted will be driven to P, so that by increasing the 

 potential between B and P and preventing the reflected 

 -electrons from reaching B we may decrease and not increase 

 the negative current received by P. And experiment shows 

 that this effect is of importance, for at the higher values of 

 V'i 2 o, the negative current received by P when U = 20 is 

 actually greater than v, that received when U = V. The 

 following figures give the ratio z 20 /zv' for various values of 

 V in states A, B, B' corresponding to the three curves of 

 fig. 2 ; if there were no reflexion the ratio would be 1 ; 

 the occurrence of values greater than 1 indicates the effect 

 of ionization at the surface of B : — 



40 80 120 160 200 240 280 320 



A 0-858 0-903 0-933 0-975 1-032 1054 1*109 1-170 



T^ B 0-835 0-894 0-913 0953 0-976 1-010 1039 1-067 



W 0-921 0-91S 0-920 0'919 0*920 0-945 0-956 0997 



At first sight these figures might seem to show that the 

 reflexion is actually less in state B' than in state A ; but 

 the difference between the figures can also be accounted for 

 by supposing that it is not the reflexion, but the ionization 

 produced by the reflected rays in B, which is the cause of 

 the change. 



It appears, then, that it is very difficult to interpret 

 certainly the quantitative results in a manner to throw new 

 light on the changes investigated. They seem to confirm 

 the view that the state B does not differ from A merely as 

 B' differs from B ; the change which produces B from B' 

 is not a mere partial reversal of the change which produced 

 B' from A. But the} r do not seem to decide without 

 ambiguity whether the explanation of the changes proposed 

 in §3 is tenable. It is not certain whether there is a change 

 in the ionization potential, and it appears that the decrease 

 of the ionization between states B and B' must be due to 



