﻿Ionization of Metals by Cathode Rays. 371 



received by B and P together will be less than the whole 

 current entering B. Another way of measuring i 2 is to 

 prevent any of the rays leaving P by making U sufficiently 

 large and positive ; it is obvious that if B is kept connected 

 to W none of the rays from W either before or after 

 striking P will strike B * ; all that enter B must come 

 to rest finally either on P or on the tube, and very few 

 will strike the tube after striking P because the tube is 

 surrounded by metal which repels the rays. This method of 

 measuring i 2 is much more satisfactory than the other, and 

 was always adopted ; but it must be noted that, unless the 

 rays entering B have all exactly the same velocity the 

 number striking P will not be the same when i 1 and when i 2 

 are being measured. For the potential of B determines the 

 potential at the mouth of the tube by which the rays enter; 

 when i 2 is being measured the rays are entering against a 

 retarding potential V 2 — 20; when i l is being measured the 

 retarding potential is Y x and is always greater. But since 

 it was found that the total current measured in this way did 

 not vary by as much as 10 per cent, when the speed of the 

 rays was varied (by means of V 2 ) from 400 volts to 2 volts, 

 there is probably little uncertainty on this account, not 

 enough at any rate to affect the conclusions it is proposed 

 to draw. 



The Changes in the Ionization. 



3. It appeared that any change in P which reduced the 

 value of the ionization for one speed of the incident rays 

 reduced it for all speeds, so that in considering the nature of 

 such changes we can speak of " the ionization " without 

 reference to the speed of the incident rays. The precise 

 variation of the ionization with the speed will be considered 

 later. 



Experiments were made with four metals : platinum, nickel, 

 copper, and aluminium. In the cases of platinum and 

 copper the changes were made both by heating the metal by 

 an electric current and by making it the electrode of u 

 discharge. In the earlier experiments on platinum and 

 nickel the apparatus shown in tig. 1 was enclosed in a vessel 

 constructed entirely of glass and protected from vapours 

 by a U-tube cooled in liquid air. In the later experiments 

 the apparatus was exposed to the vapours of sealing-wax, 

 tap-grease, and mercury. No difference between the two 

 arrangements could be found with platinum and nickel ; it 



* For reasons which it is needless to set forth in detail, B had to be 

 at a slightly lower potential than W when V was small. 



2 B 2 



