﻿I he Ionization of Metals by Cathode Rays. 369 



given of the observed effects and also of the null effect 

 obtained at very high temperatures. A method of investi- 

 gating qualitatively the thermionic emission from various 

 bodies is developed and applied to the case of the emissions 

 from wires of various alloys in air and other gases. A 

 number of these had not previously been investigated. 



In conclusion the author desires to express his indebted- 

 ness to Professor A. W. Porter, F.R.S., and to Professor 

 0. W. Bichardson, F.R.S., for their interest in the above 

 experiments, and to thank the Staff of the Electrical Depart- 

 ment of St. Bartholomew's Hospital for placing their powerful 

 X-ray installation at his disposal, and for the use of their 

 standardized amount of radium bromide. 



XL. Hie Ionization of Metals by Cathode Rays. 

 By Norman Campbell, Sc.D* 



1. TN the Philosophical Magazine for August 1914 (p. 286), 

 X some experiments on the ionization of platinum by 

 cathode rays were described. It appeared that considerable 

 changes in the amount of ionization produced might be 

 effected by heating the metal or making it the cathode of a 

 discharge through oxygen. In order to throw light on the 

 precise nature of these changes it appeared desirable to 

 extend the observations to other metals and to cathode rays 

 of higher speed. Such an extension is made in the observa- 

 tions now to be described. 



2. A slight alteration was made in the essential part of 

 the apparatus shown in fig. 1 of the last paper. Fig. 1 of 

 this paper shows the new arrangement. A and B are 

 nickel-plated brass cylinders. In A the cathode rays are 

 produced from the Wehnelt cathode W and caused to enter 

 B with a speed Y x by keeping A at a potential V x higher 

 than W. The rays fall on the plate P, the ionization of 

 which is under investigation ; the speed, V, with which they 

 strike P is varied by varying the potential Y a between A 

 and P. The electrons leaving P fall on B ; the potential 

 between B and P will be called U, and will be counted 

 positive when P is positive. 



Two quantities have to be measured : i, the total current 

 carried by the rays falling on P, and i 2? the current received 

 by P when none of the rays reflected from P or the electrons 

 emitted from P by its ionization fall back on P. R, tbe 

 reflexion coefficient, in terms of which all the results are 



* Communicated by the Author. 

 Phil. Mag. S. 6. Vol. 29. No. 171. Match 1915. 2 B 



