Section III, 1920 [19] Trans. R.S.C. 



On the Mobilities of Ions in Helium at High Pressure 

 By Professor J. C. McLennan, F.R.S., and Mr. E. Evans, M.A. 



(Read May Meeting, 1920.) 



SYNOPSIS 



1. Absence of metallic conductivity. 



The experiments described in the paper shew that pure helium at high pressures 

 does not exhibit any appreciable metallic electrical conductivity. 



2. Mobilities of ions in helium at high pressures. 



The mobilities of positive and negative ions produced in helium at a pressure of 

 81 atmospheres by alpha rays from polonium have been shewn to be respectively 

 2-52 X 10-^ cms. per sec. per volt per cm. and 4>26 x 10-^ cms. per sec. per volt 

 per cm. It has also been shewn that the mobilities do not vary inversely with 

 the pressure at high pressures. 



I. Introduction 



It has been shown by Franck^ that when argon, helium, and 

 nitrogen are carefully purified, and especially when they are freed from 

 oxygen and water vapour, there exists in them even at atmospheric 

 pressure negatively charged particles having such high mobilities that 

 one is led to conclude that they are free electrons. The values re- 

 corded are 206 cm/sec. per volt/cm. for argon, 500 cm/sec. per volt/cm. 

 for helium, and 120 cm /sec. per volt/cm. for nitrogen. Haines^ has 

 confirmed this observation in so far as nitrogen is concerned, and he 

 has also shown that the same result can be obtained with hydrogen if 

 it be highly purified. Wellisch,^ too, has shown that even in thor- 

 oughly dried air, free electrons can exist, at least for pressures as high 

 as 10 cms. of mercury. 



Quite recently a considerable supply of helium became available 

 to the writers, and in view of the results recorded above, it was 

 thought advisable to investigate the conductivity of this gas at very 

 high pressure. 



It appeared that if free electrons could exist in pure helium at 

 high pressures in any considerable amount, the gas would exhibit in 

 a measure something similar to a metallic conduction, and it would 

 consequently be suitable in special cases for high resistances or other 

 similar purposes. Some experiments were therefore made in this 

 direction, and the results are given in the following note. 



1 Franck, Ber. d. Deut. Phys. Ges. (1909). 



2 Haines, Phil. Mag. Oct., 1915. 



3 Wellisch, Am. Jour. Sc, Vol. XXXIX. May, 1915. 



