﻿Ionization by Collision in Helium. 837 



substance like carbon under bombardment by a rays furnishes 

 a new means of investigating the process by which gases are 

 occluded in carbon, and probably also in other substances. 



In conclusion, I wish to express my gratitude to Profesor 

 McLennan for his suggestions and help throughout the course 

 of this investigation. 



Physical Laboratory, 

 University of Toronto. 



LXXIX. Ionization by Collision in Helium. By E. W. 

 B. Gill, M.A., B.Sc, Fellow of Merton College, Oxford, 

 and F. B. Pidduck, II. A., Fellow of Queen's College, 



Oxford *. 



npHE theory of the ionization of gases by collision, due to 

 JL Professor J. S. Townsend, has been experimentally 

 verified for a large number of gases f , among them being 

 helium J. In the first experiments made by the authors 

 with helium, the gas was not quite pure (probably about 

 2 per cent, of impurity being present) and there was good 

 reason for supposing that these slight traces of impurity 

 exercised a very considerable effect upon the ionizing powers 

 of the ions. A few experiments were performed at the time, 

 after the gas had been purified by pumping into a chamber 

 containing charcoal and immersed in liquid air. The supply 

 of liquid air was then limited and only a few results were 

 obtained, but they were sufficient to show that the impurities 

 exercised an effect which was very large in comparison with 

 the amount present. 



The ionizing powers of both negative and positive ions, 

 especially the latter, were increased, and as a necessary 

 result the minimum sparking potential was lowered. It was 

 actually reduced from 232 volts to 203, and it was con- 

 jectured that for absolutely pure gas it would be not lower 

 than 190 volts. 



A continuous supply of liquid air having since become 

 available, the experiments were repeated and have shown that 

 with careful purification the ionizing powers of the ions are 

 increased even more than was thought possible; for example, 

 the minimum sparking potential was found to be about 160 

 volts instead of the 190 volts conjectured. 



Before giving other results and the experimental methods 



* Communicated by Prof. J. S. Townsend. 



t J. S. Townsend, 'Theory of Ionization of Gases bv Collision.' 



% Phil. Mag. [6] vol. xvi. 1908, p. 280. 



