﻿400 Dr. Norman Campbell on 



become impaired. In that case it can, if desired, be restored 

 by the use of an objective cylindrical lens (§ 22). 



The method of detecting and measuring small angular 

 displacements, which forms the subject of this paper, was 

 developed in connexion with a " search for a new property 

 of matter in motion/'' for which a grant was obtained through 

 the Royal Society. I have also to thank the Earl of 

 Berkeley for most kindly placing the resources of his 

 laboratory workshop at my disposal. 



Boars Hill, Oxford. 

 Nov. 191.1. 



XXXVI. Ionization by Collision. 

 By Norman Campbell, Sc.D* 



1. rpHE theory of ionization by collision of ions and the 

 JL application of it to explain the electrical discharge 

 through gases at low pressures is due mainly to Townsendf. 

 In the course of some recent work, described in the following- 

 paper, doubts have been thrown upon the accuracy of the 

 mathematical argument by which his formulae are derived 

 from his physical assumptions. Those assumptions are not 

 doubted, but since some of them are of sufficient interest to 

 merit special consideration, Townsend's entire argument will 

 be sketched briefly. 



2. Townsend makes the following hypotheses : — 



(1) An ion, positive or negative, formed by any means in 

 a gas, will produce a fresh pair of ions when it collides with 

 a neutral molecule, if previously to that collision it has 

 fallen freely under the action of the electric field through a 

 potential not less than some finite value V. V 1 is much 

 greater for positive than for negative ions. In what follows 

 negative ions only will be under consideration, for it will be 

 assumed that the electric fields are so small that there is no 

 appreciable ionization by collision of the positive ions. 



(2) An ion, after collision with a neutral molecule, whether 

 or not that collision results in the production of fresh ions, 

 retains no portion of its previous velocity, but starts again 

 from rest ; it retains, however, its charge. (The velocity of 

 thermal agitation is so small compared with that acquired 

 under the action of the field in all the cases considered that 

 it may be neglected.) 



* Communicated by the Author. 



t Townsend's work in this direction is summarized in his volume 

 'The Theory of the Ionization of Gases by Collision' (Constable, 

 London, 1910. Preferences will be made throughout to this book). 



