﻿[ m ] 



IX. The Variation with Distance of the Ionization produced 

 by Rontgen Rays. By E. W. B. Gill, M.A., B.Sc, 

 Fellow of Merton College, Oxford*. 



IN connexion with the still unsolved problem as to the 

 constitution of the Rontgen rays, it seemed worth while 

 to make a careful set of experiments on the variation of the 

 ionizing powers of the rays with the distance from the source, 

 to see if the law usually assumed — that the number of ions 

 produced was proportional to the inverse square of the 

 distance — was correct. 



The bearing of this law, if correct, upon the rival theories 

 put forward to explain the nature of the rays — the pulse 

 theory and the corpuscular theory — would be indecisive: but 

 if incorrect would certainly be adverse to the corpuscular 

 theory. 



For consider a beam of rays diverging from a point forming 

 a solid cone of rays. On the corpuscular theory (if there is 

 no absorption) the total number of particles, whatever mav 

 be their nature, crossing any section while the rays are on 

 must be the same. But in all the modifications of the cor- 

 puscular theory, including that of Prof. Bragg, this number 

 alone determines the number of ions produced at the section, 

 and therefore the ionization must be the same across every 

 section of the beam, that is, the inverse square law must be 

 obeyed. 



The pulse theory, on the other hand, could be satisfied 

 by any law, for although the intensity of the rays is given bv 

 the inverse square of the distance, there is no reason why the 

 numbers of ions produced should be proportional to the 

 intensity, though they may be. 



The result of the experiments to be described was that no 

 variation, other than errors of experiment, from the inverse 

 square law could be detected. 



The above argument shows that this result, which mioht 

 have been destructive to the corpuscular theory, is indecisive ; 

 but reasons will be given later tor believing that it is in some 

 respects adverse to the pulse theory. 



Arrangement of Apparatus. 



At the outset it was resolved to measure the ionization at 

 two distances from the anode, one about four times the other • 

 the longer distance being about 100 cm. The experimental 

 arrangements were devised to produce a beam of rays of 



* Communicated by Trof. J. S. Townscnd. 



