﻿Mr. Normau Campbell on Delta Rays. 61 



with those oE the work described in this and the previous 

 paper. 



15. Some explanation is necessary why no attempt has 

 been made to use the magnetic method of determining the 

 velocity of the rays which has been used by Ewers and 

 Hauser (loc. cit.). The reason is that the examination of 

 the work o£ those authors, together with a few observations 

 which 1 have made, have led me to a complete mistrust of 

 that method as a means of getting accurate results. 



The theory of the method is given very completely by 

 J. J. Thomson in his ' Conduction of Electricity through 

 Gases' (2nd ed.) sections 57, 58*. The rays are supposed 

 to be emitted from one or both of two parallel plates, in the 

 space between which a magnetic field H is established with 

 its lines parallel to the plane of the plates. The measure- 

 ments can be made in two ways. In the first a magnetic 

 field is established strong enough to prevent the rays 

 crossing between the electrodes when they are at the same 

 potential difference, and then the potential difference is 

 increased and the value of the electric field found necessary 

 to make, first some, and then all, of the rays cross. If X x 

 and X 2 are the two values of the field corresponding to these 

 two conditions, and v is the velocity of the rays 



1 X 2 — Xj , Xi+Xo 



»- 2— a- ; e/m = -TRT- 



In the second method an electric field X, strong enouoh to 

 make all the rays cross between the electrodes, is established, 

 and then H is increased from zero until first some and then 

 all of the rays are prevented from crossing. If H x and H 2 

 are the two values of the field corresponding to these two 

 conditions 



X^-HO , 2X . 



; ejm- 



It is clear that this method must fail unless all the ravs 

 have the same speed, or unless this speed is so small 

 compared with X/H that it may be put equal to zero. For 

 the values of Xj and H 2 are determined by the speed of the 

 fastest rays, whereas X 2 and H T are determined by the speed 

 of the slowest rays. Since then there is no evidence that 

 the rays are homogeneous, considerable doubt attaches to 

 the method from the start. 



* I am unable to follow completely the methods by which the authors 

 named interpret their results. The conclusion given here, based on 

 Thomson's theory, is rather different from that at which they arrive, 



