304 Prof. J. J. Thomson on Cathode Rays. 



The value of I, i. e., Up, where p is the curvature of the 

 path of the rays in a magnetic field of strength H was found 

 as follows : — The tube was fixed between two large circular 

 coils placed parallel to each other, and separated by a distance 

 equal to the radius of either ; these coils produce a uniform 

 magnetic field, the strength of which is got by measuring 

 with an ammeter the strength of the current passing through 

 them. The cathode rays are thus in a uniform field, so that 

 their path is circular. Suppose that the rays, whea deflected 

 bv a magnet, strike against the glass of the tube at E 



H 



(fig. 5), then, if p is the radius of the circular path of the 

 rays, 



-> Lull . n 



thus, if we measure CE and AC we have the means of 

 determining the radius of curvature of the path of the 

 rays. 



The determination of p is rendered to some extent uncertain, 

 in consequence of the pencil of rays spreading out under the 

 action of the magnetic field, so that the phosphorescent patch 

 at E is several millimetres long ; thus values of p differing 

 appreciably from each other will be got by taking E at 

 different points of this phosphorescent patch. Part of this 

 patch was, however, generally considerably brighter than the 

 rest ; when this was the case, E was taken as the brightest 

 point ; when such a point of maximum brightness did not 

 exist, the middle of the patch was taken for E. The uncer- 

 tainty in the value of p thus introduced amounted sometimes 

 to about 20 per cent.; by this I mean that if we took E first 

 at one extremity of the patch and then at the other, we should 

 get values of p differing by this amount. 



The measurement of Q, the quantity of electricity which 

 enters the inner cylinder, is complicated by the cathode rays 

 making the gas through which they pass a conductor, so that 

 though the insulation of the inner cylinder was perfect when 

 the rays were off, it was not so when they were passing 

 through the space between the cylinders ; this caused some 

 of the charge communicated to the inner cylinder to leak 

 away so that the actual charge given to the cylinder by the 

 cathode ravs was larger than that indicated by the electrometer. 



