302 Prof. J. J. Thomson on Cathode Rays. 



deflexion of the rays produced by a magnet diminishes, or at 

 any rate the deflexion of the rays when the phosphorescence 

 is a maximum diminishes. If an air-break is inserted an 

 effect of the same kind is produced. 



In the experiments with different gases, the pressures were 

 as high as was consistent with the appearance of the phos- 

 phorescence on the glass, so as to ensure having as much as 

 possible of the gas under consideration in the tube. 



As the cathode rays carry a charge of negative electricity, 

 are deflected by an electrostatic force as if they were negatively 

 electrified, and are acted on by a magnetic force in just the 

 way in which this force would act on a negatively electrified 

 body moving along the path of these rays, 1 can see no escape 

 from the conclusion that they are charges of negative elec- 

 tricity carried by particles of matter. The question next 

 arises, What are these particles ? are they atoms, or mole- 

 cules, or matter in a still finer state of subdivision ? To 

 throw some light on this point, I have made a series of 

 measurements of the ratio of the mass of these particles to the 

 charge carried by it. To determine this quantity, I have used 

 two independent methods. The first of these is as follows : — 

 Suppose we consider a bundle of homogeneous cathode rays. 

 Let m be the mass of each of the particles, e the charge carried 

 by it. Let N be the number of particles passing across any 

 section of the beam in a given time ; then Q the quantity of 

 electricity carried by these particles is given by the equation 



Ne=Q. 



We can measure Q if we receive the cathode rays in the 

 inside of a vessel connected with an electrometer. When 

 these rays strike against a solid body, the temperature of the 

 body is raised ; the kinetic energy of the moving particles 

 being converted into heat ; if we suppose that all this energy 

 is converted into heat, then if we measure the increase in the 

 temperature of a body of known thermal capacity caused by 

 the impact of these rays, we can determine W, the kinetic 

 energy of the particles, and if v is the velocity of the particles, 



lNmr 2 = W. 



If p is the radius of curvature of the path of these rays in a 

 uniform magnetic field II, then 



e r 



where I is written for Up for the sake of brevity. From 

 these equations we get 



