Prof. J. J. Thomson on Cathoch Rays. 303 



1 



2 



— v 2 = 

 e 



w 





v= 



2W 





m 



I 2 Q 





e 



2W 



Thus, if we know the values of Q, W, and I, we can deduce 

 the values of v and m/e. 



To measure these quantities, I have used tubes of three 

 different types. The first I tried is like that represented in 

 fig. 2, except that the plates E and D are absent, and two 

 coaxial cylinders are fastened to the end of the tube. The 

 rays from the cathode C fall on the metal plug B, which is 

 connected with the earth, and serves for the anode ; a hori- 

 zontal slit is cut in this plug. The cathode rays pass through 

 this slit, and then strike against the two coaxial cylinders at 

 the end of the tube ; slits are cut in these cylinders, so that 

 the cathode rays pass into the inside of the inner cylinder. 

 The outer cylinder is connected with the earth, the inner 

 cylinder, which is insulated from the outer one, is connected 

 with an electrometer, the deflexion of which measures Q, the 

 quantity of electricity brought into the inner cylinder by the 

 rays. A thermo-electric couple is placed behind the slit in 

 the inner cylinder ; this couple is made of very thin strips of 

 iron and copper fastened to very fine iron and copper wires. 

 These wires passed through the cylinders, being insulated 

 from them, and through the glass to the outside of the tube, 

 where they were connected with a low-resistance galvanometer, 

 the deflexion of which gave data for calculating the rise of 

 temperature of the junction produced by the impact against it 

 of the cathode rays. The strips of iron and copper were large 

 enough to ensure that every cathode ray which entered the 

 inner cylinder struck against the junction. In some of the 

 tubes the strips of iron and copper were placed end to end, 

 so that some of the rays struck against the iron, and others 

 against the copper ; in others, the strip of one metal was 

 placed in front of the other ; no difference, however, could 

 be detected between the results got with these two arrange- 

 ments. The strips of iron and copper were weighed, and the 

 thermal capacity of the junction calculated. In one set of 

 junctions this capacity was. 5 x 10 -3 , in another 3 x 10 -3 . If 

 we assume that the cathode rays which strike against the 

 junction give their energy up to it, the deflexion of the 

 galvanometer gives us W or ^Nmc 2 . 



