Prof. J. J. Thomson on Cathode Rays. 315 



Velocity of the Cathode Rays. 



The velocity of the cathode rays is variable, depending upon 

 the potential-difference between the cathode and anode, which 

 is a function of the pressure of the gas — the velocity increases 

 as the exhaustion improves ; the measurements given above 

 show, however, that at all the pressures at which experiments 

 were made the velocity exceeded 10* cm./sec. This velocity is 

 much greater than the value 2x10 which I previously 

 obtained (Phil. Mag. Oct. 1894) by measuring directly the 

 interval which separated the appearance of luminosity at two 

 places on the Avails of the tube situated at different distances 

 from the cathode. 



In my earlier experiments the pressure was higher than 

 in the experiments described in this paper, so that the velocity 

 of the cathode rays would on this account be less. The 

 difference between the two results is, however, too great to 

 be wholly explained in this way, and I attribute the difference 

 to the glass requiring to be bombarded by the rays for a 

 finite time before becoming phosphorescent, this time 

 depending upon the intensity of the bombardment. As this 

 time diriiinishes with the intensity of bombardment, the 

 appearance of phosphorescence at the piece of glass most 

 removed from the cathode would be delayed beyond the time 

 taken for the rays to pass from one place to the other by the 

 difference in time taken by the glass to become luminous ; the 

 apparent velocity measured in this way would thus be less 

 than the true velocity. In the former experiments endeavours 

 were made to diminish this effect by making the rays strike 

 the glass at the greater distance from the cathode less 

 obliquely than they struck the glass nearer to the cathode; the 

 obliquity was adjusted until the brightness of the phospho- 

 rescence was approximately equal in the two cases. In view, 

 however, of the discrepancy between the results obtained in 

 this way and those obtained by the later method, I think that 

 it was not successful in eliminating the lag caused by the 

 finite time required by the gas to light up. 



Experiments ivith Electrodes of Different Materials. 



In the experiments described in this paper the electrodes 

 were generally made of aluminium. Some experiments, 

 however, were made with iron and platinum electrodes. 



Though the value of m/e came out the same whatever the 

 material of the electrode, the appearance of the discharge 

 varied greatly; and as the measurements showed, the potential- 



