300 Prof. J. J. Thomson on Cathode Rays. 



20 volts seems slightly to decrease the rate of leak. Potential - 

 differences much exceeding 400 volts cannot be used, as 

 though the dielectric between the plates is able to sustain 

 them for some little time, yet after a time an intensely bright 

 arc flashes across between the plates and liberates so much 

 gas as to spoil the vacuum. The lines in the spectrum of this 

 glare are chiefly mercury lines ; its passage leaves very 

 peculiar markings on the aluminium plates. 



If the upper plate was charged positively, then the negative 

 charge communicated to the lower plate was diminished, and 

 stopped when the potential-difference between the plates was 

 about 20 volts ; but at the lowest pressure, however great (up 

 to 400 volts) the potential-difference, there was no leak of 

 positive electricity to the lower plate at all comparable with 

 the leak of negative electricity to this plate when the two 

 plates were disconnected from the battery. In fact at this 

 very low pressure all the facts are consistent with the view 

 that the effects are due to the negatively electrified particles 

 travelling along the cathode rays, the rest of the gas possessing 

 little conductivity. Some experiments were made with a 

 tube similar to that shown in fig. 2, with the exception that 

 the second plug B was absent, so that a much greater number 

 of cathode rays passed between the plates. When the upper 

 plate was connected with the positive pole of the battery a 

 luminous discharge with well-marked striations passed be- 

 tween the upper plate and the earth-connected plug through 

 which the cathode rays were streaming ; this occurred even 

 though the potential-difference between the plate and the 

 plug did not exceed 20 volts. Thus it seems that if we supply 

 cathode rays from an external source to the cathode a small 

 potential-difference is sufficient to produce the characteristic 

 discharge through a gas. 



Magnetic Deflexion of the Cathode Rays in Different Gases. 



The deflexion of the cathode rays by the magnetic field 

 was studied with the aid of the apparatus shown in fig. 4. 

 The cathode was placed in a side-tube fastened on to a bell- 

 jar ; the opening between this tube and the bell-jar was closed 

 by a metallic plug with a slit in it ; this plug was connected 

 with the earth and was used as the anode. The cathode rays 

 passed through the slit in this plug into the bell-jar, passing 

 in front of a vertical plate of glass ruled into small squares. 

 The bell-jar was placed between two large parallel coils 

 arranged as a Helmholtz galvanometer. The course of the 

 rays was determined by taking photographs of the bell-jar 



