298 Prof. J. J. Thomson on Cathode Rajs. 



Conductivity of a Gas through which Cathode Rays 

 are passing. 



The conductivity of the gas was investigated by means of the 

 apparatus shown in fig. 2. The upper plate D was connected 

 with one terminal of a battery of small storage-cells, the other 

 terminal of which was connected with the earth ; the oilier 

 plate E was connected with one of the coatings of a condenser 

 of one microfarad capacity, the other coating of which was to 

 earth; one pair of quadrants of an electrometer was also 

 connected with E, the other pair of quadrants being to earth. 

 When the cathode rays are passing between the plates the 

 two pairs of quadrants of the electrometer are first connected 

 with each other, and then the connexion between them was 

 broken. If the space between the plates were a non-conductor, 

 the potential of the pair of quadrants not connected with the 

 earth would remain zero and the needle of the electrometer 

 would not move ; if, however, the space between the plates 

 were a conductor, then the potential of the lower plate would 

 approach that of the upper, and the needle of the electrometer 

 would be deflected. There is always a deflexion of the elec- 

 trometer, showing that a current passes between the plates. 

 The magnitude of the current depends very greatly upon the 

 pressure of the gas; so much so, indeed, that it is difficult to 

 obtain consistent readings in consequence of the changes 

 which always occur in the pressure when the discharge 

 passes through the tube. 



We shall first take the case when the pressure is only just 

 low enough to allow the phosphorescent patch to appear at 

 the end of the tube ; in this case the relation between the 

 current between the plates and the initial difference of poten- 

 tial is represented by the curve shown in fig. 3. In this 



Fig. 3. ___^ 



figure the abscissas represent the initial difference of potential 

 between the plates, each division representing two volts, and 

 the ordinates the rise in potential of the lower plate in one 

 minute each division again representing two volts. The 

 quantity of electricity which has passed between the plates in 



