Prof. J. J. Thomson on Cathode Rays. 299 



one minute is the quantity required to raise 1 microfarad to 

 the potential-difference shown by the curve. The upper and 

 lower curve relates to the case when the upper plate is con- 

 nected with the negative and positive pole respectively of the 

 battery. 



Even when there is no initial difference of potential between 

 the plates the lower plate acquires a negative charge from 

 the impact on it of some of the cathode rays. 



We see from the curve that the current between the plates 

 soon reaches a value where it is only slightly affected by an 

 increase in the potential-difference between the plates ; this is 

 a feature common to conduction through gases traversed by 

 Rontgen rays, by uranium rays, by ultra-violet light, and, as 

 we now see, by cathode rays. The rate of leak is not greatly 

 different whether the upper plate be initially positively or 

 negatively electrified. 



The current between the plates only lasts for a short time ; 

 it ceases long before the potential of the lower plate ap- 

 proaches that of the upper. Thus, for example, when the 

 potential of the upper plate was about 400 volts above that of 

 the earth, the potential of the lower plate never rose above 

 6 volts: similarly, if the upper plate were connected with the 

 negative pole of the battery, the fall in potential of the lower 

 plate was very small in comparison with the potential-difference 

 between the upper plate and the earth. 



These results are what we should expect if the gas between 

 the plates and the plug B (fig. 2) were a very much better 

 conductor than ihe gas between the plates, for the lower 

 plate will be in a steady state when the current coming to it 

 from the upper plate is equal to the current going from it to 

 the plug: now if the conductivity of the gas between the 

 plate and the plug is much greater than that between the 

 plates, a small difference of potential between the lower plate 

 and the plug will be consistent with a large potential-difference 

 between the plates. 



fco far we have been considering the case when the pressure 

 is as high as is consistent with the cathode rays reaching the 

 end of the tube ; we shall now go to the other extreme and 

 consider the case when the pressure is as low as is consistent 

 with the passage of a discharge through the bulb. In this 

 case, when the plates are not connected with the battery we 

 get a negative charge communicated to the lower plate, but 

 only very slowly in comparison with the effect in the previous 

 case. When the upper plate is connected with the negative 

 pole of a battery, this current to the lower plate is only 

 slightly increased even when the difference of potential is as 

 much as 400 volts : a small potential-difference of about 



