428 Sir J. J. Thomson on 



if the electric force between the plates remained constant. 

 Thus we may conclude that the cathode rays at any rate 

 do not produce enough ionization to affect the electric field 

 between the plates. 



The most direct proof that the positive rays do not affect 

 this field is given by the following method. The charged 

 ions in a gas do not modify the electric field when this is 

 strong enough to send the saturation current through the 

 gas, for in this case the ions are removed as fast as they are 

 formed and have not time to congregate and affect the electric 

 field. Hence, if we prove that the potential -difference 

 between the plates used to produce the electrostatic deflexion 

 was sufficient to produce the saturation current through the 

 gas, we are safe in assuming that the electric field between 

 the plates is uniform. 



I therefore measured the current sent through the gas 

 ionized by the positive rays when the potential-difference 

 applied to the plates was the same, 100 volts and upwards, 

 as in the experiments made to determine the electrostatic 

 deflexion of the rays. 



To do this, one of the parallel plates was connected with 

 the leaf in a gold-leaf electroscope, and initially was con- 

 nected with the earth, the other plate was connected with 

 •one of the terminals of a large battery of storage-cells the 

 other terminal of which was earthed. The current between 

 the plates was measured by the movement of the gold-leaf 

 when its connexion with the earth was broken. It was found 

 that there was a very considerable current through the gas 

 ionized by the positive rays, so that to get measurable de- 

 flexions it was necessary to connect a small leyden-jar with 

 the electroscope so as to increase its capacity. The current 

 varied very greatly with the pressure: it was, which is im- 

 portant, very small when the pressure was so high as to 

 reduce the potential-difference between the terminals in the 

 discharge-tube to 3000 volts, it reached a maximum when 

 the pressure was quite low, and rapidly fell off as the pressure 

 was still further reduced. The results given in the following- 

 table show that even when the ionization was greatest the 

 current through the gas was practically the same when the 

 potential- difference between the plates was 100 volts as when 

 it was 200; so that the current was saturated by the field 

 used to deflect the Ciinalstrahlen : hence the electric field 

 between the plates must have been uniform and the electric 

 force equal to the potential-difference divided b}^ the distance 

 between the plates. 



