Mobility of the Negative Ion. 



459 



expected : The wind-pressure should at first slowly diminish 

 with increasing rate of alternations, and when the number of 

 alternations per second is large enough to prevent the normal 

 ions from reaching the grating, a sudden drop in the wind- 

 pressure should be observed. 



Fig. 8. 



900 



Alternations per Second, 



II 



The experimental results are, however, altogether different 

 from what might be expected if Wellisch's theory were true. 

 No drop can be observed in the wind-pressure when the 

 frequency of alternations reaches the critical value, which 

 decisively proves that the wind is produced only by one kind 



of ions whose mobility I as measured by the ratio -p J is 



intermediate between that of a normal ion and of a free 

 electron. The curve ( — ) (fig. 8) was drawn at 100 mm. 

 pressure, the distance between the plates being 3 cm. and 

 the p.d. between them 200 v. ; 900 alternations per second 

 is enough under these conditions to separate the normal ions 

 from the electrons ; the curve, however, shows that the 

 wind-pressure continues to diminish slowly with further 

 increase of the rate of alternation. The curve ( + ) shows 

 the wind-pressure under the same conditions, but in the 

 case when it is produced by positive ions. 



212 



