Mobility of the Negative Ion. 447 



A sample of the results of these experiments is shown in 

 Table I., where p is the pressure of air under the bell-jar, 

 v the potential in volts applied to the plate A, and c and P 

 are given in arbitrary units. 



Table I. 



^ = 600 mm., d=2Q mm. 



V 



negative. 



c. 



P. 



~-k 



V 



positive. 



c. 



P. 



~-k 



p— A ( + ) . 



280 

 400 



600 



1000 



>> 



18 

 45 

 20 

 66 

 102 

 28 



11 

 27 

 12 

 41 

 63 

 17 



1-64 

 1-66 

 1-66 

 161 

 1-62 

 1-65 



320 

 400 

 600 

 800 

 1000 



26 

 34 

 75 

 71 

 66 

 31 



22 

 28 

 65 

 60 

 56 

 26 



1-18 

 1-22 

 1-16 

 1-18 

 118 

 119 



Mean yalue of k 1*64 



Mean value of k 1*19 



h~) 1#64 



H+) 



-HS- 1 *- 



It 



is seen 



from the Table that p is constant within the 



limits of experimental error and that the value of •£-* = 1*38 



<+> 

 is in good agreement with the well-known ratio between the 



mobilities of the negative and the positive ion in air. 



Experiments of this kind, together with some other control 



experiments, which will be described later, prove the efficiency 



of the method and show that the results are reliable. 



It is easy to see that the wind-pressure produced by the 



stream of gas can be measured by the gauge only in arbitrary 



units and, consequently, the absolute value of the mobility 



cannot be deduced from equation (1). This, however, is not 



necessary for the purpose of this work, as in the results given 



below the abnormal mobility of the negative ion is expressed 



in terms of its normal mobility — a constant determined to a 



high degree of accuracy by other methods. 



Results. 



w 

 1. The mobility of an ion ~ , as measured in arbitrary 



c 

 units by the ratio p , is constant over a certain range of the 



