796 Mr. G. W. Todd on the Mobility of Positive Ions 



It has been shown that for an ion just to reach the plate 

 of the electroscope 



T 



<-5.C 



2)- 



area noxz 

 % L 



see shaded portion above, 



u v t 



_ wE 

 """ W 

 where n is the number of alternations per second, 



_ 4% <P __ lOOn 



E E 



The mobility was obtained in the usual way by plotting 

 the current to the electroscope and the maximum alternating 

 potential, while the number of revolutions of the commutator 

 per second was maintained constant. The rate of rotation 

 was tested before and after every reading with a stopwatch. 

 With this commutator one could obtain up to 450 alternations 

 per second. 



Experiments. 



There was no difficulty in making observations at much 

 lower pressures than in former experiments. The stream of 

 positive ions could be regulated by altering the heating cur- 

 rent through the platinum strips to give any convenient rate 

 of deflexion of the electroscope leaf in any gas. Using the 

 town mains as the source of alternating potential mobilities 

 up to five or six thousand centimetres per second could be 

 measured, and with the more rapid alternations produced by 

 the commutator one could measure mobilities of fifty thousand 

 centimetres per second. As the electroscope plate required 

 about 0*1 volt to carry the leaf across the timed portion of 

 the scale, readings were not taken with alternating potentials 

 having a maximum of less than one volt. A fuither limit 

 was imposed on the observations by the increase in the mean 

 free path with diminution of pressure. When the distance 

 between the plates was less than about fifty free paths, 

 readings were not taken. 



In the first experiments a curious result was found. As 

 the pressure was lowered the mobilities increased inversely as 

 the pressure until about one millimetre was reached. At this 



