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



Thus when the platinum was heated a constant stream of 

 positive ions was directed to the gauze and into the alter- 

 nating field between it and the electroscope plate P. 



If u = velocity of the ion per volt per centimetre, and 

 d = distance between the gauze and the electroscope plate, 

 then, i£ at any instant the gauze is at a potential E, the 

 velocity of the ion at that instant will be uE/d. 



Let the potential E = <j)(t) where <j>(t) is a periodic function 

 of the time. 



For an ion to reach the electroscope plate we must have 



<-jC 



2 (f> (t) . dt, 



where T is the period of alternation of the electric field. 

 In most of the experiments E = E sin 27r_ ; 



hence 



?/ 



e/i 



or 



d 7T 



_ird 2 



? '"~E r 



The voltmeter gives the 

 ting potential, hence, if the instantaneous value E = E sinp£, 

 then E = \/2.e. No correction was made for induction 

 between the plates,, the correction amounting to less than 

 one per cent. The distance between the electroscope plate 

 and the gauze was d = 5'0 cm., and the number of alter- 

 nations per second was 90, so that the mobility is given by 



5010 

 u = . 



e 



In making a determination of the mobility the pressure 

 was brought down to required dimensions by means of a 

 mercury pump and read on a McLeod gauge. The electro- 

 scope was then insulated and the alternating potential 

 switched on. If this potential was more than sufficient to 

 make the ions getting through the gauze travel the distance 

 d in T/2 the leaf would move across the scale of the electro- 

 scope. With the maximum alternating potential less than a 

 certain critical value there would be no charge communicated 

 to the electroscope. At each pressure a curve was drawn 

 showing the relation between the current communicated to 

 the electroscope and the effective value e of the alternating- 

 potential, and where this curve cut the axis (P ; fig. 2) gave 



