Ions and Electrons through Gases, 35 



communication with the case of the vessel. The lower com- 

 partment contained a gauze electrode insulated by a thin 

 ebonite ring from the partition. Two centimetres below the 

 gauze was the electrode e connected to the electrometer : this 

 electrode was surrounded by a guard-screen (W) connected 

 to earth by means of a guard tube. 



Ficr. 1. 



vwwwyw ' 



Hi]iiiir:::i.:.,jiiiii*-n 



AAAA/WWsAA/WWWW- 



to GAUZE 



Fig. 1 illustrates the method emploj^ed to effect the com- 

 mutation of potential. The commutating disks were of brass 

 wdth a number of fibre segments of equal width placed at 

 regular intervals along the periphery. The two potentials 

 V*! and — V 2 were connected across the terminals of a large 

 metal resistance R, in series with the commutator ; it was 

 not in general convenient to alter the potential V 1 except in 

 steps of 40 volts each, and on this account the potentiometer 

 device (v, r, p) was employed to effect finer gradations of 

 potential. 



g^fWhen the commutator is in action the potential of K (fig. 1) 

 should alternate between x and — "V 2 , where 



v vp + rjVt + Y!) 



" V2 + K pr + lir-p'-- 



(1) 



Owing, however, to the time involved in the establishment of 

 potential, this formula will be sufficiently valid only if care 

 be taken to maintain a satisfactory relation between the 

 frequency of commutation and the resistance R. This was 

 effected by an experimental method described later. We 

 shall assume here that the potential of the gauze is given by 

 j& and — V 2 alternately, the former potential lasting for a 



D2 



