6 Wellisch — Motion of Ions and Electrons through Gases. 



frequency commutator the value R = 200,000 afforded a read- 

 ing greater than the normal bj less than 2 per cent. Inasmuch 

 as the potential (V^^) ^^^ always chosen considerably greater 

 numerically than the advancing potential {x) the value K = 

 200,000 was sufficient to ensure the reahzation of the desired 

 conditions. In the present series of experiments this value of 

 K was chosen in preference to a smaller value because in the 

 determination of electron velocities Y^ is often small and it is 

 advisable to have c in formula (2) small compared with Y^. 



Manipulation of Sicitches. 



In general, when the gauze is raised to any potential, the 

 electrode e is raised by induction to a potential which has to 

 be taken into consideration when the electric field is estimated. 

 It was found possible, however, by a suitable manipulation of 

 the switches S, /* and g to arrange that the electrode e was 

 practically at zero potential when the potential (x) had been 

 established on the gauze, so that no correction for induction 

 was necessary. The series of operations involved in taking a 

 single reading was as follows : — 



(i) potentiometer (b) fixed at a convenient value so that the elec- 

 trometer needle should have a suitable range of deflection : k 

 closed : S closed on b side : / and g both closed : earth key K 

 open : motor and commutator running but not operating on 

 account of the short-circuit at g : capacities B and C included in 

 the system. 



(ii) k opened : S switched to the right, 

 (iii) g opened, if it is desired to work with added capacity, 

 or 



(iii) capacities B and C cut out and g then opened, if it is desired 

 to work without added capacity. 



It will be seen from theforegoingthat the effect of induction 

 was to alter only the reversed field whose value had not to be 

 known at all accurately. The electrometer needle experienced 

 always a small kick when the switch g was opened but this 

 quickly subsided and the current w^as measured with the needle 

 jn steady motion, the midpoint of the range of deflections 

 being so chosen as to coincide with the zero of the instrument. 



4. Experimental Results. 



(A). jElectro7is in Gases. 



In fig. 4 of the previous paper typical curves were given 

 showing the relation between the current due either to positive 

 or negative ions and the potential {x) for various pressures ; 

 from such curves the critical potential Y^ could be deduced 

 and the ionic mobility determined. In figs. 5 and 7 of the 



