588 KM. Wellisch — Mobilities of Ions in Air. 



ings. The motor maintained a very constant speed and the 

 error in the determination of the frequency was less than 1 per 

 cent. 



The double-throw switch f in fig 3 was employed to throw 

 either commutator into action ; g was employed to short cir- 

 cuit either commutator. The double-pole, double-throw switch 

 S, when thrown to the right, completed the connections as 

 exhibited graphically in fig. 2. When thrown to the left, con- 

 nection was made with a subsidiary potentiometer system (5); 

 it will be seen from fig. 3 that in this position the quadrants of 

 the electrometer can be commutated in potential between zero 

 and any convenient potential read off on the potentiometer. 

 This device afforded a thoroughly reliable indication as to the 

 nature of the contact between the brushes and the commutator ; 

 excellent contact was indicated by the electrometer needle 

 assuming a definite position, and the slightest irregularity in 

 contact was shown by a corresponding unsteadiness of the 

 needle. Moreover, this device was employed to ascertain the 

 value of y, the fractional duration of contact ; f is obviously 

 the ratio of the potential assumed by h when the switch g is in 

 turn opened and closed, ywas of course, approximately \ in 

 most cases but owing to wear in the brushes was often slightly 

 greater, especially when commutator 2 was employed. A third 

 use of the potentiometer device was to enable the electrometer 

 needle to be so adjusted in taking observations that the zero of 

 the instrument was also the midpoint of the range of deflec- 

 tions over which the current was estimated ; this point is of 

 especial importance when the gauze was at a small potential. 



For convenience in manipulation, a table was prepared of the 

 potentials assumed by the gauze for different values of /?, V l5 

 Y 2 and R. v was always chosen equal to 40 volts and r was 

 always 15,000 ohms. This was effected by means of formula 

 (1) which for the purpose was put in the following form : 



= V 1 +c(say) (2) 



The calculated values of c for various values of p and Y, + Y 2 

 were then tabulated and the value of x under any desired con- 

 ditions could be quickly obtained. 



The correctness of the results furnished by this formula was 

 experimentally verified by reading directly the potentials on a 

 Kelvin multicellular electrostatic voltmeter ; the smaller poten- 

 tials (below 2 volts) were read off directly on the electrometer, 

 which had a sensibility of 180 mm per volt. In all the instances 

 employed the agreement was excellent. 



