586 



E. M. Wellisch — Mohilities of Ions in Air. 



(K in diagram) ; between the commutator and V, was a poten- 

 tiometer device consisting of a tray of cells (v) joined to the 

 terminals of a Wolff potentiometer (r). In the steady state 

 when the brushes are not in metallic contact with the commu- 

 tator the potential of K is — Y 2 ; when the brushes are in con- 

 tact with the disc the potential of K is readily deducible by an 



Fig. 2. 



^MM/WWWWWWV^A 



application of Kirehhoff's laws ; if x denote the potential at K 

 we deduce 



V 2 + R 



vp + r(V 1 + V 3 ) 

 P r + R?- — p* 



0) 



When the commutator is in action the potential of K should 

 vary sensibly between x and -Y 2 provided we maintain satis- 

 factory relations between the frequency of commutation and 

 the resistance R. We shall revert to this point later on, but 

 at present the assumption will be made that the potential of 

 the gauze is given by x and — Y 2 alternately, the former po- 

 tential lasting for a fraction f of the total time. This fraction 

 can be determined experimentally. Under these conditions it 

 is easy to deduce the expression for the mobility (k) of the ion 

 under consideration after a series of experiments has afforded 

 the value of the critical potential (V ) which is just sufficient 

 to bring the ions over to the electrode e. If n be the number 

 of complete alternations per second and d the distance between 

 the gauze and the electrode, we obtain 



h 



cVn 



/y 



4?i 



for d = 2. 



