Wellisch — Motion of Ions and Electrons through Gases. 3 



{v. fig. 2) consisted of a brass cylinder divided into two com- 

 partments bj a brass partition containing^ a circular aperture. 

 In the upper compartment was a copper plug on which a layer 

 of polonium had been deposited ; great care was taken that the 

 radiation from the polonium was confined to the upper com- 

 partment. A circular electrode A was situated about 3"^^ 

 above the aperture and was in metallic communication with 

 the case of the vessel. The lower compartment contained a 

 gauze electrode insulated by a thin ebonite ring from the par- 

 tition. Two cm. below the gauze was the electrode e connected 

 to the electrometer; this electrode w^as surrounded by a guard 

 screen (W) connected to earth by means of a guard tube. 



Fig. 1 illustrates the method employed to effect the commu- 

 tation of potential.. The commutating discs were of brass with 

 a number of fiber segments of equal width placed at regular 

 intervals along the periphery. The two potentials Yj and —Y.^ 

 were connected across the terminals of a large metal resistance 

 R in series with the commutator ; it was not in general con- 

 venient to alter the potential Y^ 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. 



When the commutator is in action the potential of K (fig. 1) 

 should alternate between x and — Y^ where 



X = - V, + R «P + KV. + V.) (1) 



pr + Kr — p* 



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

 potential this formula will be sufificiently 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 x and — Y^ alter- 

 nately, the former potential lasting for a fraction/" of the total 

 time ; this fraction can be determined experimentally. Under 

 these conditions the mobility h of the ion under consideration 



is given hy k = -;^ , where n is the number of complete alter- 

 ed ' o 

 nations per second, d is the distance between the gauze and 

 the electrode 6, and Y^ is the critical potential, i. e. the value 

 of X which is just sufficient to enable the ions to reach the 

 electrode e before the field is reversed. 



The diagram of connections is exhibited in fig. 2. As in the 

 previous experiments two commutating discs were employed ; 

 one of these had 20 fiber segments while the circumference 

 of the other was half fiber and half metal. The motor was 



