Gases at Low Presures. 



795 



the critical value of e from which the mobility for that 

 pressure was determined. 



In some later experiments the alternating field was pro- 

 duced by means of the commutator shown in fig. 3. The 

 shaded portions represent metal and the unshaded portions 

 insulation. Brushes in contact at A and B were connected 

 to an insulated source of potential. Two more brushes were 

 placed at C and D at a distance apart equal to that between 

 the centres of consecutive metal strips, so that when A was in 



Fig:. 3. 



connexion with C, B would be in connexion with D. One of 

 these, say C, was connected to earth and the other, D, to 

 the potentiometer R (fig. 1), the other end of the latter being- 

 earthed. The commutator was driven by an electric motor and 

 the speed regulated by adjusting the gear and current. Such 

 an arrangement gave a curve of potential thus (fig. 4) : — 



Fig. 4. 



X 



The points m, 



were equidistant. 



