in Gases at Low P 



ressures. 



703 



Apparatus and Method. 



A diagrammatical representation of the apparatus is shown 

 in fig. 1. P was a carefully insulated metal plate surrounded 

 by a guard-ring and connected to a tilted electroscope. At 

 a distance of 5'0 cm. below P and parallel to it was a metal 

 gauze G which could be connected to a source of alternating 

 potential, of which the maximum could be altered as desired 

 by means of the potentiometer arrangement at R. This was 



Fig.]. 



I m 



■— AL| 



ih m 



a copper sulphate resistance with its ends connected to the 

 terminals of the alternating circuit, one of which was earthed. 

 In most of the observations this alternating circuit was ob- 

 tained from the town mains, but in some of the later obser- 

 vations the alternations of the field were produced by means 

 of a rotating commutator which will be described later. By 

 moving the copper disk d up or down in the solution, varying 

 alternating potentials were tapped off for the gauze G. 

 About a centimetre below the metal gauze were two thin 

 strips of platinum on which aluminium phosphate had been 

 deposited. These strips of platinum could be heated to a 

 desired temperature by passing a current through them from 

 an insulated battery of accumulators A, which was connected 

 to the positive terminal of the cells C, so that the strips were 

 always at a constant positive potential above the gauze G. 



