Mobility of the Positive Ion at Low Pressures. 165 



Wilson tilted electroscope. Below this electroscope plate 

 was a tier of ten parallel circular plates, a, a, ... , of 14 cm. 

 diameter, insulated and equidistant from each other. In the 

 centre of each there was a hole somewhat smaller than the 

 diameter of the electroscope plate. The hole in the lowest 

 plate was covered with a wire gauze G, below which, at a 

 distance of one or two centimetres, was a platinum strip P 

 covered with aluminium phosphate, which served as the 

 source of positive ions. The whole of this was enclosed in 

 a large bell-jar, which could be exhausted to the desired 

 pressures with a mercury pump and a " liquid-air-charcoal " 

 tube. From each plate a fine wire carefully insulated with 

 thin glass tubes and beads passed through the opening in 

 the bell-jar to the potentiometer L. This was an ebonite 

 tube three-quarters of a metre long containing water. It 

 was divided into ten equal parts, and at each division a wire 

 connected to its corresponding plate penetrated into the 

 water. One end of this potentiometer was earthed, while 

 the other was connected to a source of alternating potential. 

 The latter was sometimes obtained from the town mains and 

 sometimes from a specially designed commutator, described 

 fully in the author's previous paper (loc. cit.) on positive 

 ions. The maximum potential was varied by the same 

 contrivance as before, and will be sufficiently evident from 

 the diagram (D). 



In measuring mobilities by the alternating field method 

 it is essential that at every instant the field is uniform. The 

 object of the parallel plates and the potentiometer L was to 

 secure that condition. The distance between e and G was 

 21 cm., and there were ten plates 14 cm. in diameter with 

 central holes less than 4 cm. diameter. With such dimen- 

 sions it was reasonable to assume that the field at any instant 

 along the path of the ions from the gauze to the electroscope 

 plate was uniform. That it was so is supported by the 

 fact that the mobilities measured at the higher pressures 

 agreed with those obtained with two parallel plates near 

 together. 



The phosphate-covered strip was heated by the accumulators 

 A, and the temperature regulated by means of a variable 

 resistance to give a convenient number of positive ions to 

 work with. A small potential was maintained between P 

 and G to direct the ions to the gauze. 



The measurement of the mobility was made in the usual 

 way by plotting the current to the electroscope for various 

 maximum values of the alternating potential. If the latter 

 has a period T, and the distance between the gauze and the 



