[mclennan-keys] electrical CONDUCTIVITY 67 



A circumstance which seemed to point to diffusion exerting an 

 influence on the readings was a time effect which was observed es- 

 pecially when low fields were used. With fields below 60 volts a centi- 

 metre this was particularly marked and it occurred with both positive 

 and negative fields. Where such fields were applied after the various 

 parts of the apparatus had remained uncharged for some time, a 

 gradual diminution in the intensity of the current was always observed 

 in taking a series of successive readings. 



It required about half an hour to elapse after the field was applied 

 before the current reached a limiting value and the diminution in 

 current amounted to from 12 to 20%. With fields above 60 volts a 

 centimetre the effect gradually disappeared and when fields of 250 

 volts a centimetre and higher were applied no diminution in the 

 current was observed. This effect would seem to indicate that the 

 walls of the opening in the ebonite plate gradually became charged 

 by ions diffusing to them. One effect of this action would be to pro- 

 duce a partial neutralisation of the field and this in turn would account 

 for the falling off in the current. 



As this effect was absent when the higher fields were applied it 

 is not clear why such small values were obtained for the mobilities. 

 It may be added that the readings given in Talkie VI were all taken 

 immediately after the application of the field. 



It was the intention of the writers to use the apparatus to de- 

 termine the mobilities of the ions produced in liquid air by alpha rays, 

 but in view of the small values obtained with it for k^ and k, in air 

 at the particular pressure of 116 atmospheres it seemed advisable 

 to defer these measurements until more extended observations were 

 made on the mobilities of the ions in air at a series of high pressures. 

 The polonium coated plate used above was of rather small dimensions, 

 but it was all that was available at the time the experiments 

 were made. With a larger plate the effects of diffusion should be 

 lessened. 



The experiments made so far show that there should be no great 

 difficulty in finding the mobilities of ions produced in air up to and even 

 beyond 150 atmospheres. The difficulty in determining the mobili- 

 ties in liquid air will be largely one of getting rid of the formation of 

 air bubbles in the field and of preventing the liquid air from becoming 

 contaminated with ice crystals. 



In order to ascertain the mobilities of the ions in liquid air and 

 over as large a range of pressures as possible, and especially to clear 

 up any doubt regarding the results obtained in the present experi- 

 ments on the mobilities at 116 atmospheres, the investigation is being 

 continued with a polonium coated plate of larger area. 



Sec III, 1913—4 



