Nolan and Harris — lonisation in Moist and Dry Air. 



41 



ing in steepness. The ion of mobility 2'5 was present, and that of mobility 

 3 had appeared. The ion of mobility 2 was found, but on the fifth day 

 of drying no slower ion could be found with certainty. At this stage the 

 work was interrupted for eleven days. On resuming, it was at once evident 

 that a great increase in the ionisation had occurred. An effect appeared 

 also which had not been noticed before. With the accelerating voltage on 

 the upper system, and no field between the perforated plate and the electro- 

 meter plate, a considerable current passed down to the latter. We spent a 



200 



Fig. 5. 



Ionis;ition curves in dry air, alternating-field method. Two negative curves show no 

 ion of njobility less than 2'5. Positive curves show normal ions. 



good deal of time in satisfying ourselves that this current was due to the 

 diffusion downward under their mutual repulsion of ions which had come 

 through the perforated plate. Some samples of the curves now obtained are 

 given in figs. 5 and 6. It may be said at once that they differ very materially 

 from the curves obtained with moist air. Fig. 5 covers roughly the same 

 range as fig. 4, and fig. 6 includes the range corresponding to the fast ions 



