Nolan and Harris — lonisation in Moist and Dry Air. 48 



identical with that shown in the diagram, except that the distance between 

 the perforated plate and the electrometer plate was now 5 cms. instead of 

 1 cm. The constant Vu for the new apparatus has therefore the value 

 129 X 5" = 3,225. With this apparatus, after twenty-five days' drying, we 

 obtained discontinuities in the negative curve at voltages of 132, 260, 407, 

 and 568, corresponding to ions of mobilities 24'4, 12'4, 7'9, and 5'6. In 

 addition we observed indications of ions of mobilities roughly 16 and 11. 

 Similar results were obtained with the positive. These results are satis- 



FiG. 6. 

 lonisation curves in dry air, alternMting-field method. Negative and positive curves 

 show ions of mobilities 12-3, 8'2, 5-61, 4'39, 3-04, and ^-53. All negatives show 

 an ion of mobility 24 '6, and one positive curve shows the same ion. 



factory, as they show that no serious error was made in calculating mobilities 

 from the curves got by the other apparatus. The appearance of two new 

 ions of mobilities 16 and 11 has to be explained. There is no doubt that the 

 second apparatus is superior in resolving power to the first ; but this we 

 think is not the sole reason for the appearance of these new ions. In the 

 first apparatus the accelerating field was of the order of 5 volts/cm. ; with- 



