Nolan — Ionic MoUUties in Air and Hydrogen. 



77 



critical voltages determined. The curves reproduced in the second paper 

 show the manner of working. That, on allowing for the general ionization, 

 Blackwood should have found indications only of ions of normal mobility is 

 what might have been anticipated. The ions of high mobility are present in 

 small numbers, and show up only as " nicks" on current-voltage curves very 

 carefully plotted. The bulk of tlie ions have mobilities lying between 2 and 

 1'35. In Blackwood's experimental determination and separation of the 

 current due to the general ionization from that due to the localized radiation, 

 the small number of high mobility ions are obscured, and the others yield 

 only a mean value. Ions of mobilities lying so close together will not be 

 separated by any method depending on variation of current with voltage 

 unless very accurate current readings are taken, and very small increments 

 in voltage used. The results given by the author's original method have 

 since, of course, been amply supported by the evidence obtained from the 

 Eutherford-Franck alternating-field method, and are extended in the present 

 paper to ionization in hydrogen. In addition it is now proposed to give some 

 evidence obtained by a somewhat different alternating-field method which 

 was referred to but not described in the original paper.' 



Alternating-Field Method ivith Variable Periodic Time. 



The method employed will be clear from the diagram (fig. 1). Ions are 

 produced by a source of a-rays in the space above a sheet of metal gauze 



^v 



-qy-^- 



f- 



? p 



R 



AAA/VW 



Fig. 1. 



through which ions of one sign are caused to diffuse by a steady field. Below 

 the gauze and parallel to it is a metal plate insulated, furnished with a 



Proc. Royal Irish Acjideiuy, J 920, p. 43. 



