E. M. Wellisch — Mobilities of Ions in Air. 589 



Use was also made of the electrostatic voltmeter to ascertain 

 that the conditions in operation admitted of an effectively 

 instantaneous establishment of the withdrawing potential -Y 2 

 through the resistance R. It was found that with the highest 

 speed of commutation employed the value R = 200,000 was 

 sufficiently small to ensure exact mobility determinations in 

 practically all measurements. R was chosen to be 50,000 ohms, 

 so that a large factor of safety was provided for. 



The switches S, f and g were manipulated when observations 

 were being taken in such a way that the electrode e was prac- 

 tically at zero potential when the (advancing) potential (%) was 

 established on the gauze; no induction correction was there- 

 fore necessary. 



For large current values readings were taken with the capaci- 

 ties B and C added to the electrometer system ; the capacity 

 was then increased 17^ times. 



Throughout the experiment the potential V 2 was always 

 chosen considerably greater numerically than x in order to 

 ensure the complete withdrawal from the field of any ions 

 which failed to reach the electrode e during an alternation. 



The temperature was throughout approximately 20° C. 



3. Experimental Results. 



For each pressure employed, a complete curve was in general 

 obtained expressing the current registered by the electrometer 

 as a function of the "advancing" potential x. The alterna- 

 tions of potential assumed by the gauze were so extremely 

 rapid that no sign of unsteadiness on the part of the elec- 

 trometer needle was noticeable, the current being measurable 

 with the same ease as if produced by a direct field. 



As the pressure of the air was reduced, there was relatively 

 little reduction in the value of the ionization current passing 

 between the gauze and the electrode e ; the diminution of the 

 ionization produced in the region above the gauze at the lower 

 pressure was to a large extent compensated by the increased 

 rate at which the ions diffused through the gauze. The cur- 

 rent, for example, was 5'32 in arbitrary units when the pressure 

 was 121*5 mm ; under otherwise identical conditions, the current 

 was 2*28 at a pressure of 8'09 mm . At the lowest pressures 

 employed when the mean free path of the ion was of the order 

 of the distance between the meshes of the gauze the ionization 

 current decreased rapidly with diminishing pressure. 



In iig. 4, there are shown typical curves giving the relation 

 between the electric current and the " advancing " potential 

 under various conditions. These curves were produced to cut 

 the axis of potential and in this way the critical potential Y 



