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



elaboration and the experiments are being continued in other 

 directions. 



2. Experimental Method and Procedure. 



Method. — The method employed to determine the mobili- 

 ties of the ions was that devised by Franck and Polil.* In 

 this method the ions are produced in the region above a gauze 



Fig. 1. 



To M c .LE0D GAUGE 

 & TOPLE.R PUMP 



to P 2 3 , CaCl 2) 



ID DUST FILTERS 



and under the action of a steady field are drawn to the gauze ; 

 some of the ions diffuse through the meshes and come under 

 the influence of an alternating electric field which is established 

 between the gauze and a parallel electrode connected to an 

 electrometer. If the frequency of alternation of the field be 

 kept constant and the current which is conveyed to the elec- 

 trometer be determined for different values of the alternating 

 held, the mobility of the ion under the given conditions can in 

 general be estimated by means of certain formulae given later. 

 Vessel. — The vessel employed in these experiments (fig. 1) 

 consisted of a brass cylinder (10 cnas in diameter) divided into 

 two compartments by a brass partition containing a circular 



* Franck and Pohl, Verh. Dents. Phys. Ges., ix, p. 69, 1907. 



