590 



E. M. Wellisch — Mobilities of Ions in Air 



was determined ; e. g., with p = 121*5 mm we get V = 46*0 

 volts. The speed counter indicated 10,000 revolutions in 229*2 

 seconds and /was exactly equal to \. The mobility of the 



positive ion is given by k x = -=- = 7 # o9 cm/sec at a pressure 



J 



of 121*5 mm . Assuming pJc = constant, we deduce K = 1/213 

 as the mobility corresponding to a pressure of T60 mm . 



Fig. 4. 



100 

 Volts 



At the higher pressures, the curves obtained were all of the 

 type shown "in fig. 4; but the curve obtained in the case of the 

 negative ion at lower pressures was of an essentially different 

 character, especially when the rapid alternations of commuta- 

 tor 2 were employed. Such a curve is shown in fig. 5, corre- 

 sponding to a pressure of 12*22 mm ; this curve is typical of many 

 others which were obtained under various conditions. The 

 curve is readily seen to be compounded of two simple curves, 

 e. g., the curve E, which is obtained by a free prolongation of 

 the initial part of the experimental curve, when compounded 

 with the curve I is equivalent to the original curve. That the 

 curve E is at least in part due to electrons is shown by the fact 

 that it cuts the potential axis so near to the origin, in other 



120 



