270 TME lONISATION OF THE AIR. 



C V 1 



> 



\ 



A^ 4 TT y I 5. 300 



C V 

 or ^i-l > 00018. 



A' 



(C and V are here expressed in electrostatic units, N is measured 

 in c.c. per sec. ; and v is therefore reduced to cms. per sec. per 

 electrostatic unit per cdl.m.) 



Similarly, if the apparatus is to collect all ions of the second 

 class, i.e., ions of average mobility, .038 cm. per sec. per volt 

 per cm., 



C V 1 1 



> 



N 4 TT X 038 300 



C V 

 or ^-1 > 007. 



And if the ions collected are to include the Langevin ions, 



C V 1 1 



> 



A' 4 TT X 00055 300 



or £-K > -48. 



A^ 



Now, in the case of the Ebert apparatus, C is 17 cms.; V is 

 of the order of fgg electrostatic units, and an average value 



C V 

 for iV is 1,150 c.c. per sec. This gives for —^ a value .0095- 



O V 

 In the case of the Gerdien apparatus -rp is about .22. 



A comparison between these numbers and those derived 

 above shows that results obtained with the two instruments are 

 not comparable. For whereas the Gerdien type of instrument 

 must collect some Langevin ions in addition to all others, the 

 Ebert certainly does not. 



The Ebert apparently collects all the small ions and all ions 

 of the second or intermediate class. Values obtained with the 

 Ebert for the number of ions per c.c. and their average mobility 

 otherwise confirm this. During a series of observations the 

 number of positive ions per c.c. varied between 1,134 and 1,800; 

 and the corresponding numbers for the negative ions between 

 1,157 ^^'^^ 1. 68 1. The ratio of the number of positive to the 

 number of negative ions varied between the limits 1.08 and 1.02, 

 and was therefore fairly constant. The values of the mobilities 

 of ions of either sign varied between .058 and .736. i\lso, 

 when the number of ions per c.c. was small, the mobility was 

 high ; while the mobility diminished as the number of ions 

 increased. Now .058 and .736, the limits for the observed 

 mobilities, are values intermediate between .038 and 1.5, the 



