624 Dr. G. C. Simpson on the Wilson- Gerdien 



every negative ion has water deposited on it when it reaches 

 the condensation layer, and is retained there, while every 

 positive ion passes through. In this way we shall find the 

 absolute maximum rate at which the charge at the condensation 

 layer can increase with naturally ionized air. 

 Let the upward velocity of the air 



=V cm. sec. 



Then the vol. of air which crosses the sq. cm. under con- 

 sideration in one second 



= Vc.c. 



The time taken for a given small vol. of air to pass from 

 the cloud to the condensation layer, 2,500 metres above 



2-5 Xl0 5 

 = ^ sec. 



The number of ions of each sign generated in a ex. in 

 this time 



25x2-5Xl0 5 6-2xl0 G 



The total number of negative ions which are retained at 

 each sq. cm. of the condensation layer each second 



6-2 xlO 6 T7 , „ 1A6 

 = y — x ^ = 6*2 x 10 6 . 



Now the charge on 3 X 1 9 ions is equal to 1 electrostatic 



unit of electricitv. 

 Hence the maximum rate at which the charge would grow 



on the condensation layer 



6* 9 x 10 6 



= 2 x 10" 3 els. unit 



per sq. cm. per sec. 



It is interesting to note that this maximum charge possible- 

 is quite independent of the upward velocity of the air, and 

 therefore, from ordinary considerations, independent of the 

 rate of rainfall. This does not mean, however, that if the 

 process as here sketched actually takes place in nature the 

 rate of separation of the electricity would be independent of' 

 these factors ; for we have neglected the rate of recombina- 

 tion of the ions, which would be considerable if the rate of 

 rise were so small that it took an appreciable time for the air 

 to pass through the supersaturated stratum ; all factors, how- 

 ever, which we have neglected could only reduce, and not. 

 increase, the charge separated at the condensation layer. 



