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



(ii.) We will now consider the charge which rain formed 

 in the condensation layer could carry to the earth with it. 

 In order to treat again of the maximum effect possible we 

 will assume that the whole water deposited in the condensa- 

 tion layer is deposited on the negative ions, and that every 

 negative ion becomes the nucleus of a drop. 



We will consider a cubic metre of air rising from the 

 lower cloud to the condensation layer, where it deposits its- 

 water vapour on the negative ions which it contains. 



We can accurately calculate the amount of water deposited 

 under these conditions from a cubic metre of air. Let us 

 assume that the temperature of the air on the ground is- 

 25° C, and that the height of the top of the lower cloud is 

 2,000 metres. Under these conditions the temperature of 

 the air when it leaves the cloud will be about 15° C, and the 

 pressure approximately 600 mm. From these initial con- 

 ditions, according to the method described on pp. 122-124 

 of Professor J. J. Thomson's " Conduction of Electricity 

 through Gases," we find that when condensation takes place 

 at four-fold supersaturation 6* 5 grams of water will be de- 

 posited from each cubic metre of air. Now if we know the 

 number of ions on which this amount of water is deposited 

 we can determine the quantity of electricity contained in 

 each c.c. of water. 



So far we have not found it necessary to determine how 

 many ions are actually contained in the air when it reaches 

 the condensation layer ; we have only discussed the rate of 

 formation of the ions, and this will not help us, for we do 

 not know the upw r ard velocity of the air, and so the time 

 taken in traversing the supersaturated stratum. We must 

 find the probable state of ionization from other considerations.. 

 Now at the surface of the earth numerous measurements of 

 the ionization of the air have shown that under normal con- 

 ditions a cubic metre of air contains about a quarter of an 

 electrostatic unit of free electricity of each kind. From, 

 balloon ascents it has been found that this amount tends to 

 increase as one rises from the surface, but for reasons given 

 above it is very unlikely that the air in the supersaturation 

 stratum would ever contain so many ions as have been found 

 at the corresponding heights under normal conditions *. We 

 will, however, for the sake of a round number, and in order 

 to take the data used by Gerdien, assume that the air just 

 before entering the condensation layer contains one electro- 

 static unit of each kind of electricity in the form of free 

 ions. We shall then have approximately 3x10° ions of each. 

 * L. c. pp. 13 and 15. 



