Electricity of Atmospheric Precipitation. 3 



A. Elster and Geitel's " Influence " Theory. 



The influence theory is based on what Elster and Geitel 

 •consider to be two facts : 



(a) That small drops, in particular cloud particles, rebound 



from large drops without uniting with them ; 



(b) That although the small drops do not unite with the 



larger ones, electrical contact takes place. 



Before going further it is necessary to examine these two 

 premises, for it appears to me that they are not sufficiently 

 well established to form the foundation of an important 

 theory. 



There can be little doubt that (a) is true for pure un- 

 charged water. Every one who has rowed on a river of 

 clear water, or watched the drops falling into the basin of a 

 fountain, will have seen small drops running on the surface 

 for an appreciable time before they are absorbed. On the 

 other hand, the great effect of an electrical charge in causing 

 water-drops to unite has been observed by most physicists 

 since it was first pointed out by Lord Rayleigh in 1879*. 

 In a thunderstorm the drops are so highly charged rela- 

 tively to one another that it is difficult to believe that there 

 can be any large amount of impact without the drops uniting 

 more or less completely. Indeed most writers have used 

 this fact to account for the dark colour of the clouds and 

 the heavy rain associated with thunderstorms. 



The second foundation of the theory, that electrical contact 

 takes place although the drops do not unite, is based on 

 experiments made by Elster and Geitel in 1885. As these 

 experiments have never been repeated, while the results 

 have been accepted without question for thirty years, it 

 seems desirable to examine them in the light of modern 

 knowledge. 



The experiments were shortly described by Elster and 

 Geitel f, and on account of their importance a literal trans- 

 lation is given here. 



" The following method was used to prove that small 

 water-drops can come into electrical contact with a large 

 water surface without uniting with it. 



" The metal nozzle of a 'sprayer' was directed at an 

 angle towards an insulated water surface which was con- 

 nected to a Thomson quadrant electrometer. Over the 



* Rayleigh, Proc. Eov- Soc. xxviii. p. 406 (1879). 

 t Elster & Geitel, Wied. Ann. xxv. p. 129 (1885). 



B2 



