Electrical Processes in Thunder- clouds. 35 



provided with an efflux-tube directed downwards and ending 

 in a narrow aperture. If now a small charge of + electricity 

 is imparted to the tube A, while at the same time the insu- 

 lated vessel B filled with water is held over it, the drops from 

 B become negatively electrified, but fall through the tube A 

 without coming in contact with the sides. By this means B 

 itself is positively charged, and in an extremely short time 

 acquires a potential which is not much lower than that of A, 

 when the difference between B and A is not too great. If 

 now B, while still held by the insulating handle, is rapidly 

 inserted in "A, and touches it momentarily, the electricity of 

 B passes almost completely over to A, according to known 

 electrostatic laws. If B be withdrawn to its original position, 

 the same operation may be repeated. It must be remem- 

 bered that A has now a greater charge than before, and that 

 accordingly B is also more strongly influenced. It follows 

 accordingly that, by repeating the operation, the charge of A 

 increases in geometrical progression. In practice the limit is 

 soon reached. With such a duplicator it is easy to show a 

 considerable increase in the strength of the charge; it is only 

 necessary to connect A during the experiment with a gold-leaf 

 electroscope and to be somewhat quick in moving B. We 

 observed almost always a self-excitation of the apparatus, so 

 that after fortyfold oscillation of the vessel B a spark could be 

 taken from the tube A. 



The process in a thunder-cloud may be considered to be 

 analogous. Suppose that a given charge of + electricity is 

 imparted to one place in the lower layer. As long as the cloud 

 is not dissolved in rain this electricity remains in nearly the 

 same place, or only slowly spreads with decreasing potential 

 over the entire cloud according to the conductivity of the 

 mass of vapour. The case is different when the cloud dissolves 

 in rain, where it may be assumed that the formation of drops 

 must take place in the upper, that is the colder, layers of air. 

 As far as the cloud rains it will acquire positive electricity ; 

 and if the formation of rain does not begin at too great a height 

 above the electrical layer, it is of not much lower potential 

 than the latter. The negatively electrified drops fall to the 

 earth through the lower inducing layer. 



But now, as has often been pointed out (so far as we know, 

 first by Hermann J. Klein), the contraction of surface, which 

 accompanies the formation of rain, must produce an increase 

 of potential. As the entire mass of cloud which was pre- 

 viously charged withpositive electricity by induction aggregates 

 together, and the individual particles of vapour coalesce to 

 form larger drops, the electricity is confined with increasing 



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