8 Dr. Gr. C. Simpson on the 



in all probability break up into smaller drops than either of 

 them was before the collision. It does not appear possible 

 to deny some amount of breaking of drops, the only question 

 is, how much takes place ? 



Assuming that drops do break, we will now consider what 

 the result of the splashing would be. We will again take 

 the case of the formation of a rain cloud on a day with the 

 normal potential gradient. Until the rain commences to fall 

 there is no separation of electricity in the cloud and the 

 normal potential gradient remains unaffected. The potential 

 gradient indicates a negative charge on the ground, and the 

 corresponding positive charge is in the form of a volume 

 charge in the air above. Now wdien the rain commences 

 and collisions take place, the rain becomes positively charged 

 and the negative charge remains behind in the cloud or in 

 the air beneath it. Before very long the rain has brought 

 down sufficient positive electricity to the ground and left 

 sufficient negative charge behind to reverse the normal field 

 Thus during steady rain one would expect positively charged 

 rain and a reversed electrical field, and this is exactly what 

 is found by observation. If the air from the rain area is 

 carried over the surrounding country by the wind, it may 

 have such a large volume charge of negative electricity that 

 it will reverse the field by itself. This accounts for the fact 

 that as a rain shower moves across the country, the normal 

 field is often reversed before the rain actually commences at 

 a station. We see from this that the theory accounts for the 

 facts qualitatively, but is it sufficient quantitatively ? The 

 want of accurate measurements makes a definite reply to 

 this question difficult. 



Schindelhauer * found that with non-thunderstorm rain 

 (Landregen) 92 per cent, of his observations showed a posi- 

 tive charge, of which 81 per cent, gave a vertical current of 

 between 1 and 5 x 10~ 15 amp./cm. 2 Balditf found for the 

 same kind of rain 85 per cent, of positively charged rain, of 

 which 86 per cent, gave a vertical current of between '1 and 

 10 X 10~ 15 amp./cm. 2 From these figures it will be safe to 

 say that the average current produced by the descending 

 positively charged rain is about 2 x 10~ 15 amp./cm. 2 This is 

 equivalent to 6 x 10" 6 E.S.U. per cm. 2 per second. Now the 

 normal charge on the ground, assuming a potential gradient 

 of 100 volts per metre, is -3 x 10" 4 E.S.U. per cm. 2 Thus 

 we see that the potential gradient would be reversed in 



3 x 10~ 4 



- — 77T— ^ sees. = 50 sees. Hence the reversal of the field is 

 6xl0" 6 



* Schindelhauer, Ver. Kon. Preuss. Met. Institut^o. 263, p. 27(1913). 

 t Baldit, Le Radium, ix. March, 1912. 



