RAIN AND ITS ORIGIN IN THUNDERSTORMS. 393 



(2) That there was no relationship between the sign of the potential gradient and 

 the sign of the rain electricity. 



(3) That the excess of negative potential gradient over positive potential gradient 

 was somewhat less when the rain was charged than when it was uncharged, the 

 percentages of negative potential gradient in the two cases being 63 and 72 

 respectively. 



Before summing up the results of this work, it is desirable to point out the sources 

 of error which might be held to influence the results : 



(1) A certain number of raindrops are bound to fall on the rim at the top of the 

 cylinder B (see fig. 1, p. 380), and part at least of each drop will probably splash into 

 the receiver A. If, now, this rim has high charges induced on it by the influence of 

 the earth's field, the drops which break on it will take away some of the induced 

 charge and give it up to the receiver when they fall into it. 



This difficulty was guarded against by carrying up the walls of the shed to a height 

 of about 1'5 metres above the level of the top of the cylinder B, thus reducing the 

 earth's field in the neighbourhood of B as much as possible. But the best proof that 

 the results were not affected by this source of error is to' be found in Table VII, which 

 shows that the sign of the rain electricity was independent of the sign of the potential 

 gradient, which it would not have been if the water which entered the receiver had 

 obtained its main charge from the rim. 



(2) The possibility of the " Lenard effect " being a source of error has also to be 

 considered. LENARD showed that when a drop of pure water falls on a surface and 

 splashes, a separation of electricity takes place, the water retains a positive charge, 

 and the air takes a negative charge. If steps are taken to remove the charged air 

 from the water by a blast of air, the positive charge on the water can be measured ; 

 but if the splashing takes place at the bottom of a fairly deep vessel, not artificially 

 ventilated, there is no appreciable separation of electricity. It was for this reason 

 that the receiver A was made 3 1 cm. deep. With such a vessel the Lenard effect 

 could not play any appreciable part. 



But, again, the results themselves are the best test, for they show positive charges 

 which could not be given to water by a single splashing under the most favourable 

 conditions in a laboratory. LENARD* found that when a stream of water, in small 

 drops of 2 mm. diameter, impinged on a metal plate with a velocity of 18 metres a 

 second, and great care was taken to obtain complete separation of the electricity by 

 artificial ventilation, each drop developed '2 X 10~ 13 coulomb of electricity. From 

 this we find that a cubic centimetre of water developed 0'15 els. unit of electricity. 

 It therefore appears unlikely that with raindrops falling on to the bottom of the 

 receiver A, and without any ventilation to separate the electricity, anything like such 

 a large charge as O'l els. unit per cubic centimetre of water could be given to the 

 rain by the Lenard effect. Now a charge of O'l els. unit per cubic centimetre of 



* Loc. cit., p. 626. 



VOL. OCIX. A. 3 E 



