70 Proceedings of the Roijal Irish Academij. 



Disetission. 



On the luaiu issue of these experiments our work is in agi-eement with 

 that of Simpson' and Baldit ;- theiQ ap^jeai-s to be no doubt that a large 

 excess of positive eleetricitj" is brought down to the Earth by rain. This 

 holds true, whether the rain is accompanied by thunderstorms as in Simpson's 

 work, or during normal electrical conditions as in the case of our observations. 

 In some details there is not complete agreement Ijetween the observations 

 of Simpson, Baldit. and our own. Simpson and Baldit found the highest 

 positive and negative charges in the case of light i-ain. Our exjjerieuee has 

 been that the high charges per c.e. are iu most cases associated with heavy 

 rain. Again, Baldit states that during light I'ains the highest charges are 

 generally negative. "When the i-ain consists of exceedingly minute drops, 

 and such rain is usually light, we find the. charge always negative, but 

 genei-ally we could not say that the highest charges are usually n^ative on 

 light rain. As appears fiom the previous pages, we have found it easier ta 

 analyse oui- results by using the si^e of the di'ops leather than the rate 

 of rainfalL 



The great e.xcess of positive charge on i-ain is the common feature of 

 all recent work on the subject, and must be taken into account in future 

 attempts to found a theory of atmospheric electricity. That there should be 

 such an excess under the normal electrical conditions as well as during 

 thunderstorms increases the difficulties of the subject when Aaewed in the 

 light of our present very imperfect knowledge of atmospheric electric 

 constants. The air-to-earth current due to conductivity in the air is positive 

 under the normal potential gradient, and we have now another current of 

 the same sign due to i-ain. 



The theory of thunder-storms which Simpson has put forward, that the 

 separation of electricity and the charge on r-ain during thundei-storms is due 

 to the breaking up of raindrops in veiiical air-eiuTents, appeai-s to afford an 

 explanation of many of the phenomena of thunderstorms. Our work now 

 shows that the rain occurring under normal electrical conditions is charged, 

 generally speaking, in a similar manner to thundei'storm rain, and it is a 

 question whether the conditions such as vertical air-cun-ents which Simpson 

 assimies can te supposed to exist imder normal conditions, ilore experimental 

 work is required on the breaking up of drops by air-ciuTents and on the 

 exact manner in which the separation of positive and negative eleetrieity 

 occurs. Again, while we have extensive mea.'mrements of the small ions in 



' Simpson, Phil. Trans, ccix, 1909, p. 379: Proc. Eor. Soc.. Isxsiii, 191", !•. 394. 

 -. Baldit. I* Radium. .4vril, 19 U : Mars, 1912. 



