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V. 



THE ELECTEIC CHAEGE OX EAIN. 



By PEOFESSOE J. A. McCLELLAND, D.Sc, F.E.S., and 

 J. J. NOLAN, M.A., M.Sc, University College, Dublin, 



Read llth Decembek, 1911. Published February 5, 1912. 



The determination of the sign and magnitude of the electric charge brought 

 down by rain to the Earth is obviously of great importance from its bearing 

 on problems of atmospheric and terrestrial electricity. Measurements have 

 been made by many observers ; but their results have exhibited some discor- 

 dance. The earlier observers, Elster and Geitel,^ at Wolfenbiittel and 

 Gerdien- at Gottingen, obtained results which seemed to be readily capable of 

 explanation, and to harmonize with what was already known about atmospheric 

 electricity. In general they found that the rain brought down an excess of 

 negative electrification. It was supposed that condensation took place around 

 the negative ions of the atmosphere as in the experiments of C. T. E. Wilson f 

 a negative charge was thus brought to Earth, and the positive ions remained 

 in the atmosphere. The maintenance of the normal positive potential gradient 

 above the surface of the Earth was thus accounted for. Measurements of the 

 potential gradient during the fall of rain were also made by these observers. 

 It was found that, in general, the sign of the potential is opposite to that of 

 the charge on the rain. 



The results of Weiss,* however, were not in agreement with those of the 

 foregoing observers. He is supported by Kahler,' working at Potsdam in 1908, 

 and by Simpson/ working at Simla in 1908-'09. Kahler found (1) that the 

 rain was more frequently charged positively than negatively ; (2) that the 

 sign of the charge had no relation to the sign of the potential gradient ; 

 (3) that the intensity of the rain and the quantity of electrification brought 

 down did not correspond. 



' Elster and Geitel, Wien. Ber., vol. xcix, 1890, p. 421 ; Terr. Mag., vol. iv, 1899, p. 15. 



- Geidien, Phys. Zeit., vol. iv, 1903, p. 837. 



3 Wilson, Phil. Trans., 1897, p. 265 ; 1899, p. 403 ; 1900, p. 289. 



* Weiss. Wien. Ber., cxv, 190G, p. 1S25. 



' Kahler, Publ. of Met. Instit. of Prussia, ccxiii, 1909. 



« Phil. Trans., ccix, 1909, p. 379 ; Proc. Roy. Soc. Ixxxiii, 1910, p. 394. 



E.I. A. PKOC, VOL. XXIX., SECT. A. [11] 



