130 ANNUAL REPORT SMITHSONIAN INSTITUTION, 1937 



over a much larger region of the cloud in the upper part and through- 

 out most of the body of the cloud. 



Simpson's measurements of the charge on rain drops are in agree- 

 ment with his conception in that they show that as storm clouds 

 pass overhead the falling rain consists first of large drops charged 

 positively; next of a mixture of negatively charged and positively 

 charged rain; and, finally, a steady downfall of moderate to small- 

 sized negatively charged drops. 



ELSTER AND GEITEL's INFLUENCE THEORY 



Elster and Geitel * ^ have postulated a very different theory of 

 cloud electrification, in which the upward air currents of thunder- 

 storms play as prominent a part as in Simpson's theory. In arriving 

 at an understanding of their theory, one can visualize the carrying 

 up of atmospheric moisture by the upward air currents, the forma- 

 tion of drops, their combination and their fall much as described in 

 Simpson's theory. Since the earth is normally negatively charged, 

 a separation of electricity will occur on each water drop by induction 

 even though it is remote from the earth. The bottom of the drop 

 will be positively charged and the top negatively charged. As the 

 large drops fall through tlie upward moving air stream, smaller drops 

 being carried upward come in contact with the lower surface of the 

 large drops. As the negative top of each droplet contacts the posi- 

 tive bottom of a larger drop, an exchange of charge occurs. The 

 smaller droplet gains a positive charge while losing an equal amount 

 of negative charge to the larger drop. The smaller drops continue 

 their upward journey gaining positive charge at each contact while 

 the larger drops proceed downward gaining negative charge as they 

 go. Thus an accumulation of negative charge occurs near the bot- 

 tom of the cloud while the uj)pcr region becomes positively charged. 

 This theory is of interest in view of the fast-accumulating evidence * 

 that a very large majority of the discharges to transmission lines 

 are from negatively charged clouds. 



c. T. R. Wilson's theory 



C. T. R. Wilson ^ *° has suggested still another theory of thunder- 

 cloud electrification. He explains his theory by following the progress 

 of the water drops through the rising air currents of the thunderstorm 

 and attributes their electrification to contact with air ions. 



< Elster and Geitel, Bemerkungen Ueber den Eloctrischen Vorgang in den Gewitterwolken, AVicd. Ann., 

 vol. 25, p. 116, 1885. 

 ' Qeitel, H., On the origin of precipitation electricity, Physik. Zeitschr., vol. 17, p. 455, 1916. 



• Lewis, W. W., and Foust, C. M., Lightning investigation on transmission lines: V, Amer. Inst. Elec. 

 Eng. Trans., vol. M, pp. 934-942, 1935. 



• Wilson, C. T. R., Investigations on lightning discharges and on the electrical field of thunderstorms, 

 Phil. Trans. Roy. Soc, ser. A, vol. 221, pp. 73-115, 1920. 



'0 M'ilson, C. T. R., Some thundercloud problems, Joum. Franklin Inst., vol. 208, pp. 1-12, July 1929. 



