354 CARNEGIE INSTITUTION OF WASHINGTON. 



Recent results on the diurnal variation of atmospheric electricity from observations aboard 

 the Carnegie.^ S. J. Mauchly. [Abstracts] Phys. Rev. vol. 18, No. 2, pp. 

 161-162 (1921), and Jour. Wa.sh. Acad. Sci. vol. 11, No. 16, pp. 398-399. 



As noted in the various annual reports of the Director of the Department of 

 Terrestrial Magnetism, the atmospheric-electric work aboard the Carnegie 

 has since 1915 included observations for determining the diurnal variation 

 of atmospheric electricity. 



The first results of such observations were published in 19 17^ and from the 

 limited number of series of observations then available it appeared that the 

 diurnal variation of the potential gradient over the ocean was of the type quite 

 generally observed at land stations, especially in temperate latitudes and in 

 summer, namely, that the curve representing the diurnal variation showed two 

 distinct maxima and two minima, with the greater amplitude ascribed by 

 Fourier analysis to the 12-hour "wave." However, the data upon which this 

 result was based included a considerable proportion of series which could not 

 be continued throughout a full day on account of unfavorable weather. 

 Swann,^ later, discussing results derived from 20 series made by the observers 

 aboard the Car/ieg/e, during the year ending February 20, 1917, with a much 

 greater proportion of complete series at his disposal than for the earlier 

 report, stated that "the afternoon minimum, formerl}' found about 3 p. m. 

 now appears somewhat later, about 6 p. m., and is less marked than before," 

 and "the effect of the 12-hour Fourier wave is less important in the present 

 curves than in those already published." 



In order that the present results should be free from the obvious error 

 which would result from the inclusion of data from incomplete series of ob- 

 servations, the reduction was limited strictly to series which were complete or 

 which required only a small amount of interpolation. On this basis of selec- 

 tion there are available, from the data for July 1915 to February 1921 inclusive, 

 45 series of potential-gradient observations, each representing a sequence of 

 phenomena which actually occurred during 24 consecutive hours. Of these 

 series, 30 were obtained in the Pacific Ocean, 10 in the Indian, and 5 in the 

 Atlantic; together they represent about half of the Earth's surface. 



Separate mean curves were prepared for each of the oceans represented, 

 showing the mean variation of the potential gradient with local time. Com- 

 parison of these curves shows: (1) that the observed diurnal variation is due 

 primarily to a single wave of 24-hour period ; (2) that the three curves are all 

 similar in form but differ greatly with regard to the phase of the variation 

 for a given local time. For example, the predominating maximum occurs 

 several hours before noon on the Pacific curve, several hours after noon on the 

 Atlantic, and not until after midnight on the Indian. 



These phase differences correspond closely enough to the differences be- 

 tween the respective mean longitudes of all positions at which the observations 

 were made in each of the several oceans to suggest the possibility of a simulta- 

 neous occurrence of maximum at all places. Since no results of simultaneous 

 ocean observations are available, the material in hand was utilized by referring 

 all observations to Greenwich mean time (civil) and preparing new mean 



'Summary of papers read before the Section of Terrestrial Magnetism and Electricity of the 

 American Geophysical Union, April 18, 1921; the American Physical Society, April 23, 1921; 

 and the Philosophical Society of Washington, May 21, 1921. 



^Researches of the Department of Terrestrial Magnetism, Vol. Ill, pp. 416-420, Washington 

 (1917). 



'Supplementary report on atmospheric-electric observations made aboard the Carnegie from 

 May 17, 1916, to March 2, 1917, by W. F. G. Swann, in "Annual Report of the Director of the 

 Department of Terrestrial Magnetism" for the year 1917. Year Book of the Carnegie Institu- 

 tion of Washington, 1917, p. 282. 



