Atmospheric Electricity in Egypt

Abstract

IN Paper No. 10 of the Survey Department of Egypt Mr. H. E. Hurst discusses two years' results of atmospheric electric potential obtained from a Kelvin water-dropper electrograph at Helwan from March, 1906, to February, 1908. From observations made with a Kelvin portable electrometer, and experiments on the disturbing effect due to the presence of instrument and observer, a factor was obtained, multiplication by which transfers curve readings to potential gradient in the open (volts per metre of height). The mean value found for the potential gradient from the two years was 113, a value lower than is usually encountered in Europe. In the second year, however, owing to the more open scale employed, there was at times considerable loss of trace, and an allowance which Mr. Hurst makes for this would bring up the value of the potential gradient for that year from 119 to 129, and the mean for the two years from 113 to 118. The curves were not smoothed, and were measured only at the even hours, and there is rather excessive irregularity in the diurnal inequality curves which are given for individual months of the year. All show a prominent minimum in the early morning from 4 a.m. to 6 a.m., and some a secondary minimum in the early afternoon, but successive months differ in this respect rather widely. In the mean diurnal inequality for the year there is little variation in the potential from 10 a.m. to 10 p.m.; the value at 8 p.m. is the absolutely largest in both years. One very exceptional phenomenon is that the potential is decidedly highest in summer. The mean potential gradient from the. four months June to September was 136, while that from the four months November to February was only 98, no allowance being made in either case for loss of trace. Curiously, however, the mean range of the diurnal inequality was 81 for the four mid-winter months as compared to 50 for the four months June to September.