ATMOSPHERIC ELECTRICITY

Abstract

IN the third of the series of Sir Halley Stewart Trust Lectures on “Scientific Progress”, on October 29, Prof. E. V. Appleton dealt with recent advances in the study of atmospheric electricity. The surface of the earth is negatively charged in fine weather, while the air immediately above possesses a positive charge. Measurements made over the oceans by American observers in the non-magnetic ship Carnegie showed that there is a maximum effect all over the world at the same universal time (7 p.m. G.M.T.). It is now believed that the maintenance of the earth's charge is due to thunderstorms, which send an appreciable amount of negative electricity into the ground. In the upper atmosphere, the electrification is a thousand times denser than it is in the lower atmosphere. The marked solar control of the density indicates unmistakeably that the ionisation is due to solar radiation, and radio observations show that the cause of the electrification is ultra-violet light. A difference in the behaviour of the lower and upper levels of the ionosphere has been recognised recently. The lower stratum is found to be about twice as dense in summer as in winter, due to the more direct influence of sunshine. This is exactly the amount of variation predicted by theory. But for the higher region there is not the expected increase in summer. To account for this anomaly the theory has been put forward that, at a height of 150-200 miles, the atmosphere is raised to a high temperature by the sun. Expansion results from this heating, so that the electricity is attenuated in density. To account for the observed facts it appears necessary to assume that at this level the summer noon temperature is at least 2000° F. The density of the ionosphere appears to follow the sunspot cycle of 11 years. The minimum of solar activity occurred about the latter half of 1933, and both magnetic and radio observations now show that activity is increasing again. It is expected, as a result, that the radio-engineer will find substantial differences in the wireless wave-lengths best suited for long-distance communication.