On the nature of atmospherics—IV

Abstract

The modern study of the rapid changes of the earth’s electric field associated with lightning flashes was initiated by C. T. R. Wilson in 1916. Wilson made observations on the net changes of the earth’s field due to the destruction of near-by thundercloud moments by lightning flashes and, since interest was centred solely in the magnitude of the initial and final values of the field, a relatively sluggish electrical indicator (capillary electrometer) was found most convenient. In observations of essentially the same character, Appleton, Watt, and Herd, using a string electrometer as well as a capillary electrometer, made measurements at greater distances from the discharge channel. The work of Wilson, at distances usually within the region of audible thunder, had shown that the net changes of the earth’s field associated with lightning flashes were more frequently positive than negative in sign. On the other hand, Appleton, Watt, and Herd found the opposite preponderance, and this led them to conclude that a thundercloud is frequently, if not always, bipolar and, further, that in order to account for the signs of the field changes it must be assumed that a very frequently occurring type of bipolar thundercloud is one with the positive charge uppermost. These conclusions were later confirmed by Schonland and Craib in observations made in South African thunderstorms. The measurements made by Appleton, Watt, and Herd on disturbances of the earth’s electric field also included observations on the wave-form of those naturally occurring electric waves known to radio-engineers as atmospherics. In this series of observations the potential variations developed across a condenser or resistance included in a damped wireless antenna were examined visually by means of a sensitive cathode-ray oscillograph. The results of a statistical analysis of a large number of individual drawings of wave-forms showed the type of atmospheric observed to be a relatively long aperiodic or quasi-periodic electrical disturbance of duration 2 to 3 milliseconds (msec.) and intensity 0·1 volt per metre (v. /m.). High-frequency ripples on the main gross structure of quasi-frequencies up to 10 kilocycles per second (kc. /s.) were, however, noted and the interferent effects of atmospherics were attributed to such high-frequency components rather than to the effect of the relatively slow main disturbance.