On the nature of atmospherics.—I

Abstract

The present paper gives an account of some experiments recently carried out with the aim of ascertaining the intensity and duration of those naturally occurring electric waves known as atmospherics. It has been found possible to determine these quantities with some precision and also to accumulate information regarding the temporal variation of the electric field during many typical atmospherics. Such information is of fundamental importance to the radiotelegraphist, one of whose main concerns in the present state of radio-communication is the elimination of the effects of atmospherics from his receiving apparatus. He has, until now, had to consider as more or less unknown the quantities determined in these experiments. thus progress in the development of anti-atmospheric devices has necessarily been made empirically. The information is also of interest to the meteorologist in view of the fact that many atmospherics originate in thunderstorm areas; for the "wave form" of such an atmospheric originating in a lightning flash gives us information regarding the variation of the electric moment of the thundercloud during the flash. An estimate of the duration of lightning flashes has previously been made by de Blois, who, using an ordinary wireless aerial of feeble damping (and natural periodicity 500,000), examined the electromotive forces developed in it by local discharges. An oscillograph (of natural periodicity 5000 to 6000) was used as indicator of these transient electromotive forces, its behaviour being examined photographically or by means of a rotating mirror. De Blois came to the conclusion that the discharges were essentially aperiodic with a duration of approximately 1500 micro-seconds. The members of the United States Bureau of Steam Engineering have criticised the results of de Blois adversely, pointing out that, since the antenna used was feebly damped, oscillations of its natural period would be produced by any type of excitation. Thus the oscillograph record could not possibly be a faithful reproduction of the initial electric field variations. Moreover, an oscillograph of such low natural period would be entirely unsuitable for the recording of impulses as transient as those dealt with.