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The various ways of delineating photographically the random transient changes in the earth's electric field by means of a cathode ray oscillograph are described together with a convenient method of recording the time history of all the disturbances that arise from a thundercloud discharge. An interpretation of these records and observations on the rate of change of field follows. It is found that a positive field change is almost invariably initiated by a volley of pulsations which is succeeded by a sequence of violent changes of similar characteristic structure and indicates the discharge of negative electricity from the cloud to ground by the familiar intermittent lightning flash. The results obtained give the magnitude and nature of the currents involved in the various stages of the flash. Negative field changes observed under the same conditions are recorded as a relatively slow variation of the field carrying minor pulsations and provide evidence relating to the different nature of discharges within the cloud. Following an analysis of the field change due to a component stroke of the intermittent flash an explanation is given to account for the rate of dispersal of charge in the thundercloud towards the end of the lightning stroke which requires a conductivity of the order of 10 2 e.s.u. The records obtained at great distances from the storm centre show that the strongly radiated pulse from each lightning stroke is accompanied by a succession of echoes due to multiple reflections between the ionosphere and the ground.
F. W. Chapman (Fri,) studied this question.