The mean electric field of the Earth is characterized by a negative surface‐charge and a positive space‐charge in the atmosphere. The mean distribution of the positive space‐charge can be inferred from the values of the potential gradient at different elevations, since d2V/dh2 = −4πρ. Approximate computation shows that 50 per cent of the charge is in the first kilometer. Variations in distribution will cause variations in potential gradient at any point by changing the relative magnitude of the spacer‐charge (a) in horizontal layers and (b) in vertical columns. Expansions or contractions and convection‐currents will produce such changes in distribution and hence determine the character of changes in potential gradient. Continuous records of space‐charge and potential gradient have been obtained for a year 7.5 meters above the ground and their diurnal variations have been compared with the diurnal system of convection as inferred from the variation in the meteorological elements. It is concluded that for this set of records the diurnal variation of the potential gradient is satisfactorily accounted for on this theory.
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Joseph G. Brown (1930) studied this question.