On the observations of Earth potential‐gradients at Ebro

In a recent paper1 the writers have re‐investigated the mathematical relations between the observed terrestrial magnetic variations and the external and internal (induced) magnetic fields which give rise to them. In the course of this paper we compared the diurnal variation of electric potential‐gradient within the earth, given for Berlin, in arbitrary units, by Weinstein, with that derived from the potential of the field of the diurnal magnetic variations, as previously determined by one of us.2 A moderate similarity in type was found; the question of agreement in absolute magnitude could not be tested, however, Weinstein's unit being unknown. 2. The recent appearance of Dr. Bauer's discussion of the measurements of earth potential‐gradients at Ebro3 affords the opportunity for further examination of the question. It may be said at once that the results prove extremely puzzling. The observed potential‐gradients are, as in Weinstein's case, nearly of the type to be expected from the magnetic variations, but are five or six times as large in amplitude. As Dr. Bauer's tables refer to the means for groups of five magnetically quiet days, it seemed just possible that allowance for the factor 5 might have been overlooked, but on enquiry Dr. Bauer kindly confirmed our expectation that this was not the cause of the discrepancy.