A review of Mr. George W. Walker's magnetic survey(1915)

1. Recent magnetic surveys, whether extended over the whole surface of the earth or confined to limited areas, seem to indicate that the observed magnetic forces are not entirely deducible from a potential function, the curl round a vertical axis having finite value. The evidence is strong, and were it not for the extreme difficulty of finding a rational explanation it would probably be accepted as decisive. Exclusive of unknown effects, the observa­tions can only be accounted for by electric currents cutting the surface of the earth. Such currents are indeed known to exist, but their intensity is only an insignificant fraction of that deduced from the magnetic forces. Grave objec­tions may also be raised against the alternative explanation assigning the effect to a penetrating corpuscular radiation. The principal argument in favour of the real existence of an earth-air current is supplied by the world surveys, which show an apparent regularity depending on latitude, being upwards in the polar regions and generally downwards in lower altitudes. According to Dr. Bauer, upward currents of the same order of magnitude are found in terrestrial districts within the United States. Rücker and Thorpe obtained no decisive results in Great Britain, while in the districts covered by Mr. Walker’s survey, he gives six with currents agreeing in direction with that found by Dr. Bauer and two in the opposite direction.