Earth‐movements and terrestrial magnetic‐variations

It is shown that a general contraction of the Earth or a movement of a small part of it may give rise to appreciable disturbing magnetic fields. The movement of a conducting region inside the Earth across the Earth's magnetic field sets up electromotive forces which may produce large current‐systems. These current‐systems can give rise to the observed magnetic variations if the conductivity of the earth's core is sufficiently large. Earlier determinations of the resistivity of the Earth's core are found to be to several orders of magnitude too high, due to the neglect of skin‐effects. It seems probable that the electrical time‐constant of the Earth's core is not less than 1,000 years, and therefore magnetic diurnal‐variation data cannot be used to calculate its conductivity.