Notes on Isomagnetic Charts: VI—Earth‐Air Electric Currents and the Mutual Consistency of the H and D Isomagnetic Charts

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Abstract
Summary—Atmospheric‐electric observations indicate that the electric currents crossing the Earth's surface are too weak to have an observable effect on the distribution of the geomagnetic field, which should therefore (to a high degree of accuracy) be derivable from a scalar potential. This question has been examined in various ways: By means of line‐integrals of the horizontal magnetic intensity; by drawing magnetic parallels; by spherical harmonic analysis; and (by Schuster) by calculating the vertical component of the curl of the field at particular points. A discussion of these various methods leads to the conclusion that the curl method is the most generally useful. A new formula is given for calculating the curl directly from the isomagnetic charts for declination (D) and horizontal intensity (H), the elements that are most usually observed and charted. The new process is applied to the 1922 Admiralty charts. The error of estimation of the curl, or of the resulting inferred vertical curren‐tintensity i, is also examined.The non‐zero values of i are due partly to errors of estimation; but the larger values are ascribed mainly to a lack of mutual consistency between the D and H charts in particular localities, due to errors in one or both charts. It is suggested that when new charts of D and H are constructed, the value of i should be taken as zero (on the atmospheric‐electric evidence), and the mutual consistency of the charts should be tested, and ajusted, by the methods here developed.

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