A vertical‐intensity magnetometer

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Magnetic observations, and in particular those necessary for the control of magnetograph records, at stations in high magnetic latitudes have been greatly handicapped, because of unfavorable magnetic conditions as well as temperature conditions at such stations, for lack of a suitable instrument to determine rapidly and with precision the vertical intensity of the Earth's magnetic field. The establishment of the Godhavn Magnetic Observatory in Greenland, where the magnetic inclination is about 82°, led the author to consider electrical methods of determining the magnetic vertical‐intensity of the Earth's field. An experimental apparatus was used during a trip to Greenland in 1925 to select a site for the Observatory, and as a result of these experiments a new type of vertical‐intensity magnetometer was developed and was sent in August 1926, after trial at the Rude Skov and Potsdam observatories, to the Godhavn Observatory for use there regularly.The essential part of the new instrument is a precise standard for mutual inductance, that is, an inductor with two sets of windings, and very strongly built, By rotaiing the standard 180° on a horizontal axis the vertical intensity of the Earth's magnetic field gives rise to electromotive forces in the windings. One winding is connected with a ballistic galvanometer which, through the arrangement described below, measures the induced effect; the sensitivity of the galvanometer is such that rotating the inductor 180° would cause a deflection quite beyond the scale‐limit of the galvanometer, for example, at Rude Skov for a scale‐distance of 2 meters, a deflection of 10 meters. This arrangement, a sort of zero‐method, by which the deflection of the galvanometer never exceeds a few centimeters even for highest sensitivity of the apparatus, utilizes an accurately determined electric current which, during the turning of the inductor, may be made or broken in the second winding. The making or breaking in this current through the second winding induces an electromotive force in the winding connected with the galvanometer, and the connections are such that the electromotive force induced by the variation of the “compensating current” is opposite to the electromotive force induced by the vertical component of the Earth's magnetic field during a turn of the inductor. Thus, the resulting deflection of the galvanometer corresponds to the difference of the two induced electromotive forces. By again turning the coil and using some other compensating current, a second galvanometer reading is obtained. By means of two such readings the strength of a compensating current, which should give just the zero‐deflection of the galvanometer, is readily computed.