Measurements at sea of the vertical gradient of the main geomagnetic field during theGalatheaexpedition

Abstract

During the Danish Galathea expedition, 1950–52, investigations were made of the vertical gradient of the geomagnetic field, in an attempt to test the probability of the new “fundamental” theory for the main geomagnetic field proposed by P. M. S. Blackett in 1947. Three relative, self-recording magnetometers—one needle instrument measuring the Z-component, one needle and one rotating-coil instrument measuring the H-component—are described. All instruments may be lowered to extreme depths in the sea in non-magnetic containers. In the Pacific Ocean, a series of trial measurements were carried out, and at least one of the H magnetometers proved to be of the desired accuracy of 10γ during a single relative measurement. All trial stations were situated in places with locally disturbed geomagnetic field, which makes the gradient results unreliable, and a breakdown of the technical equipment of the surveying vessel prevented measurements at stations better suited to magnetic work. However, summed up, the trial results indicate a case against the fundamental theory—in accordance with gradient measurements ashore in mines and collieries.