| Summary: | Charts marked with the lines of magnetic variation have been published since Halley's Atlantic chart of 1701. It was already known that the location of the magnetic poles shifted over time, and that the north and south poles were not diametrically opposite. As more seafarers penetrated the Southern Ocean, isogons on the charts were extended southwards with greater confidence. At sea variation was measured by comparing compass direction with the Sun's midday shadow. In polar regions, where horizontal force is too weak to attract a compass needle, the location of the pole was sought by observing the inclination of a dip needle swinging in the magnetic meridian, which would hang vertically at the pole. The Fox dip circle, developed in 1834, was the first instrument capable of measuring dip and intensity at sea, and allowed James Clark Ross to predict the location of the South Magnetic Pole. In 1902 Discovery's crew landed an observatory ashore, but a trek on to the plateau failed to reach the magnetic pole. Success came in 1909 during Shackleton's Nimrod expedition, when T. Edgeworth David's party reached the zone of maximum dip. Over the following years data from photographic magnetometers recording declination, vertical and horizontal intensity were routinely made at the various national bases round Antarctica; they contributed to our knowledge of the Earth's internal magnetism and on the solar influences.
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