It has been known for many hundreds of years that a suitably suspended bar magnet will turn itself so that its axis lies roughly in a north‐south direction. This unexpected and useful property early attracted the attention of navigators and was among the first subjects to be studied after the rebirth of science in Europe in the fifteenth and sixteenth centuries. These studies culminated in the work of William Gilbert, Queen Elizabeth's physician, who wrote in 1600, “Magnus magnes ipse est globus terrestris,” the earth itself is a great magnet.

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