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Navigation is one of the uses of the Earth's magnetic field -there are many others - but why does a compass point north? The answer is that 'the Earth itself is a great magnet', as William Gilbert of Colchester wrote in 1600 (Figure 3). He based this conclusion on his own experiments with magnets and on the work of Robert Norman. Norman pivoted a needle so that it could swing up and down rather than from side to side. He was suprised to find that, when it was magnetised, it chose to point nearer to vertical than to the horizontal (see Figure 4). What he had discovered and measured was the magnetic dip. At the north magnetic pole a magnetised needle points straight down, near the equator it stays horizontal, at the south magnetic pole it points straight up, and in between it dips to intermediate angles (for example, in London the dip is about 66 degrees below the horizontal). Like declination, dip changes slowly with time as well as with position.

Gilbert's experiments involved a spherical magnet or 'terella' (see Figure 5) to represent the Earth, and a number of small, pivoted magnetic needles which he moved around the terella and found that they behaved in the way described above (see Figure 3). So we can agree that the Earth behaves as though it were a magnet, but this is only a start. What sort of a magnet is it, and why is it there?