The peculiar properties of the oxide of iron Fe3O4 were known to the ancient world. It was called lodestone or leading stone because of its power of attracting iron, and its ancient name λίθος Mαγνῆτις was attributed by Lucretius to Magnesia in Thessaly, where the ore was found in large quantities. The modern science of Magnetism may be said to date from William Gilbert of Colchester who published his great work De Magnete in 1600.

If a body such as a steel needle has been rubbed with a lodestone, and so acquired magnetic properties, and is then suspended so that it can turn freely about its centre of gravity, it sets itself in equilibrium so that a definite line in the body—called the axis of the magnet—is parallel to a definite direction in space—called the direction of the earth's magnetic field, roughly north and south. If the ends of the axis are marked on the magnet, it is found that, when the magnet is free to move, one end always points in a northerly direction and the other southerly. The azimuth, or direction of the magnetic field west of north, is called the variation or the magnetic declination, and the angle between the direction of the earth's magnetic field and the horizontal plane is called the magnetic dip. The earth's magnetic field is completely specified by its intensity, declination and dip.