24 - The geomagnetic field

Summary

Preamble

The study of geomagnetism has a longer history than other branches of geophysics, partly because of its use as an aid to navigation. In the earliest times, the spontaneous alignment of magnetized iron oxide (lodestone) was believed to be due to an extra-terrestrial influence. Properties of spherical lodestones were described in a letter, written in 1269 by Petrius Peregrinus (Pierre de Maricourt of Picardy in France). He introduced the word ‘poles’ in connection with magnets because their influence was believed to be derived from the celestial poles. Recognition of the similarity of the magnetic fields of lodestones to the field of the Earth appears to have awaited the work of Robert Norman and William Gilbert of England in the sixteenth century. A review of their evidence that the Earth is a great magnet, with its axis approximately north–south, was presented, in Latin, in Gilbert's book De Magnete, published in 1600. An English translation by P. F. Mottelay appeared in 1893 and has been reprinted (Dover Publications, 1958) as an early milestone in scientific literature.

Magnetic declination, the difference between the direction of the field, as indicated by mariners' compasses, and true (geographic) north, was indicated on navigational maps by the mid-sixteenth century, but was attributed to a misalignment of magnetic and geographic axes and not to a flaw in Gilbert's concept of a dipole field.