how we discovered the Earth is a magnet

The Mechanism

William Gilbert was born on 24 May 1544 in Colchester, Essex, the eldest son of a recorder of the town. He took his BA at St John's College, Cambridge, in 1561, his MA in 1564, and his doctorate in medicine in 1569, and by the 1570s had established a successful medical practice in London on Wingfield Street near St Peter's Cornhill. He was elected a Fellow of the Royal College of Physicians in 1573, censor in 1581 and 1587, treasurer in 1587, and president of the College in 1600. That same year — aged 56, at the height of his medical prestige — he published in London, at his own expense, a Latin folio titled *De Magnete, Magneticisque Corporibus, et de Magno Magnete Tellure* (On the Magnet, Magnetic Bodies, and the Great Magnet of the Earth). It was the product of eighteen years of private experiment. Its central claim was a break with two thousand years of philosophical opinion. In the Aristotelian and scholastic tradition the pointing of a compass northward was explained by long-distance sympathy: the lodestone was attracted, said Peter of Maricourt in 1269, to the pole of the heavens; the compass needle pointed toward the Pole Star. Sailors in Gilbert's day still believed there was a mountain of loadstone at the North Pole that pulled the needle. Gilbert cut a piece of natural lodestone into a sphere about a hand across, polished it, and called it a *terrella* — a "little Earth." He rested magnetised iron needles at points around the sphere and measured what they did. The needles arrayed themselves along lines running from one pole of the terrella to the other; a small compass moved across the surface deflected in ways that matched, quantitatively, the pattern of dip and declination that Norman and Borough had measured with dip-circles in the Thames estuary during the previous twenty years. If a compass on the Earth behaved exactly as a compass on the terrella did, then the Earth itself was a magnet — a great magnet, whose interior force reached out through the surrounding space to align the small magnets of navigators, and did not require a distant star or a fictional mountain. He proposed also that the Earth's magnetic force reached beyond the Earth into space and could act between celestial bodies at a distance — a proto-Newtonian conjecture that would help Kepler, twenty years later, argue for a physical (rather than kinematic) explanation of planetary motion. Gilbert was one of the first natural philosophers to insist explicitly on experiment over authority. *De Magnete* — 240 folio pages, six books, more than a hundred engraved illustrations, the first great English scientific book — was dedicated on its opening leaf to Queen Elizabeth I, whose physician-in-ordinary Gilbert was appointed in early 1601. She died in March 1603; Gilbert followed in November that year, of the plague. His fortune of £160 in books, globes, minerals, and instruments — probably the first true experimental-physics laboratory in England — was bequeathed to the Royal College of Physicians. The College's library, and the entire bequest, was destroyed in the Great Fire of London in 1666. What remains of Gilbert's research programme is his book. Galileo cited it, Kepler cited it, the *Novum Organum* cited it, Newton cited it. The mathematical theory of the geomagnetic dipole would not be worked out until Gauss in Göttingen in 1839, and the actual mechanism — a self-exciting fluid-iron dynamo in the outer core — would not be understood until the mid-twentieth century. But the fact that Gilbert first stated in 1600 — the Earth is a magnet, and this is why compasses point north — has never had to be corrected.