Demystifying Magnetism

Article excerpt

Published in 1600, De Magnete, William Gilbert's book on the properties of magnets, bridges the divide between medieval scholarship and modern science.

Without the magnetic compass, Columbus, da Gama, Magellan, Drake, and other great navigators could never have conducted their voyages. The magnetic compass was and is a simple device--a magnetized needle suspended on a pivot--and yet up to 1600, no one had any idea what made it point north.

Magnetism itself was a mystery. According to legend, a Greek shepherd near the city of Magnesia found that his iron-tipped staff stuck to a peculiar outcropping of rock, a natural magnet later named lodestone. The Chinese knew that steel rubbed against a lodestone became magnetic, that both attracted iron, and that a magnetic needle had two different poles. Around the year 1000, some unknown Chinese placed a lodestone on a "boat," set in afloat, and observed that it always turned to point to a fixed direction.

Centuries passed. The familiar pivoted needle was developed and became a standard navigation tool, but no one knew how it worked. Was it attracted by a magnetic mountain at the North Pole, a mountain that no ship built with iron fittings or nails should ever dare to approach too close? Or was it, as Columbus thought, the attraction of the pole star, the northern pivot around which the heavens seemed to rotate, even the sun as it rose and set?

Hardly anyone conducted serious experiments with magnets, in part because the tradition of science, which today we take for granted, almost died out in the Middle Ages. History told of an ancient golden age, when Greek philosophers knew wisdom. The books that had survived from that era were copied, embellished, and cited, but few people studied nature itself anymore. If some ancient Roman authority had written that the sharp smell of garlic destroyed magnetism, this was widely accepted. And since a ship's safety might depend on its magnetic needle, it also became a flogging offense for the helmsman to be caught eating anything with garlic in it.

All this was greatly changed by a Latin book that appeared in London in 1600 titled De Magnete (On the magnet). Its author was William Gilbert (see drawing), president of the Royal College of Physicians, and it described the results of nearly 20 years of study. Gilbert was fascinated by magnets. He collected and read books discussing magnetism and examined the truth of their claims, finding most of them false. Garlic had no effect, and neither did diamonds (as was reputed).

Being a physician, he also examined medical claims, finding them equally illusory: "The application of lodestones to all sorts of headaches no more cures them (as some make out) than would an iron helmet or a steel cap."

Robert Norman's experiment

We do not know what started Gilbert's lifelong interest, but it might have been a remarkable experiment published in 1581--the year Gilbert began his studies--by Robert Norman, a British compass maker.

In those days a craftsman would build a compass as follows: He would fashion a flat steel needle, balance it on a pivot, and then magnetize it by stroking it with a lodestone or a strong magnet, always in the same direction, until it, too, became magnetic. But a strange thing was noted: When the magnetized needle was placed back on its pivot, its north-pointing end always hung down, as if it had become heavier. To restore the balance, the craftsman then had to snip a piece off that end.

The story is told that when Norman one day spoiled a compass needle by snipping off too much, he decided to investigate. Taking a ball of cork, he stuck a steel needle into it and floated the cork ball in a goblet of water. Next he whittled down the cork bit by bit until the cork and needle just hovered in the water, and then after evening out the position of the needle in the cork, he magnetized the needle by stroking. …