Gilbert: On the Loadstone

Have you ever held a compass? The needle always swings to point north. About 400 years ago, sailors used compasses to cross the oceans, but no one knew why the needle points north. Some thought a magnetic star pulled it. Some thought there was a giant magnet mountain at the top of the world.

William Gilbert, a doctor in London, had a better idea. He took a natural magnet stone, called a lodestone, and carved it into a ball. He called it a terrella, which means "little earth". Then he moved a tiny compass needle around the ball. The needle acted just like a ship's compass on the real Earth! It pointed toward the ball's poles, and it tipped down more and more as it got near a pole.

So Gilbert said: the Earth itself is a giant magnet. That is why compasses work everywhere.

Who, when and why

William Gilbert (1544–1603) was a physician from Colchester who worked in London, became president of the College of Physicians, and served as a physician to Queen Elizabeth I. For many years he tested magnets and iron in his own rooms. In 1600 he published On the Magnet, Magnetic Bodies, and the Great Magnet the Earth (in Latin, De Magnete). It is one of the first books built mainly on experiments, and it gave English its words electric and, later, electricity.

The problem Gilbert faced

The compass had changed sea travel, but what moved the needle was a mystery. Books on magnets mixed real knowledge with old tales: that garlic or a diamond takes away a lodestone's power, that magnets cure headaches, that a magnetic mountain or the pole star pulls the needle. Navigators had also found two puzzles. The needle doesn't point exactly at true north (it is off by different amounts in different places), and a balanced needle, once magnetized, dips below the horizon. Gilbert wanted to clear away the tales and explain the real effects.

Idea 1: the little earth

Gilbert's key tool was a model. If the Earth is a magnet, then a round magnet should behave like a small Earth, and experiments on it should show what happens on the real one.

He found its poles by laying a short iron wire on it and marking where the wire lay; the lines meet at two poles, like the meridians of a globe. Then he carried a tiny needle (his versorium, a "turner") over the surface. At the stone's equator the needle lay flat. Moving toward a pole, it tipped more and more, and at the pole it stood straight up and down.

Idea 2: the Earth is a great magnet

The same pattern of dip shows up on the Earth: near London a needle dips steeply, and sailors near the equator found it nearly level. Gilbert concluded that the needle is not pulled by the sky but turned by the Earth itself. Book I, chapter 17 is titled "That the globe of the earth is magnetick, & a magnet." He even suggested that the dip could tell a sailor his latitude on a cloudy night.

Idea 3: test the tales

Idea 4: electric attraction is different

In Book II Gilbert studied a different kind of pull: amber, rubbed, picks up chaff and bits of straw. He built a light pivoted needle (any metal, not magnetized) to detect it, and found that many things besides amber do it: glass, sulfur, sealing wax and gems. He called them electrica, from the Greek word for amber, ēlektron. He showed that this rubbing-attraction is not magnetism: it pulls many materials, not just iron, and damp air weakens it. The English word electricity was first used a few decades later, by Thomas Browne in 1646.

Getting the history right

• The dip was found by Robert Norman. A London instrument maker, Norman described the dip and measured it at London as about 72° in his book The Newe Attractive (1581). Gilbert praised him and built on his work. Gilbert's new step was the explanation: the Earth as a magnet.
• The garlic test was older, too. Giambattista della Porta, in Naples, reported in his Natural Magic (1589) that he had rubbed a lodestone with garlic juice and it worked as well as ever. Gilbert's book adds the reasoning and many tests of its own.
• Gilbert's reach went too far in places. He believed magnetism made the Earth spin once a day, and he spoke of a magnetic "soul" in the Earth. Francis Bacon complained (Novum Organum, Book I, aphorism 54) that Gilbert built a whole philosophy out of the magnet.

What later science kept, and what it changed

Kept: the Earth has a magnetic field shaped much like a bar magnet's; compasses and dip needles respond to it; magnetism and static electricity are separate effects of rubbing and of lodestones (until the 1800s, when Ørsted, Ampère and Faraday showed they are deeply linked). Changed: the Earth is not a solid lodestone. Its field is made by moving molten iron in the outer core, a natural dynamo, and it slowly shifts: the magnetic poles wander, and the field has reversed many times over the ages, a record kept in volcanic rocks. Gilbert's idea that magnetism spins the Earth was dropped. Kepler took up Gilbert's idea of a magnetic force from the Sun to drive the planets; Newton replaced it with gravity.

The power of a model. Gilbert couldn't walk around the whole Earth with a dip needle, but he could walk a needle around a ball in his hand. Using a small, controllable copy of something too big to study directly is now a basic tool of science: wind tunnels, wave tanks, computer models. What must be true for a model to teach us about the real thing?

Gilbert and the Copernicans. Gilbert accepted that the Earth turns daily on its axis, and he used magnetism to explain why. Kepler read him eagerly and imagined the Sun turning and sweeping the planets along with a magnet-like force. Galileo praised Gilbert in his Dialogue (1632). A book about compasses became part of the argument about the shape of the heavens.

Experiment as a habit. Gilbert's preface tells readers to repeat his experiments "not carelessly" and marks his own discoveries with stars in the margin, large and small. He treats the reader as a fellow tester, not just a listener.

Big questions. When an old book and a simple test disagree, why do so many people still believe the book? What makes a test fair? Is it enough to show an idea is wrong, or must we also explain why people believed it?