De Magnete, 1600. William Gilbert, an English physician, published De Magnete in 1600 — a treatise on magnets, magnetic bodies, and "that great magnet the Earth". It contains some fifty distinct experiments and it argues from them rather than from authority, which is why it is so often named as one of the first genuinely experimental works of science.

The terrella. Gilbert turned lodestone into spheres he called terrellae, little Earths. Carrying a compass across one, he showed the needle aligns north-south everywhere on the surface; carrying a dip needle — balanced on a horizontal axis so it can tip — he showed the inclination varies with position on the sphere exactly as magnetic inclination varies with latitude on Earth. From that correspondence he concluded that the Earth is itself a great magnet.

What it replaced. The prevailing explanations were that the compass was drawn to the pole star or to a magnetic island or mountain in the north. Gilbert's model does something those cannot: it reproduces the whole pattern of compass behaviour — direction and dip together, at every latitude — rather than just the fact that needles point north. That is the strength of a model argument, and it is why the dip needle is the crucial instrument rather than the ordinary compass.

What he got wrong. Gilbert believed the Earth's magnetism was a permanent property of its solid substance. It is not: it is generated by convecting molten iron in the outer core, and it reverses at irregular intervals of hundreds of thousands of years. Being wrong about the mechanism does not damage the finding — that the Earth is a magnet is correct, and it was established by building a model and measuring it.

The word persists. Physicists still call a magnetised sphere used to model planetary magnetism a terrella, and such devices are still used to study aurorae. It is a rare case of a four-hundred-year-old piece of apparatus keeping both its name and its job.

Electromagnetism

Blueprints (2)

Gilbert's Terrella

Leyden Jar