A Rock That Points North. People Had No Idea Why for Two Thousand Years

The stone is a naturally occurring piece of magnetite, an iron oxide, that carries a permanent magnetic field strong enough to attract iron and to align itself with the field of the Earth when free to turn. Most magnetite is not magnetised, so these pieces are uncommon, and for most of history they were the only source of magnetism anybody had. Suspended on a thread or floated on wood, such a stone turns until one end points roughly north, which is what made it useful.

The explanation arrived long after the use:

• •Magnetite is iron oxide with a particular crystal structure
• •That structure can hold a permanent magnetic alignment
• •The Earth's own field is far too weak to produce one
• •Lightning strikes generate enormous brief fields
• •Ore near a strike is magnetised strongly by it
• •Which is why they are found at or near the surface

The practical history runs through navigation and predates any theory. Chinese sources describe a magnetised spoon used for divination by the first century and a floating needle for navigation by the eleventh. European references to a magnetised needle appear from the late twelfth century. In both traditions the stone was used to magnetise iron needles by stroking, which transferred the property to a shape that was more convenient than the rock. The stone itself remained valuable equipment on ships, because a needle loses its magnetism and must be restored.

Having a needle that points north is only the start of navigating with one, and the practical difficulties took centuries to solve. The needle points to the magnetic pole rather than the geographic one, and the difference varies by location and drifts over decades, so a correction must be known and updated. Iron in a ship deflects the needle, which required compensating magnets and iron spheres mounted beside the compass. A needle on a moving ship swings, so it was mounted in fluid and on gimbals. And a compass indicates direction without indicating position, which is why longitude remained unsolved long after the compass was ordinary.

Explaining the behaviour occupied serious thinkers for centuries and most of the attempts are wrong in interesting ways. Attraction was variously attributed to a sympathy between substances, to invisible emanations, and to the stone reaching towards mountains at the pole. William Gilbert published a study in 1600 arguing from experiment that the Earth itself is a magnet, which he demonstrated with a magnetised sphere, and that is the first correct account. Why magnetism exists at all required quantum mechanics and was not settled until the twentieth century.

A naturally magnetised piece of magnetite was the only magnet available for most of history, magnetised almost certainly by lightning, since the Earth's own field is far too weak. Stroking an iron needle with one transferred the property to a convenient shape. Gilbert demonstrated in 1600 that the Earth is itself a magnet, and why magnetism exists needed quantum mechanics.