Why a Needle Knows North

A planet with a dynamo inside it

About 2,900 km beneath your feet the Earth's outer core is liquid iron at roughly 5,000°C. It convects — hot metal rises, cools, sinks — and because the planet is spinning, those currents are twisted into rolls. Moving molten metal carries electric current, and electric current makes a magnetic field, which in turn keeps the currents flowing. This self-sustaining loop is called the geodynamo, and it is the reason the Earth has a magnetic field at all.

The field is feeble — about 50 microtesla at the surface, some 200 times weaker than a fridge magnet. But it fills the whole space around the planet, and it has done so, on and off, for at least 3.5 billion years.

Every atom is already a magnet

Inside a piece of iron, each atom behaves as a tiny magnet — a dipole, with a north end and a south end. This comes from unpaired electrons: an electron has an intrinsic magnetic moment called spin, and in most materials those moments pair off and cancel. Iron, cobalt and nickel are unusual in having unpaired electrons whose moments line up with their neighbours' over small regions called domains.

In an ordinary nail, the domains point in every direction and their fields cancel. The nail is not a magnet. Bring a real magnet near, or cool the iron slowly inside a magnetic field, and the domains rotate into agreement. Now the little fields add instead of cancelling, and the iron is magnetised.

Heat undoes it

Above a particular temperature — 770°C for iron, called the Curie point after Pierre Curie — thermal jostling overwhelms the alignment and the domains scramble. The magnet dies. Cool it back down inside a field and it re-forms. This is not a curiosity: it is how the sea floor recorded the Earth's magnetic history. Lava erupting at mid-ocean ridges cools through the Curie point and freezes the field direction of that moment into the rock. Reading those stripes outward from the ridge is how we know the field has reversed hundreds of times.

And the compass

Hang a magnetised needle so it can turn freely and it rotates until its own field agrees with the planet's. That is all a compass is. Two honest complications: the needle points to magnetic north, not the geographic pole — the difference is called declination and varies by where you stand; and the field also dips into the ground, steeply near the poles, which is why a needle balanced for one hemisphere drags in the other.

Your phone has no needle. It uses a magnetometer — typically a Hall-effect or magnetoresistive sensor, a few square millimetres of silicon — measuring the field along three axes, and combines that with the gyroscope and accelerometer to work out which way it is being held. The question it asks the world is exactly the one the needle asked.