How Does a Compass Work? A Needle, the Earth's Core and a Pole That Wanders
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The Earth is a magnet, weak but enormous, and a sliver of magnetised steel balanced on a pin will swing to lie along its field, which is why a compass points north. That much was known to Chinese diviners by the first century AD and to Chinese and Mediterranean sailors by the twelfth, a thousand years before anyone knew why the Earth should be magnetic at all. The answer lies 3,000 kilometres down, in an ocean of liquid iron, and the field it makes is neither fixed nor permanent: the magnetic pole moves tens of kilometres a year, and every few hundred thousand years the whole field turns over.
The needle
A compass needle is a small permanent magnet, with a north-seeking pole and a south-seeking pole, pivoted so that it can turn freely. Placed in a magnetic field it experiences a torque that turns it until its own field lines up with the external one, and it settles pointing along the field. In the Earth's field that means roughly north, and the end of the needle that points north is, strictly, a south magnetic pole, since opposite poles attract, which is a confusion of naming that has been tolerated for centuries. The needle also tries to tip downward, following the field's dip towards the ground, which in Britain is about 67 degrees, and compasses are balanced with a small weight for the latitude they will be used in.
The dynamo
The Earth's field is not made by a lump of magnetised rock at the centre; the core is far above the temperature at which iron loses its magnetism. It is made by motion. The outer core is a shell of liquid iron and nickel 2,200 kilometres thick, kept churning by heat escaping from the solid inner core and by the Earth's rotation, which twists the flow into columns aligned with the axis. Moving molten metal in a magnetic field generates electric currents, and those currents generate a magnetic field that sustains the flow's own field, a self-exciting dynamo of the kind that also powers the Sun and Jupiter. The field at the surface is about a twentieth of that of a fridge magnet, and it is what deflects the solar wind around the planet and makes the auroras.
Why north is not north
The magnetic pole does not coincide with the geographic one, and the difference between where a compass points and true north is called declination or variation:
- •The north magnetic pole was in the Canadian Arctic when it was first reached in 1831, and has since drifted about 2,000 kilometres towards Siberia, accelerating to over 50 kilometres a year in the 2000s before slowing
- •Declination varies from place to place and year to year; in London it was 24 degrees west in 1820, zero around 2020, and is now creeping east
- •Maps print the declination and its annual change so that a bearing taken from the map can be corrected for the compass, and pilots repaint runway numbers as the magnetic bearing drifts
- •Local iron ore, steel hulls, and the electronics in a phone all deflect a needle, which is why ships carried correcting magnets and why a phone's compass asks to be waved in a figure of eight to calibrate
When it flips
Rocks record the field they cooled in, and the record shows that the Earth's magnetic north and south have swapped places hundreds of times, most recently about 780,000 years ago, at irregular intervals averaging a few hundred thousand years. A reversal takes a few thousand years, during which the field weakens and breaks into several poles before re-forming the other way up; the striped pattern of magnetisation on the ocean floor, symmetrical about the mid-ocean ridges, was the evidence that confirmed sea-floor spreading in the 1960s. The field has weakened by about nine percent since 1840, and a patch over the South Atlantic where it is especially weak lets more radiation through to satellites, but whether a reversal is beginning nobody can say; the field has dipped and recovered before. Life has survived every previous flip, and a compass would still work during one, if pointing somewhere unhelpful.
Compasses now
The magnetic compass has been joined rather than replaced. Ships and aircraft use the gyrocompass, a spinning wheel that aligns itself with the Earth's rotation axis and so finds true north, unaffected by iron or by the field's wandering; satellite positioning gives direction from movement; and the compass in a phone is a magnetometer, a chip that measures the field in three directions and combines it with the accelerometer to draw an arrow. The needle on a card is still carried in every lifeboat and by every hillwalker, because it needs no power, no satellites and no calibration beyond a glance at the map's margin, and because a field generated by 3,000 kilometres of flowing iron is not something that can be switched off.
The takeaway
A compass works because its needle is a magnet that turns to align with the Earth's magnetic field, which is generated by the churning of molten iron in the outer core acting as a self-sustaining dynamo. The magnetic pole sits away from the geographic pole and wanders by tens of kilometres a year, so a bearing must be corrected by the local declination; the field has reversed hundreds of times over geological history and is weakening now, and the needle, which needs no power, still rides alongside the gyrocompass and the satellite.