Chapter 3 · Magnetism: poles, field lines and the compass
Magnets and the Earth's field
Poles and field lines
A lump of lodestone, a natural magnet made of the mineral magnetite, sits in the scene while iron filings settle around it. They line up in curves that run from one end, the north pole, round to the other, the south pole. Those curves are the field lines, and the field is strongest where they crowd together. Cut a magnet in half and you do not get a lone north and a lone south; you get two smaller magnets, each with both poles. A single isolated magnetic pole has never been observed.
Magnets meet magnets
Bring two magnets together. North against north pushes apart, north against south pulls together: like poles repel and opposite poles attract. The force rises quickly as the gap shrinks. A typical fridge magnet makes a field of about 5 mT at its surface, while the Earth's field at the ground is about 50 µT, and it varies between roughly 25 and 65 µT. Divide: 5 × 10⁻³ / 50 × 10⁻⁶ = 100. The fridge magnet is about a hundred times stronger than the planet, but only within a few centimetres, because its field fades fast with distance.
The compass
A magnetised needle on a pivot swings to point roughly north. Chinese writers describe magnetised needles in the 11th century, and Shen Kuo's Dream Pool Essays, about 1088, notes that such a needle does not point exactly north and south. Ships carried compasses soon after. Earth itself is a magnet, and scientists think its field is made by moving liquid iron in the outer core. A compass finds magnetic north, which is not the geographic north of the spin axis. The gap is called declination: at 10°, a 5 km march along the compass bearing ends 5 × tan 10° ≈ 0.9 km to the side.
Domains inside iron
Zoom into iron and the scene shows tiny regions called domains, each a crowd of atoms whose own small magnets point the same way. In an ordinary nail the domains point every which way and cancel out. A magnet is iron whose domains have lined up. Hammering or heating shakes them out of line. Above the Curie point, which Pierre Curie studied in the 1890s, iron stops being magnetic. For iron it is about 770 °C, which is 770 + 273.15 ≈ 1043 K. Once cooled, the domains usually do not line up again by themselves.
A shield around the planet
Earth's field reaches far into space and forms the magnetosphere. It bends the charged particles of the solar wind around the planet. Scientists think Mars, which lost its global field long ago, also lost much of its air to the solar wind. Some particles spiral down the field lines near the poles and meet the thin upper air, where they make the glowing curtains of the aurora. In the scene the field lines wrap round the globe like the skin of an onion, with a stream of particles sliding past.
Stripes in the rock
Lava that cools on the seabed locks in the direction of the field at that moment. Maps of the ocean floor made in the 1950s and 1960s showed stripes of rock magnetised in alternating directions on either side of mid-ocean ridges. The best explanation, published in 1963 by Fred Vine and Drummond Matthews and independently by Lawrence Morley, is that the field has reversed many times while the seabed spread like two conveyor belts. The last full reversal was about 780,000 years ago. At 2 cm a year the seabed has moved 0.02 × 780,000 = 15,600 m, about 16 km, from the ridge since then.
In numbers
- ≈ 770 °C
- the Curie point of iron, where it loses magnetism
- ≈ 50 µT
- the Earth's field at the ground (25–65 µT)
- ≈ 780,000 yr
- since the last full reversal of the field