What is magnetism?

Every magnet has two ends called poles: a north pole and a south pole. Hang a bar magnet from a thread and its north pole swings round to point north. Bring two magnets together and the rule is simple: like poles repel, unlike poles attract. North pushes north away; north pulls south in.

A magnet reaches out through empty space with a magnetic field. You can't see it, but you can map it. Sprinkle iron filings on a card and they line up into curves from pole to pole. Those curves are field lines. By custom they run out of the north pole and into the south pole. Where they crowd together the field is strong; where they spread out it is weak.

A compass is just a tiny magnet on a pin. Its needle turns to lie along the field line where it sits, north end pointing the way the line runs. Far from any magnet, it lines up with the Earth's own field and points north.

Field strength is measured in tesla (T), named after Nikola Tesla. One tesla is a lot. The Earth's field is only 25 to 65 microtesla (millionths of a tesla). A fridge magnet makes about 5 millitesla. The face of a strong neodymium magnet reaches about 0.5 T, and an MRI scanner makes 1.5 to 3 T all the way round your body. Labs measure fields with a Hall probe, a chip whose voltage changes in a field; your phone has one inside to act as a compass.

Magnetic force falls off steeply with distance. Double the gap between two magnets and the pull drops to a small fraction, often less than a quarter. That is why a magnet snaps onto the fridge at the last moment.

In 1820 Hans Christian Ørsted noticed that a compass needle swung when he switched on a current in a nearby wire. It was the first proof that electricity makes magnetism. The field of a straight wire goes round in circles around it, strongest close in: B = μ₀ I ÷ (2π r).

Which way round? Use the right-hand grip rule: grip the wire with your right hand, thumb along the current, and your curled fingers show the direction of the field.

Wind the wire into a coil and the circles from every turn add up inside it. A long coil, or solenoid, has a strong, even field inside, B = μ₀ n I, where n is the number of turns per metre and I the current. It behaves just like a bar magnet with a north and a south end. Push an iron core inside and the iron's own tiny magnets line up with the coil's field, multiplying it hundreds of times, until the iron saturates at about 1.6 T. That is an electromagnet: a magnet you can switch on, switch off and turn up.

Electromagnets are everywhere. A scrapyard crane lifts old cars with one and drops them by cutting the current. An electric bell pulls an iron hammer, which breaks its own circuit and springs back, over and over. A relay uses the same trick so a tiny current can switch a big one. A magnetic door lock holds a door shut with the force of a small car's weight, and lets go if the power fails. Motors (next chapter), the magnetron in a microwave and the MRI scanner all start here.

Put a wire carrying a current inside a magnetic field and the wire gets a push. The push is at right angles to both the field and the current, and its size is

F = B × I × L

where B is the field in tesla, I the current in amps and L the length of wire in the field, in metres. Michael Faraday built the first electric motor on this idea in 1821.

To remember the directions, use Fleming's left-hand rule. Hold your left thumb, first finger and second finger all at right angles. First finger along the Field (north to south), seCond finger along the Current, and your thuMb shows the Motion.

A DC motor puts a coil in the field. One side is pushed up, the other down, so the coil turns. After half a turn the pushes would swap and stop it, so a split-ring commutator with two carbon brushes reverses the current in the coil every half turn, and it keeps spinning the same way. As it speeds up the coil also acts as a generator and makes a back-EMF that fights the battery, so the current falls. That is why a motor draws a big current at start-up and much less at full speed.

You own dozens of these. A mixer grinder's universal motor uses brushes like this and screams along at up to about 22,000 rpm with no load (see MixerClear). A ceiling fan uses an induction motor with no brushes at all (see FanClear). An electric car uses a permanent-magnet motor packed with neodymium magnets (see CarClear). And every loudspeaker and headphone is the same trick backwards and forwards: a coil in a magnet's gap pushes a paper cone in and out with the music, as at the end of a guitar amp (see GuitarClear) or a synth (see SynthClear).

An MRI scanner is built round a giant electromagnet. Its coil is made of niobium–titanium wire cooled by liquid helium to −269 °C, where it becomes a superconductor with zero resistance, so the current goes round and round without a battery. It makes 1.5 or 3 tesla down the tunnel, tens of thousands of times the Earth's field. The field lines up the hydrogen nuclei in your body, which then wobble at a radio frequency the scanner can listen to (see MRIClear). Loose steel near the magnet can fly in like a missile, which is why you leave everything metal outside.

A microwave oven's magnetron has two ring magnets that make about 0.17 T along its axis. Electrons leaving the hot cathode are bent into curls by the field, so they sweep past a ring of copper cavities instead of flying straight to them. That swirl makes the cavities ring at 2.45 GHz, and out come microwaves (see MicrowaveClear).

An electric guitar's pickup is a magnet wrapped in thousands of turns of wire. The magnet magnetises the steel string above it; when the string wiggles, the field through the coil wiggles too, and that makes a voltage (see GuitarClear, and Faraday's law in FaradayClear).

A transformer is two coils on one iron ring. Alternating current in the first coil makes a changing field in the iron, which makes a voltage in the second. Change the turns and you change the voltage: V₂ ÷ V₁ = N₂ ÷ N₁. The home inverter in UPSClear uses one to turn 12 V from its battery into 230 V.

Hard drives and the stripe on a bank card store data as tiny magnetised patches, north one way for a 1 and the other way for a 0. A hard disk packs about a trillion bits into a square inch, each about 25 nanometres long.

The Earth is a giant magnet. Molten iron churning in the outer core makes a field that is 25 to 65 microtesla at the surface. A compass points along it. But the magnetic pole is not the geographic North Pole. The field is tilted by about 9°, and the spot where a compass needle points straight down wanders: it is now near 86°N and drifting from Canada towards Siberia by tens of kilometres a year. Oddly, the pole near geographic north is magnetically a south pole: it is what pulls in the north end of your compass.

Rocks record the past field, and they show that it has flipped hundreds of times. The last full reversal was about 780,000 years ago. Nobody can predict the next one.

The field also shields us. Far out in space it forms the magnetosphere, which turns aside the solar wind of charged particles from the Sun. Some are funnelled down near the poles, where they make the air glow as the aurora. In the strong storm of May 2024, aurora were photographed even from Hanle in Ladakh.

Why is iron magnetic? Each iron atom is a tiny magnet. In small regions called domains they all line up. In an ordinary nail the domains point every which way and cancel; a magnet nearby lines them up, which is why a nail sticks. Heat iron past its Curie point, 770 °C, and the jiggling of the atoms wins: the domains fall apart and the nail drops off the magnet.

Some materials, cooled far enough, become superconductors and push magnetic fields out. A magnet can then float above one, locked in place.

Myth: break a magnet and you get a north piece and a south piece. No: you get two smaller magnets, each with its own north and south. Nobody has ever found a lone magnetic pole. Myth: magnet bracelets heal. Careful trials find no convincing evidence. The field of a bracelet magnet fades to almost nothing a centimetre or two into your wrist, and the iron in your blood is locked inside haemoglobin and is not attracted to magnets at all.