Magnetism and electromagnetism

A magnet can move a paperclip without touching it. The space around a magnet, where iron or another magnet feels a push or pull, is its magnetic field.

A field cannot be seen, but it can be mapped. Lay a sheet of paper over a bar magnet, sprinkle iron filings on top and tap the paper. The filings line up in curves that leave one pole, loop round and return to the other. These curves are called field lines. Where they crowd together, at the poles, the field is strongest.

The field also weakens quickly with distance. Far from a small magnet, doubling the distance cuts the field to about an eighth. That is why a paperclip lies still until a magnet comes close, and then jumps the last few millimetres.

A fridge magnet clings to a steel door but slides off aluminium or copper. Only a few metals are strongly drawn to a magnet. The commonest by far is iron, the main ingredient of steel, and nickel and cobalt are two of the others. Iron answers so strongly because each of its atoms is itself a tiny magnet, thanks to its electrons.

In a piece of iron, neighbouring atoms line up together in small regions called domains, most of them far smaller than a millimetre. In an ordinary nail the domains point every which way, and their pulls cancel. Bring a strong magnet near, or stroke the nail with one, and they come into line and point the same way, so the nail becomes a magnet itself. Heat undoes this. Above 770 °C iron’s atoms jostle too hard to stay in line, and it stops being magnetic.

Why does a hanging magnet turn to point north at all? In 1600 the English physician William Gilbert answered in his book De Magnete. He shaped a ball of lodestone, a rock that is naturally magnetic, and carried a small needle over its surface. The needle turned and tilted just as compass needles do in different parts of the world. The Earth itself, he concluded, is a giant magnet.

A compass needle is a small magnet free to turn, and it lines up with the Earth’s field. Its north pole is drawn towards the Arctic. Since opposite poles attract, the magnetic pole in the Arctic must really be a south pole, whatever the maps call it. It lies a few hundred kilometres from the geographic North Pole, and it drifts tens of kilometres a year.

A needle rubbed with lodestone becomes a magnet. Mounted to turn freely, it is a compass, and with one a crew can hold a course out of sight of land, with no sun or stars to steer by.

The earliest clear accounts come from China. In 1088 the scholar Shen Kuo described a needle rubbed with lodestone that points south, and a Chinese book written between 1111 and 1117 records ships’ pilots steering by one in dark weather. In Europe, the English scholar Alexander Neckam described sailors’ needles around 1190. In the Muslim world the first secure mention is in a Persian book of stories from about 1232, and in 1282 Baylak al-Qibjāqī described a needle floated in a bowl of water on a voyage from Tripoli in Syria to Alexandria in 1242. Whether the compass spread west from China or was invented again in Europe is still argued.

Muslims face the Kaʿba in Mecca in every prayer, and that direction is the qibla. From the ninth century, Muslim astronomers worked out the qibla of each city from its latitude and longitude and those of Mecca, and gave it as an angle from the north–south line. On the ground, that line still had to be found.

The compass offered a quick way. Around 1290 al-Ashraf ʿUmar, a prince of Yemen’s Rasulid dynasty who later reigned briefly as sultan, ended his book on the astrolabe with a chapter on a compass bowl. A steel needle, its tip rubbed with a magnetic stone, floats on water in a bowl marked round its rim, and settles roughly along the north–south line. From it the qibla angle is counted off. His is the earliest known account of a compass used to find the qibla.

For centuries electricity and magnetism were studied as two separate things. In 1820 the Danish physicist Hans Christian Ørsted passed a current through a wire held just above a compass. The needle swung away from north. When he reversed the current it swung the other way, and when he stopped the current the needle swung back. An electric current, with no magnet anywhere near, makes a magnetic field of its own.

The field has an unexpected shape. It does not point towards or away from the wire but wraps around it in circles, which is why the needle turned sideways. Ørsted’s short paper was read across Europe within months, and electricity and magnetism came to be seen as one force, electromagnetism, now counted among the fundamental forces of nature.

The field around one straight wire is weak, but it can be gathered. Wind the wire into a coil and the circles of field from every turn add together, so the coil acts like a bar magnet, with a north and a south pole. Put a bar of iron inside and the iron’s domains swing into line with the coil’s field, making it up to thousands of times stronger. This is an electromagnet. In 1824 the English experimenter William Sturgeon wound wire round a varnished iron horseshoe weighing about 200 grams and lifted about 4 kilograms with it.

An electromagnet has one great advantage over a permanent magnet: its pull can be switched off. A scrapyard crane lifts a wrecked car with one, swings it over a pile and cuts the current, and the car drops.

If a current makes a magnetic field, a wire carrying current should feel a push when it lies in the field of a magnet, and it does. In 1821 Michael Faraday used that push to make a wire carrying current swing round and round the pole of a magnet. It was the first electric motor, a machine that turns electrical energy into motion.

The simplest modern motor puts a coil of wire between the poles of a magnet. Current makes the coil a magnet too, and the pull between its poles and the magnet’s turns it. Left alone, the coil would stop after half a turn, facing the poles it is drawn to. So a rotating switch called a commutator reverses the current at every half turn, and the coil keeps going. Motors in fans, pumps, lifts and trains use more than 40 per cent of the world’s electricity.

If a current can make magnetism, can magnetism make a current? In 1831 Faraday found that it can, though not in the way anyone expected. A magnet resting inside a coil of wire does nothing at all. Push it in, and a meter joined to the coil flicks one way; pull it out, and it flicks the other. Current flows only while the magnet is moving.

What matters is change. When the magnetic field passing through a coil grows or shrinks, it drives a current around the coil. This is electromagnetic induction, which the American physicist Joseph Henry also found at about the same time. Moving the coil instead of the magnet works just as well. Induction charges a phone laid on a charging pad with no plug, and it makes nearly all of the world’s electricity.

A generator puts induction to work. It turns a coil between the poles of a magnet, or spins a magnet inside fixed coils, so the field through the coils keeps changing and current keeps flowing. Steam from burning fuel or a nuclear reactor, falling water or wind does the turning. In 2024 generators made about 93 per cent of the world’s electricity, and solar panels nearly all the rest.

As the coil turns, each side passes first one pole and then the other, so the current reverses every half turn. This is alternating current, and Europe’s power stations keep it swinging back and forth 50 times a second.

Current drawn from the coil makes a field of its own that opposes the turning, so the more lamps are lit, the harder the generator is to turn. Every unit of electrical energy it gives out is paid for with work.

Alternating current has one great advantage: its voltage is easy to change. A transformer is two coils of wire wound on one iron core. The alternating current in the first coil makes a field that keeps changing, and the changing field induces a current in the second. Give the second coil ten times as many turns and the voltage comes out ten times higher. A battery’s steady current makes a steady field, which induces nothing.

Why raise the voltage? The power a line carries is its voltage times its current, so a tenfold rise in voltage lets the same power flow with a tenth of the current. Less current means far less energy lost as heat in the wires. In Britain, power stations generate at about 25,000 volts, transformers raise it to 400,000 for long-distance lines, and others lower it to 230 for homes.

Gilbert pictured the Earth as a magnetised ball, but it cannot be one. The Earth’s core is mostly iron, yet it is thousands of degrees hotter than the 770 °C at which iron stops being magnetic. It is too hot to hold a magnet’s lined-up order.

The field is made instead by a dynamo. From about 2,900 kilometres down, the outer core is liquid iron, stirred as heat rises through it and swirled by the Earth’s spin. Molten iron carries electricity, so as it flows through the magnetic field, currents are induced in it, as in a generator, and those currents make the field that keeps the process going.

A field made this way is not fixed. Old lavas record that it has reversed many times, north swapping places with south, at irregular intervals. The last reversal came about 780,000 years ago.

Faraday had shown that a changing magnetic field drives a current. In the 1860s the Scottish physicist James Clerk Maxwell gathered all that was known of electricity and magnetism into one set of equations, and added the reverse: a changing electric field makes a magnetic one. Then each field can keep the other going, and a ripple of the pair can travel out through empty space.

Maxwell worked out how fast such a ripple should travel, using only numbers measured in experiments on electricity and magnetism. It came out at about 300,000 kilometres a second, the speed of light. Light itself, he concluded, is an electromagnetic wave. He died in 1879, before anyone had made such waves on purpose. In 1887 the German physicist Heinrich Hertz did, sending invisible waves from a spark across his laboratory. Today we call them radio waves.