Magnetism

People discovered magnetism through lodestones, naturally magnetised pieces of the mineral magnetite. Chinese writers described them more than two thousand years ago, and magnetic compasses were used for navigation in China by the eleventh century and in Europe soon afterward.

In 1600 the English physician William Gilbert published De Magnete, arguing from experiments that Earth itself behaves like a giant magnet, which explains why compass needles point north.

Every magnet has a north pole and a south pole. Like poles repel and opposite poles attract. Cut a bar magnet in half and you do not get a separate north and south pole; you get two smaller magnets, each with both poles.

A magnet is surrounded by a magnetic field, which can be visualised by sprinkling iron filings around it. Field lines run from the north pole to the south pole outside the magnet, and they are closest together where the field is strongest.

Magnetism in materials comes from electrons, which behave like tiny magnets because of a quantum property called spin. In most materials these tiny magnets point in random directions and cancel out.

In ferromagnetic materials, including iron, nickel and cobalt, neighbouring atoms line up their magnetic directions in regions called domains. When the domains are aligned, the material becomes a magnet. Heating a magnet above a temperature known as the Curie point scrambles the domains and destroys its magnetism.

In 1820 Hans Christian Ørsted noticed that an electric current deflected a nearby compass needle, showing that electricity produces magnetism. Winding current-carrying wire into a coil concentrates the field, creating an electromagnet that can be switched on and off.

In 1831 Michael Faraday showed the reverse: a changing magnetic field produces an electric current. James Clerk Maxwell later combined these discoveries into a single theory of electromagnetism in the 1860s, which also showed that light is an electromagnetic wave.

• Electric motors use magnetic forces on current-carrying wires to turn electricity into motion.
• Generators use Faraday's induction to turn motion into electricity.
• Transformers use changing magnetic fields to raise or lower voltages.
• MRI scanners use very strong magnetic fields and radio waves to image the inside of the body.
• Hard drives, speakers and some high-speed maglev trains depend on magnetism.

Earth's magnetic field is generated by the motion of molten iron in its outer core, a process called the geodynamo. The field extends far into space, forming the magnetosphere, which deflects most of the charged particles streaming from the sun in the solar wind.

Where some of those particles are channelled toward the poles, they collide with the upper atmosphere and create auroras. The magnetic poles wander slowly, and the field has reversed its direction many times over Earth's history, a pattern recorded in magnetised rocks on the ocean floor that helped confirm the theory of plate tectonics.