1.03 · The Magnet in Your Pocket

The Magnet in Your Pocket
An invisible force is working for you all day. Here’s where to find it.

You’re carrying a compass right now. Inside almost every phone, there’s a sensor, smaller than a grain of rice, that measures the Earth’s magnetic field and tells your map which way you’re facing. It works because our planet behaves like a giant magnet. Deep inside the Earth, the outer core is liquid metal, mostly iron, and its constant movement produces a magnetic field around the whole planet. If a magnet can turn freely, it lines up with this field, and one end points north. Sailors have trusted that simple fact for more than 800 years.

So what makes a material magnetic? Here’s the key point: in iron, each atom behaves like a tiny magnet. Usually, groups of these atoms point in different directions, so their forces cancel out. But when a strong magnet comes near, the groups turn to face the same way, and the iron becomes a magnet. Heat can undo all this: if you heat iron to about 770 °C, the tiny magnets point in all directions again, and the magnetism disappears. This temperature is called the Curie point, after the French scientist Pierre Curie.

The most useful magnets are the ones you can switch on and off. Wrap a copper wire around an iron nail, connect the wire to a battery, and the nail becomes a magnet. Disconnect the battery, and most of the force disappears at once. This is called an electromagnet, and industry couldn’t work without it. In a junkyard, a crane lifts an old car with the current on and drops it the moment the current goes off. At home, you’ll find at least three more: in a traditional doorbell, in a washing machine and in your speakers.

In hospitals, magnetism saves lives without touching anyone. An MRI scanner uses a magnetic field thousands of times stronger than the Earth’s. MRI stands for magnetic resonance imaging. Together with pulses of radio waves, the field makes hydrogen atoms in your body send back tiny signals. A computer turns these signals into detailed images of the brain, muscles and organs, without surgery or X-rays. The field is so strong that patients must leave everything made of metal outside the room. Provided that they do this, and tell the staff about any metal inside their body, the scan is very safe.

Engineers are pushing magnetism further. Since 2004, a maglev train, short for “magnetic levitation,” has carried passengers between Pudong Airport and a subway station in eastern Shanghai. It floats just above its track, lifted and pushed forward by magnetic fields. Until 2021, it carried passengers at up to 431 kilometers per hour. With no wheels touching the track, there’s almost no friction to slow it down. So next time your map shows you the way, remember: the force that guides your map also lifts cars, looks inside the human body and makes trains float.