Science Puzzle
Why Does a Red-Hot Nail Fall Off a Magnet?
Hang an iron nail from a magnet and heat the nail in a gas flame. For a while nothing changes. Then, once the nail glows bright red-hot, it suddenly drops off. Let it cool down, and the magnet will pick it up again.
Why does the red-hot nail fall off?
The Answer
Hot enough, iron stops being magnetic. At about 770 °C the heat scrambles the lined-up atoms that make iron magnetic, so the magnet has nothing to grip. Cool the nail and they line up again.
Each iron atom behaves like a tiny magnet. In iron, neighbouring atoms team up and point the same way, in regions called domains. A nail on its own has domains pointing in all directions, but bring a magnet near and the domains swing round to line up with it, so the nail is pulled in.
Heat makes atoms jiggle. As the nail gets hotter, the jiggling fights harder against the atoms' tendency to line up, and at about 770 °C for iron the jiggling wins. The atoms' tiny magnets point every which way, the iron can't be magnetised and the nail falls. Expanding doesn't make it heavier: a hot nail weighs exactly the same as a cold one.
This temperature is called the Curie point, after the French physicist Pierre Curie, who studied it in the 1890s. Every magnetic material has its own: nickel's is about 358 °C. It matters to geologists too. When lava cools through the Curie point of its magnetic minerals, around 580 °C for magnetite, they lock in the direction of Earth's magnetic field at that moment, keeping a record of it for millions of years.
The principle: The Curie point. Above a certain temperature, heat jostles a material's atoms too much for their magnetic alignment to survive, so iron stops being magnetic until it cools.