English: An illustration showing one of the first methods of magnetizing iron, from one of the earliest scientific works on magnetism: Die Magnete written by William Gilbert in 1600. A blacksmith heats a piece of iron red hot, then holds it with its long axis in a north-south direction and hammers it as it cools. The magnetic field of the Earth would align the magnetic domains of the iron as it cooled through its Curie temperature, leaving the iron a weak magnet.