What is a good permanent magnet? We need a magnet with a very concentrated magnetic field. We also need a magnet that can maintain magnetism. To understand how we measure the strength and permanence of permanent magnets, it is helpful to imagine how we manufacture magnets.

We start with a piece of iron or another ferromagnetic material. When we find it, we assume that it is not magnetized. To magnetize if, we place it in a magnetic field generated by another magnet. We will place our magnetic field in a magnetic field generated by an electromagnet, so that we can change the strength of the magnetic field and see what happens to iron.

When the magnetic field intensity is low, the number of force lines concentrated in iron is also small. As the magnetic field strength increases, more small magnets (called magnetic domains) in iron align with the external magnetic field. The number of force lines in iron increases with the strength of the magnetic field. This will continue until all magnets in the iron align with the external magnetic field. At this point, we say that iron is saturated. It cannot accommodate any more power lines. When we increase the strength of the outfield, we cannot see a further increase in the number of force lines in the iron.

Assuming we start reducing the intensity of the external electric field now. At room temperature, the heat inside iron compresses the small magnetic domains inside, some of which are readjusted to be inconsistent with the external magnetic field. But some areas still remain in place and remain consistent with the field. When we completely remove the external magnetic field, these stuck magnetic domains will be left behind, and we have a permanent magnet.

Assuming we now reverse the external magnetic field and gradually increase its strength. At first, the trapped magnetic domain resisted rotation to align with the external magnetic field, and the number of force lines concentrated in the iron changed slowly. When the external magnetic field gradually submerges these stuck areas, they will flip over. In the end, the external magnetic field is strong enough to flip them over and the iron saturates again (but the poles are now reversed).