Magnetism is the name of the force field generated by magnets. Through it magnets attract certain metals from a distance, making them move closer without any apparent cause. It is also the means by which magnets affect each other. All magnets have two poles, called the “north” and “south” poles. Like magnetic poles attract each other, while unlike magnetic poles push each other away. There are many different kinds of magnets with a great variety of levels of strength. Some magnets are barely strong enough to hold paper to a refrigerator. Others are strong enough to lift cars.

To understand what makes magnets strong you must understand something of the history of the science of magnetism. In the early 19th century, the existence of magnetism was well-known, as was the existence of electricity. These were generally thought of as two wholly separate phenomena. However, in 1820, the physicist Hans Christian Oersted proved that electric currents generate magnetic fields. Soon after, in 1855, another physicist, Michael Faraday, proved that changing magnetic fields could generate electric currents. Thus it was shown that electricity and magnetism are part of the same phenomenon.

All matter is made of atoms, and all atoms are made of tiny electric charges. At the center of each atom sits the nucleus, a small dense clump of matter with a positive electric charge. Surrounding each nucleus is a slightly larger cloud of negatively charged electrons, held in place by the electrical attraction of the atom's nucleus.

Electrons are constantly on the move. They are spinning as well as moving around the atoms they are a part of, and some electrons even move from one atom to another. Each moving electron is a tiny electric current, because an electric current is just a moving electric charge. Therefore, as Oersted showed, each electron in each atom generates its own tiny magnetic field.

In most materials these tiny magnetic fields point in many different directions and therefore cancel each other out, according to Kristen Coyne of the National High Magnetic Field Laboratory. North poles are next to south poles as often as not, and the net magnetic field of the whole object is close to zero.

When some materials are exposed to an external magnetic field, this picture changes. The external magnetic field forces all of those little magnetic fields to line up. Its north pole pushes all the little north poles in the same direction: away from it. It pulls all the little magnetic south poles towards it. This makes the tiny magnetic fields inside the material add their effects together. The result is a strong net magnetic field in the object as a whole.

The more powerful the external magnetic field that is applied, the greater the magnetization that results. This is the first of the factors that determines how strong a magnet becomes. The second is the type of material the magnet is made of. Different materials produce magnets of different strengths. Those with a high magnetic permeability (which is a measurement of how responsive they are to magnetic fields) make the strongest magnets. For this reason, pure iron is used to make some of the strongest magnets.