Theory of Magnetism
The effects of magnetism have been known and used for centuries. Yet scientists still do not know exactly what magnetism is. The theory of magnetism that follows is based on one proposed by Pierre Weiss, a French physicist, in the early 20th century.
Every magnetic substance contains domains, groups of molecules that act as magnets. Before a substance is magnetized, these domains are arranged randomly, so that the magnetism of one is cancelled by the magnetism of another. When the substance is brought within a magnetic field, the domains line up parallel to the lines of force, with all the N poles facing in the same direction.
When the magnetic field is removed, the like poles tend to repel each other. In a substance that is easily magnetized, the domains turn easily, and will return to random ordering. In a substance that is difficult to magnetize, the domains will not have enough force to disarrange themselves and the substance will remain magnetized. In modern versions of this theory, the magnetism of the domains is attributed to the spin of electrons.