A completely satisfactory theory of magnetism has still not been formulated. At present it is assumed that magnetism is due to the movement of the electrons. The electrons circulate around the nucleus of the atom. They also spin around an axis through their centres. These two movements set up magnetic field about the electrons. As a result, the atoms act as magnets. Now ferromagnetic metals like iron, cobalt etc. have crystal structure and appear to have minute regions showing strong resultant magnetism. These regions are called 'domains'. In each domain the atomic magnets are in alignment as shown in figure.
It will be noticed that different domains have magnetic axes in random directions. As a result, the metal as a whole does not show any magnetic property. When the iron bar is magnetized, the atomic magnets turn to bring the domain magnetic axes in direction of magnetizing field. When the process of magnetization is complete, the magnetic axes of all the domains is brought into the best possible alignment and the metal is said to be magnetically saturated.
