In the construction of this soft-iron instrument it is essential that the fragment of iron should be as small and as well annealed as possible and not touched with tools after annealing; also it should be preferably not too elongated in shape so that it may not acquire permanent magnetization but that its magnetic condition may follow the changes of the current in the coil.
Ferromagnetism was explained by Ampere on the hypothesis that the magnetization of the molecule is due to an electric current constantly circulating within it.
The general character of curves of magnetization and of induction will be discussed later.
The magnetization retained by a long thin rod, even when its coercive force is small, is sometimes little less than that which was produced by the direct action of the field.
In anisotropic bodies, such as crystals, the direction of the magnetization does not in general coincide with that of the magnetic force.
There are, however, always three principal axes at right angles to one another along which the magnetization and the force have the same direction.
Principal axes of the crystal, the actual magnetization will be the resultant of the three magnetizations along the axes.
A magnet consisting of a series of plane shells of equal strength arranged at right angles to the direction of magnetization will be uniformly magnetized.
The availability of the energy of magnetization is limited by the coercive force of the magnetized material, in virtue of which any change in the intensity of magnetization is accompanied by the production of heat.
It can be shown that if a current i circulates in a small plane circuit of area S, the magnetic action of the circuit for distant points is equivalent to that of a short magnet whose axis is perpendicular to the plane of the circuit and whose moment is iS, the direction of the magnetization being related to that of the circulating current as the thrust of a right-handed screw to its rotation.