If the force *X*_{2} acts in the negative direction it will begin to produce a permanent effect as soon as θ_{2} + β_{2} becomes less than θ_{1} + β_{0}, that is, as soon as *X*_{2} becomes greater than *X*_{1}. But if *X*_{2} acts in the positive direction it will begin to remagnetize the iron as soon as θ_{2} + β becomes less than θ_{1} + β_{0}, that is, when θ_{2} = θ_{1} + 2 β_{0}, or while X_{2} is still much less than *X*_{1}.

It appears therefore from our hypothesis that—

When a piece of iron is magnetized by means of a force *X*_{0}, its magnetism cannot be increased without the application of a force greater than *X*_{0}. A reverse force, less than *X*_{0}, is sufficient to diminish its magnetization.

If the iron is exactly demagnetized by a reversed force *X*_{1}, then it cannot be magnetized in the reversed direction without the application of a force greater than *X*_{1}, but a positive force less than *X*_{1} is sufficient to begin to remagnetize the iron in its original direction.

These results are consistent with what has been actually observed by Ritchie^{[1]}. Jacobi^{[2]}, Marianini^{[3]}, and Joule^{[4]}.

A very complete account of the relations of the magnetization of iron and steel to magnetic forces and to mechanical strains is given by Wiedemann in his *Galvanismus*. By a detailed comparison of the effects of magnetization with those of torsion, he shews that the ideas of elasticity and plasticity which we derive from experiments on the temporary and permanent torsion of wires can be applied with equal propriety to the temporary and permanent magnetization of iron and steel.

447.] Matteucci^{[5]} found that the extension of a hard iron bar during the action of the magnetizing force increases its temporary magnetism. This has been confirmed by Wertheim. In the case of soft bars the magnetism is diminished by extension.

The permanent magnetism of a bar increases when it is extended, and diminishes when it is compressed.

Hence, if a piece of iron is first magnetized in one direction, and then extended in another direction, the direction of magnetization will tend to approach the direction of extension. If it be compressed, the direction of magnetization will tend to become normal to the direction of compression.

This explains the result of an experiment of Wiedemann's. A