Page:A Treatise on Electricity and Magnetism - Volume 2.djvu/117

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447.]
85
MAGNETISM AND TORSION.

If the force X2 acts in the negative direction it will begin to produce a permanent effect as soon as θ2 + β2 becomes less than θ1 + β0, that is, as soon as X2 becomes greater than X1. But if X2 acts in the positive direction it will begin to remagnetize the iron as soon as θ2 + β becomes less than θ1 + β0, that is, when θ2 = θ1 + 2 β0, or while X2 is still much less than X1.

It appears therefore from our hypothesis that—

When a piece of iron is magnetized by means of a force X0, its magnetism cannot be increased without the application of a force greater than X0. A reverse force, less than X0, is sufficient to diminish its magnetization.

If the iron is exactly demagnetized by a reversed force X1, then it cannot be magnetized in the reversed direction without the application of a force greater than X1, but a positive force less than X1 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

  1. Phil. Mag., 1833.
  2. Pog., Ann., 1834.
  3. Ann. de Chimie et de Physique, 1846.
  4. Phil. Trans., 1855, p. 287
  5. Ann. de Chimie et de Physique, 1858.