One other effect of torsion remains to be noticed. If a longitudinally magnetized wire is twisted, circular magnetization is developed; this is evidenced by the transient electromotive force induced in the iron, generating a current which will deflect a galvanometer connected with the two ends of the wire. The explanation given of the last described phenomenon will with the necessary modification apply also to this; it is a consequence of the aeolotropy produced by the twist. There are then three remarkable effects of torsion:
A. A wire magnetized longitudinally and circularly becomes twisted.
B. Twisting a circularly magnetized wire produces longitudinal magnetization.
C. Twisting a longitudinally magnetized wire produces circular magnetization.
And it has been shown earlier that—
D. Magnetization produces change of length.
E. Longitudinal stress produces change of magnetization.
Each of these five effects may occur in two opposite senses. Thus in A the twist may be right-handed or left-handed; in B the polarity of a given end may become north or south; in C the circular magnetization may be clockwise or counter-clockwise; in D the length may be increased or diminished; in E the magnetization may become stronger or weaker. And, other conditions remaining unchanged, the “sense” of any effect depends upon the nature of the metal under test, and (sometimes) upon the intensity of its magnetization. Let each of the effects A, B, C, D and E be called positive when it is such as is exhibited by moderately magnetized iron, and negative when its sense is opposite. Then the results of a large number of investigations may be briefly summarized as follows:
| (W) = weakly magnetized. | (S) = strongly magnetized. | ||
| Metal. | Effects. | Sign. | |
| Iron (W) | A, B, C, | D, E | + |
| Unannealed Cobalt (S) | A, | D, E | + |
| Nickel-Steel (W) | A, | D, E | + |
| Nickel | A, B, C, | D, E | − |
| Annealed Cobalt | D, E | − | |
| Iron (S) | A, C, | D, E | − |
| Unannealed Cobalt | A, | D, E | − |
Several gaps remain to be filled, but the results so far recorded can leave no doubt that the five effects, varied as they may at first sight appear, are intimately connected with one another. For each of the metals tabulated in the first column all the effects hitherto observed have the same sign; there is no single instance in which some are positive and others negative. Until the mysteries of molecular constitution have been more fully explored, perhaps D may be most properly regarded as the fundamental phenomenon from which the others follow. Nagaoka and Honda have succeeded in showing that the observed relations between twist and magnetization are in qualitative agreement with an extension of Kirchhoff’s theory of magnetostriction.