the current passes from the fixed to the free end, then the direction
of the resulting magnetization will be such as to make the
free end a north pole. The twist effect exhibited by iron under
moderate longitudinal magnetization has been called by Knott
a positive Wiedemann effect; if the twist were reversed, the other
conditions remaining the same, the sign of the Wiedemann effect
would be negative. An explanation of the twist has been given
by Maxwell (Electricity and Magnetism, § 448). The wire is
subject to two superposed magnetizations, the one longitudinal,
the other circular, due to the current traversing the wire; the
resultant magnetization is consequently in the direction of a
screw or spiral round the wire, which will be right-handed or
left-handed according as the relation between the two magnetizations
is right-handed or left-handed; the magnetic expansion
or contraction of the metal along the spiral lines of magnetization
produces the Wiedemann twist. Iron (moderately magnetized)
expands along the lines of magnetization, and therefore for a
right-handed spiral exhibits a right-handed twist. This explanation
was not accepted by Wiedemann,[1] who thought that the
effect was accounted for by molecular friction. Now nickel
contracts instead of lengthening when it is magnetized, and an
experiment by Knott showed, as he expected, that caeteris
paribus a nickel wire twists in a sense opposite to that in which
iron twists. The Wiedemann effect being positive for iron is
negative for nickel. Further, although iron lengthens in fields
of moderate strength, it contracts in strong ones; and if the wire
is stretched, contraction occurs with smaller magnetizing forces
than if it is unstretched. Bidwell[2] accordingly found upon trial
that the Wiedemann twist of an iron wire vanished when the
magnetizing force reached a certain high value, and was
reversed when that value was exceeded; he also found that the
vanishing point was reached with lower values of the magnetizing
force when the wire was stretched by a weight. These
observations have been verified and extended by Knott, whose
researches have brought to light a large number of additional
facts, all of which are in perfect harmony with Maxwell’s
explanation of the twist.
Maxwell has also given an explanation of the converse effect,
namely, the production of longitudinal magnetization by twisting
a wire when circularly magnetized by a current passing through
it. When the wire is free from twist, the magnetization at any
point P is in the tangential direction PB (see fig. 26).
Fig. 26.
Suppose the wire to be fixed at the top and twisted
at the bottom in the direction of the arrow-head T;
then the element of the wire at P will be stretched
in the direction Pe and compressed in the direction
Pr. But tension and compression produce opposite
changes in the magnetic susceptibility; if the metal
is iron and its magnetization is below the Villari
critical point, its susceptibility will be greater along
Pe than along Pr; the direction of the magnetization
therefore tends to approach Pe and to recede
from Pr, changing, in consequence of the twist,
from PB to some such direction as PB′, which has a vertical
component downwards; hence the lower and upper ends will
respectively acquire north and south polarity, which will disappear
when the wire is untwisted. This effect has never been
actually reversed in iron, probably, as suggested by Ewing,
because the strongest practicable circular fields fail to raise the
components of the magnetization along Pe and Pr up to the
Villari critical value. Nagaoka and Honda have approached
very closely to a reversal, and consider that it would occur if
a sufficiently strong current could be applied without undue
heating.
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.
The effects of magnetization upon the torsion of a previously twisted wire, which were first noticed by Wiedemann, have been further studied by F. J. Smith[3] and by G. Moreau.[4] Nagaoka[5] has described the remarkable influence of combined torsion and tension upon the magnetic susceptibility of nickel, and has made the extraordinary observation that, under certain conditions of stress, the magnetization of a nickel wire may have a direction opposite to that of the magnetizing force.
8. Effects of Temperature upon Magnetism
High Temperature.—It has long been known that iron, when raised to a certain “critical temperature” corresponding to dull red heat, loses its susceptibility and becomes magnetically indifferent, or, more accurately, is transformed from a ferromagnetic into a paramagnetic body. Recent researches have shown that other important changes in its properties occur at the same critical temperature. Abrupt alterations take place in its density, specific heat, thermo-electric quality, electrical conductivity, temperature-coefficient of electrical resistance, and in some at least of its mechanical properties. Ordinary magnetizable iron is in many respects an essentially different substance from the non-magnetizable metal into which it is transformed when its temperature is raised above a certain point (see Brit. Assoc. Report, 1890, 145). The first exact experiments demonstrating the changes which occur in the permeability of iron,
