New volumes of the Encyclopædia Britannica > Volume 30, K-MOR

MAGNETISM
444
certain value of the load for which the magnetization is
a maximum, the maximum occurring at a smaller load the
stronger the field. In very strong fields the maximum
may even disappear altogether, the effect of the smallest
stress being to diminish the magnetization; on the other
hand, with very weak fields the maximum may not have
been reached with the greatest load that the wire can
support without permanent deformation. When the load
on a hardened wire is gradually increased, the maximum
value of I is found to correspond with a greater stress
than when the load is gradually diminished, this being an
effect of hysteresis. Analogous changes are observed in
the residual magnetization which remains after the wire
has been subjected to fields of different strength. The
effects of longitudinal pressure are opposite to those of
traction; when the cyclic condition has been reached,
pressure reduces the magnetization of iron in weak fields
and increases it in strong fields (Ewing, Magnetic Induc¬
tion (1900), p. 223).
The influence of traction in diminishing the suscepti¬
bility of nickel was first noticed by Kelvin (Thomson), and
has been more recently investigated by Ewing and Cowan.
The latter found the effect to be enormous, not only upon
the induced magnetization, but in a still greater degree
upon the residual. Even under so “ moderate ” a load as
33 kilogrammes per square mm., the induced magnetiza¬
tion of a hard-drawn nickel wire in a field of 60 fell from
386 to 72 units, while the residual was reduced from
about 280 to 10. Ewing has also examined the effects
produced by longitudinal compression upon the suscepti¬
bility and retentiveness of nickel, and found, as was to be
expected, that both were greatly increased by pressure.
The maximum susceptibility of one of his bars rose from 5"6
to 29 under a stress of 19‘8 kilos per square mm. There
were reasons for believing that no Villari reversal would
be found in nickel. Ewing and Cowan looked carefully
for it, especially in weak fields, but failed to discover
anything of the kind.1 In some experiments by Heyd-
weiller (Wied. Ann. vol. lii. (1894), p. 462, and Electrician,
vol. xxxiv. p. 143) the magnetization of a nickel wire in
weak fields (corresponding to 1 = 5 or thereabouts) was
first diminished and then augmented under increasing
tensile stress • but since a cyclic condition does not appear
to have been even approximately established before the
observations were made, they can hardly be regarded as
demonstrating a true Villari effect. Nagaoka and Honda
[Phil. Mag. vol. xlvi. (1898), p. 26) found that the
magnetization of a nickel rod under feeble pulling stress
was increased by a little more than half a unit in a weak
field. The loads applied were equal to O’19 and 0‘38 kilo¬
gramme per square mm., the smallest employed by Ewing
having been 5'5. If, therefore, a Villari reversal does
occur in the case of nickel, the phenomenon is a very
inconspicuous one.
The effects of longitudinal pressure upon the magnetiza¬
tion of cobalt have been examined by Chree {Phil. Trans.
vol. elxxxi. (1890), p. 329) and also by Ewing {Magnetic
Induction (1892), p. 210). Chree’s experiments were
undertaken at the suggestion of J. J. Thomson, who, from
the results of Bidwell’s observations on the magnetic
deformation of cobalt, was led to expect that that metal
would exhibit a Villari reversal opposite in character to
that observed in iron. The anticipated reversal was duly
found by Chree, the critical point corresponding, under the
moderate stress employed, to a field of about 120 units.
1 Tomlinson found a critical point in the “temporary magnetiza¬
tion” of nickel (Proc. Phys. Soc. vol. x. (1890), pp. 367, 445), but
this does not correspond to a Villari reversal. Its nature is made
clear by Ewing and Cowan’s curves [Phil. Trans, vol. clxxix. (1888),
places 15, 16).
Ewing’s independent experiments showed that the magnet¬
ization curve for a cobalt rod under a load of 16‘2 kilos
per square mm. crossed the curve for the same rod when
not loaded at H = 53. Both observers noticed analogous
effects in the residual magnetization. Meyer ( Wied. Ann.
vol. lix. (1896), p. 134) appears to have found a Villari
effect of the opposite sign in feebly magnetized cobalt
under pulling stress (I = 3"64).
It has been shown by J. J. Thomson {Applications of
Dynamics to Physics and Chemistry, p. 47) that on dyna¬
mical principles there must be a reciprocal relation between
the changes of dimensions produced by magnetization and
the changes of magnetization attending mechanical strain.
Since, for example, stretching diminishes the magnetization
of nickel, it follows from theory that the length of a nickel
rod should be diminished by magnetization and conversely.
So, too, the Villari reversals in iron and cobalt might have
been predicted—as indeed that in cobalt actually was—
from a knowledge of the changes of length which those
metals exhibit when magnetized.2
Xagaoka and Honda {Phil. Mag. vol. xlvi. (1898), p. 261) have
investigated the effects of hydrostatic pressure upon magnetization,
using the same pieces of iron and nickel as were employed in their
experiments upon magnetic change of volume. In the iron cylin¬
der and ovoid, which expanded when magnetized, compression
caused a diminution of magnetization; in the nickel rod, which
contracted when magnetized, pressure was attended by an increase
of magnetization. The amount of the change was in both cases
exceedingly small, that in iron being less than 0T C.G.S. unit
with a pressure of 250 atmospheres and H = 54. It would hardly
be safe to generalize from these observations; the effects may
possibly be dependent upon the physical condition of the metals.
In the same paper Xagaoka and Honda describe an important
experiment on the effect of transverse stress. An iron tube,
having its ends closed by brass caps, was placed inside a compress¬
ing vessel into which water was forced until the pressure upon the
outer surface of the tube reached 250 atmospheres. The experi¬
ment was the reverse of that made by Kelvin with a gun-barrel
{Ency. Brit. vol. xv. p. 269), and the results were also the reverse.
Under increasing magnetizing force the magnetization first increased,
reached a maximum, and then diminished until its value ultimately
became less than when the iron was in the unstrained condition.
The relations between torsion and magnetization have
been discussed in Ency. Brit. vol. xv. p. 270. Some of
the curious phenomena there referred to are direct con¬
sequences of the known effects of longitudinal stress upon
magnetization and of magnetization upon form. An ex¬
planation due to Maxwell {Electricity and Magnetism,
§ 448) is given of the fact discovered by G. Wiedemann,
that if an electric current is passed through a longitudin¬
ally magnetized iron wire which is fixed at one end and
free at the other, the free end twists in a certain direction.
According to Maxwell, the wire becomes helically magnetized,
and the expansion of the iron along the lines of magnetiza¬
tion causes the twist. This explanation was not accepted
by Wiedemann {Phil. Mag., July 1886, p. 50), who thought
the effect was to be accounted for by molecular friction.
Now nickel contracts instead of lengthening when it is
magnetized longitudinally, and an experiment made by
Knott showed, as he expected, that ceteris paribus a
nickel wire twists in a sense opposite to that in which iron
twists. Further, although iron lengthens in fields of
moderate strength, it contracts in strong ones; and if the
wire is stretched, contraction occurs with smaller magnet¬
izing forces than when it is unstretched. Bidwell {Phil.
Mag., September 1886, p. 251) accordingly found upon trial
that the Wiedemann twist of an iron wire vanished when
the magnetizing force reached a certain rather high value,
and was reversed when that value was exceeded; he also
found that the vanishing point was reached with lower
2 In Ency. Brit. vol. xv. p. 271, the first footnote is equivalent to
a prediction of the changes of length in magnetized iron which were
afterwards observed.