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

MAGNETISM
442
H0 — 400, recovered its original length at H0 = < 50 ;
beyond this point there was extension, the amount of
which was still increasing fast when the experiment was
stopped at H0 = 1400. Similar results were obtained with
three different samples of the metal. Roughly speaking,
therefore, cobalt behaves oppositely to iron.
Joule and others experimented with hardened steel, but
failed to find a key to the results they obtained, which are
rather complex, and have been thought to be inconsistent.
The truth appears to be that a hardened steel rod generally
behaves like one of iron or soft steel in first undergoing
extension under increasing magnetizing force, and recover¬
ing its original length when the force has reached a certain
critical value, beyond which there is contraction. But
this “ critical value ” of the force is found to depend in an
unexpected manner upon the hardness of the steel; the
critical value diminishes as the hardness becomes greater
up to a certain point, corresponding to a yellow temper,
after which it increases and with the hardest steel becomes
very high. For steel which has been made red-hot,
suddenly cooled, and then let down to a yellow temper,
the critical value of the magnetizing force is smaller than
for steel which is either softer or harder; it is indeed so
small that the metal contracts like nickel even under weak
magnetizing forces, without undergoing any preliminary
extension that can be detected.
Joule also made experiments upon iron wires under
tension, and appears to have formed the conclusion (which
has been often quoted as if it were a demonstrated fact)
that under a certain critical tension (differing for different
specimens of iron but independent of the magnetizing
force) magnetization would produce no effect whatever
upon the dimensions of the wire.1 What actually happens
when an iron wire is loaded with various weights is clearly
shown in Fig. 21. Increased tension merely has the
effect of diminishing the maximum elongation and hasten¬
ing the contraction; with the two greatest loads used in
the experiment there was indeed no preliminary extension
at all.2 The effects of tension upon the behaviour of a
nickel wire are of a less simple character. In weak fields
the magnetic contraction is always diminished by pulling
stress; in strong fields the contraction increases under
a small load and diminishes under a heavy one. Cobalt,
curiously enough, was found to be quite unaffected by
tensile stress.
Certain experiments by Knott on magnetic twist, which
will be referred to later, led him to form the conclusion
that in an iron wire carrying an electric current the
1 In Ency. Brit. vol. xv. p. 269 it is stated that in the case of a
certain wire loaded with a weight of 600 lb there was neither extension
nor contraction, even with a very large current. This, however, was
merely a conjecture of Joule’s which an experiment would have dis¬
proved.
2 The loads were successively applied in decreasing order of magni¬
tude. They are indicated in Fig. 21 as kilos per sq. cm.
magnetic elongation would be increased. This forecast
was shown by Bidwell to be well founded. The effect
produced by a current is exactly opposite to that of
tension, raising the elongation curve instead of depressing
it. In the case of a wire 0'75 mm. in diameter the
rrmyirrmm elongation was nearly doubled when a current
of two amperes was passing through the iron, while the
“critical value” of the field was increased from 130 to
200. Yet notwithstanding this enormous effect in iron,
the action of a current upon nickel and cobalt turned out
to be almost inappreciable.
Some experiments were next undertaken with the view
of ascertaining how far magnetic changes of length in iron
were dependent upon the hardness of the metal, and the
unexpected result was arrived at that softening produces
the same effect as tensile stress; it depresses the elon¬
gation curve, diminishing the maximum extension, and
reducing the “critical value” of the magnetizing force.
A hard drawn wire showed a maximum elongation of 45
ten-millionths, the critical field being (by extrapolation)
560. After the wire had been annealed, its maximum
elongation was only 7 and the critical field 140. A
thoroughly well annealed ring of soft iron showed no
extension at all, beginning to contract under the smallest
magnetizing forces ; in a field of 506, when the experiment
had to be stopped owing to the heating of the coil, the
retraction had reached 75 ten-millionths, and appeared to
be still far from its limiting value. A ring made of the
same iron, but unannealed, showed a maximum elongation
of 33, the critical value of the field being 420. The
experiments were not sufficiently numerous to indicate
whether, as is possible, there is a critical degree of hardness
for which the height of the elongation curve is a maximum.
Finally, experiments were made to ascertain the effect
of magnetization upon the dimensions of iron rings in
directions perpendicular to the magnetization, and upon
the volume of the rings. Four brass rods were hard-
soldered to a short cylindrical iron ring, two of them
forming prolongations of a diameter, while the other two
were attached to the edges, opposite to one another and
parallel to the axis of the ring. The first pair served to
communicate to the measuring instrument the effects of
changes along the lines of magnetization, the other pair
those perpendicular to the magnetization. It was found
that the curve showing the relation of transverse changes
of dimensions to magnetizing force was similar in general
character to the familiar elongation curves, but the signs
were reversed ; the curve was inverted, indicating at first
retraction, which, after passing a maximum and vanishing
in a critical field, was succeeded by elongation. The curve
showing the circumferential (or longitudinal) changes was
also plotted, and from the two curves thus obtained it
was easy, on the assumption that the metal was isotropic
in directions at right angles to the magnetization, to
calculate changes of volume; for if circumferential elonga¬
tion be denoted by lv and transverse elongation by
then the cubical dilatation (+ or — ) = ^ + 2/2 approxi¬
mately. If lx were exactly equal to - for all values
of the magnetizing force, it is clear that the volume of
the ring would be unaffected by magnetization. In the
case of the ring in question, the circumferential changes
were in weak fields less than twice as great as the
transverse ones, while in strong fields they were more;
under increasing magnetic force therefore the volume of
the ring was first diminished, then it regained its original
value (for H = 90), and ultimately increased. It was also
shown that annealing, which has such a large effect upon
circumferential (or longitudinal) changes, has almost none
upon transverse ones. Hence the changes of volume
undergone by a given sample of wrought iron under