The observations of Baur and Rayleigh have been confirmed and discussed by (amongst others) W. Schmidt (Wied. Ann., 1895, 54, 655), who found the limiting values of κ to be 7.5 to 9.5 for iron, and 11.2 to 13.5 for steel, remaining constant up to H = .06; by P. Culmann (Elekt. Zeit., 1893, 14, 345; Wied. Ann., 1895, 56, 602); and by L. Holborn (Berl. Ber., 1897, p. 95, and Wied. Ann., 1897, 61, 281). The latter gives values of the constants a and b for different samples of iron and steel, some of which are shown in the following table:—
| κ = a + bH | ||
| Metal. | a | b |
| English tungsten steel | 8.90 | 0.264 |
| Tungsten steel, hardened | 2.23 | 0.032 |
| Silver steel | 8.66 | 0.384 |
| Tool steel | 8.30 | 0.400 |
| Refined steel | 11.28 | 1.92 |
| Cast iron | 3.16 | 0.236 |
| Soft iron | 16.6 | 18.6 |
| Hard drawn iron | 5.88 | 1.76 |
For most samples of steel the straight-line law was found to hold approximately up to H = 3; in the case of iron and of soft steel the approximation was less close.
The behaviour of nickel in weak fields has been observed by Ewing (Phil. Trans., 1888, 179A, 325), who found that the initial value of κ was 1.7, and that it remained sensibly constant until H had reached a value of about five units. While therefore the initial susceptibility of nickel is less than that of iron and steel, the range of magnetic force within which it is approximately constant is about one hundred times greater. Ewing has also made a careful study (Proc. Roy. Soc., 1889, 46, 269) of “magnetic viscosity” under small forces—the cause of the magnetometer “drift” referred to by Rayleigh. On the application of a small magnetizing force to a bar of soft annealed iron, a certain intensity of magnetization is instantly produced; this, however, does not remain constant, but slowly increases for some seconds or even minutes, and may ultimately attain a value nearly twice as great as that observed immediately after the force was applied.[1] When the magnetizing current is broken, the magnetization at once undergoes considerable diminution, then gradually falls to zero, and a similar sudden change followed by a slow one is observed when a feeble current is reversed. Ewing draws attention to a curious consequence of this time-lag. By the alternate application and withdrawal of a small magnetizing force a cyclic condition may be established in an iron rod. If now the alternations are performed so rapidly that time is not allowed for more than the first sudden change in the magnetization, there will be no hysteresis loss, the magnetization exactly following the magnetizing force. Further, if the alternations take place so slowly that the full maximum and minimum values of the magnetization are reached in the intervals between the reversals, there will again be no dissipation of energy. But at any intermediate frequency the ascending and descending curves of magnetization will enclose a space, and energy will be dissipated. It is remarkable that the phenomena of magnetic viscosity are much more evident in a thick rod than in a thin wire, or even in a large bundle of thin wires. In hardened iron and steel the effect can scarcely be detected, and in weak fields these metals exhibit no magnetic hysteresis of any kind.
6. Changes of Dimensions Attending Magnetization
It is well known that the form of a piece of ferromagnetic
metal is in general slightly changed by magnetization. The
phenomenon was first noticed by J. P. Joule, who in 1842 and
1847 described some experiments which he had made upon
bars of iron and steel. His observations, were for the most
part confirmed by a number of subsequent workers, notably
by A. M. Mayer; but with the single exception of the discovery
by W. F. Barrett in 1882 that a nickel bar contracts when
magnetized, nothing of importance was added by Joule’s results
for nearly forty years. Later researches have however thrown
much new light upon a class of phenomena which cannot fail
to have an important bearing upon the complete theory of
molecular magnetism.[2] According to Joule’s observations,
the length of a bar of iron or soft steel was increased by magnetization,
the elongation being proportional up to a certain
point to the square of the intensity of magnetization; but
when the “saturation point” was approached the elongation
was less than this law would require, and a stage was finally
reached at which further increase of the magnetizing force
produced little or no effect upon the length. From data
contained in Joule’s paper it may be calculated that the strongest
external field H0 produced by his coil was about 126 C.G.S.
units, but since the dimensional ratio of his bars was comparatively
Fig. 24.
small, the actual magnetizing force H must have
been materially below that value. In 1885 it was shown
by Bidwell, in the first of a series of papers on the subject,
that if the magnetizing force is pushed beyond the point at
which Joule discontinued his experiments, the extension of
the bar does not remain unchanged, but becomes gradually
less and less, until the bar, after first returning to its original
length, ultimately becomes actually shorter than when in the
unmagnetized condition. The elongation is generally found
to reach a maximum under a magnetizing force of 50 to 120
units, and to vanish under a force of 200 to 400, retraction
occurring when still higher forces are applied. In order to
meet the objection that the phenomenon might be due to
electromagnetic action between the coil and the rod, Bidwell
made some experiments with iron rings, and found that the
length of their diameters varied under magnetization in precisely
the same manner as the length of a straight rod. Experiments
were afterwards made with rods of iron, nickel, and
cobalt, the external field
being carried up to the
high value of 1500 units.
The results are indicated
in Fig. 24. It appears
that the contraction
which followed the initial
extension of the iron
reached a limit in fields
of 1000 or 1100. Nickel
exhibited retraction
from the very beginning (as observed by Barrett), its greatest
change of length considerably exceeding that undergone by
iron; in a field of 800 the original length was diminished by
as much as 1/40,000 part, but stronger forces failed to
produce any further effect. The curve for cobalt is a very
remarkable one. Little or no change of length was observed
until the strength of the field H0 reached about 50; then
the rod began to contract, and after passing a minimum at
H0 = 400, recovered its original length at H0 = 750; 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.
- ↑ The same phenomenon is exhibited in a less marked degree when soft iron is magnetized in stronger fields (Ewing, Phil. Trans., 1885, 176, 569).
- ↑ Principal publications: J. P. Joule, Scientific Papers, pp. 46, 235; A. M. Meyer, Phil. Mag., 1873, 46, 177; W. F. Barrett, Nature, 1882, 26, 585; S. Bidwell, Phil. Trans., 1888, 179A, 205; Proc. Roy. Soc., 1886, 40, 109 and 257; 1888, 43, 406; 1890, 47, 469; 1892, 51, 495; 1894, 55, 228; 1894, 56, 94; 1904, 74, 60; Nature, 1899, 60, 222; M. Cantone, Mem. d. Acc d. Lincei, 1889, 6, 487; Rend. d. Acc. d. Lincei, 1890, 6, 252; A. Berget, C.R., 1892, 115, 722; S. J. Lochner, Phil. Mag., 1893, 36, 498; H. Nagaoka, Phil. Mag., 1894, 37, 131; Wied. Ann., 1894, 53, 487; C. G. Knott, Proc. Roy. Soc. Ed., 1891, 18, 315; Phil. Mag., 1894, 37, 141; Trans. Roy. Soc. Ed., 1896, 38, 527; 1898, 39, 457; C. G. Knott and A. Shand, Proc. Roy. Soc. Ed., 1892, 19, 85 and 249; 1894, 20, 295; L. T. More, Phil. Mag., 1895, 40, 345; G. Klingenberg, Rostock Univ. Thesis, Berlin, 1897; E. T. Jones, Phil. Trans., 1897, 189A, 189; B. B. Brackett, Phys. Rev., 1897, 5, 257; H. Nagaoka and K. Honda, Phil. Mag., 1898, 46, 261; 1900, 49, 329; Journ. Coll. Sci. Tokyo, 1900, 13, 57; 1903, 19, art. 11; J. S. Stevens, Phys. Rev., 1898, 7, 19; E. Rhoads, Phys. Rev., 1898, 7, 5; Phil. Mag., 1901, 2, 463; G. A. Shakespear, Phil. Mag., 1899, 17, 539; K. Honda, Journ. Coll. Sci. Tokyo, 1900, 13, 77; L. W. Austin, Phys. Rev., 1900, 10, 180; Deutsch. Phys. Gesell. Verh., 1904, 6, 4, 211; K. Honda and S. Shimizu, Phil. Mag., 1902, 4, 338; 1905, 10, 548.
