Page:Encyclopædia Britannica, Ninth Edition, v. 15.djvu/291

Rh MAGNETISM 273 ipen- ental ffi- tlties. Development of Heat during Magnetization. Reasoning on purely thermodynamic principles from the results of Faraday, as to the influence of temperature on the mag netic properties of bodies, Thomson 1 has concluded (1) that a piece of soft iron at a moderate or low red heat, when drawn gently away from a magnet, experiences a cooling effect, and, when allowed to approach, a heating effect, and that nickel at ordinary temperatures and cobalt at high temperatures (between the melting point of copper and some lower temperature) experience the same kind of effect ; (2) that cobalt at ordinary temperatures and up to the temperature of maximum permeability experiences a cooling effect when allowed to approach a magnet, and heating when drawn away ; (3) that a crystal in a magnetic field experiences cooling when the axis of greatest paramagnetic or of least diamagnetic suscepti bility is turned from along to across the lines of force, and vice versa. Besides these considerations, the fact that those who adopt the molecular magnet theory are obliged to assume something of the nature of a frictional resistance to the turning of the magnetic molecules, and generally, without reference to any particular theory, many of the phenomena of coercive force, 2 lead us to suppose that some specific development of heat may accompany magnetization and demagnetization. The experimental verification of this suspicion is, however, a matter of great difficulty, owing to the enormous generation of heat arising secondarily from induced currents in the mass of the metal. The develop ment caused by magnetization and demagnetization was taken advantage of by Joule in one of his determinations of the mechanical equivalent of heat, but he makes no attempt to separate the effect of the two causes, indeed it did not concern his purpose to do so. 3 Notwithstanding that several experimenters have attacked the problem, it cannot be said that it is yet completely solved. It will therefore be best simply to call the reader s attention to some of the papers that have been published on the subject, and leave him to form his own judgment. See Yon Breda, Fogg. Ann., 1846 ; Grove, Phil. Mag., 1849 ; Ecllund, Pogg. Ann., 1864; Villari, N. dm., 1870; Cazin, Comptes Rcndus, 1874; Henvig, Wied. Ann., iv., 1878; Trowbridge, Wied. Beibl., 1879. Miscellaneous Relations of Magnetism to other Physical Properties. According to Maggi 4 the thermal conductivity of magnetized iron is less along the lines of force than across them. Naccari and Bellati 5 were unable to verify this result ; Tomlinson, 6 however, found that the con ductivity of iron and steel bars was diminished by longitudinal and increased by transversal magnetization. Abraham, Edlund, Mousson, and Wartmann all made experiments in search of a magnetic alteration of the electric conductivity of iron. Thomson seems, however, to have been the first to arrive at any definite result. 7 He found the conductivity to be diminished along the lines of magnetization and increased across them. Beez 8 verified the former result, but doubts the latter, which he is inclined to explain as a secondary effect caused by the compression of the iron arising from the external magnetic action on the plates used in Thomson s experiments. Thomson also found 9 that the thermoelectric quality of iron was affected by magnetization ; the thermoelectric current flowed from unmagnetized to longitudinally magnetized, and from transversely magnetized to un magnetized or longitudinally magnetized iron through the 1 Phil. Mag., 1878 ; or Nichol s Cyclopaedia, 2d ed., 1860. 2 See the paper of Warburg quoted above, p. 260. 3 Joule, Phil. Mag., 1843. 4 Archives de Gentve, 1850 ; or Wied., Galv., ii. 510. 5 Wied. Beibl., 1877. c Proc. Roy. Soc. Lond., 1878. 7 Phil. Trans., 1856. 8 Pogg. Ann., 1866. 9 Loc. cit. hot junction. In the case of nickel, the current flowed from longitudinally magnetized to unmagnetized through the hot junction, i.e., nickel behaved oppositely to iron. Thomson s results have been in part confirmed by a recent investigation of Strouhal and Barus. 10 A relation between magnetism and light was first Magnet- established by Faraday s discovery of the magnetic rotation ism an d of the plane of polarization of a ray passing along the hght- lines of force. This subject belongs more properly to physical optics, but there is one magnetic phenomenon apparently closely connected with it which falls to be mentioned here. This is Hall s discovery u that, if an Hall s electric current flow in a thin metallic strip in a direction pheno- AB, the effect of placing the strip in a magnetic field with menon&amp;gt; its plane perpendicular to the lines of force is to cause a transverse electromotive force perpendicular to AB, which changes in sign when the direction either of the current or of the magnetic field is changed. This transverse electro motive force is proportional to the product of the current intensity and the strength of the magnetic field ; ceeteris paribus, its direction in the case of iron is opposite to that in other metals, and its magnitude is also greatest with iron. This discovery establishes the existence of the rotatory coefficient of resistance mentioned by Maxwell 12 in his discussion of aeolotro pic conductivity ; and Rowland has shown that the phenomenon is probably due to the same cause as the magnetic rotation of the plane of polarization. 13 If, as modern physicists suppose, magnetism be a dyna- Effect of mical phenomenon, time must enter as a conditioning time - element. The question has been raised how long any magnetizing force takes to develop the maximum magnetiza tion that it is capable of producing. There are many facts that go to prove that this time is very small, or, at all events, that any force develops a very large fraction of the total magnetization due to it in a very short period of time. Perhaps the most wonderful evidence on this head is the fact that the telephone, which depends essentially on varying magnetic action, can reproduce the sounds of human speech even to the consonants. 14 Experiments bearing directly on the subject have been made by Villari. 15 A flat circular disk of flint glass was placed between the poles of a Ruhmkorff s apparatus for measuring the magnetic rotation of the plane of polarization. The axis of the disk was perpendicular to the axial line, so that rotation brought the different radii successively into the line of sight. When the disk was at rest the magnetic action in one experiment caused a rota tion of 19 divisions; spinning the disk at the rate of 110, 121, 143, and 180 turns per second reduced the magnetic rotation of the plane J of polarization by 2, 5, 10, and 17 divisions respectively ; the reduction was less the greater the magnetic force. From this Villari concluded that in flint glass not less than 001 244 second is required to produce such a diamagnetic intensity as can be observed by the rotation of the plane of polarization, and that 00241 second at least is required to develop the greatest diamagnetization of which this substance is capable ; he also states that the diamagnetism lasts for less than O OOOIS second after the inducing force is withdrawn. A series of interesting experiments on the oscillation of the plane of polarization caused by the oscillatory discharge from a Leyden jar recently made by Bichat and Blondlot 10 led them to a different conclusion, viz., that if any lagging of the induced magnetization behind the magnetizing force 10 Wied. Ann., xiv., 1881. &quot; Phil. Mag. [5], ix. and x., 1880. 12 El. and Mag., vol. i. 303. See also Stokes, Camb. and Dub. Math. Jour., vi., 1851 ; and Thomson, Trans. R.S.E., vol. xxi. p. 165, 1854. 13 Am. Jour, of Math., 1880 ; Phil. Mag. [5], x., 1880, and xi., 1881. 14 See also an article by the writer, Phil. Mag. [5], ii. 1876. 15 Pogg. Ann., 1879. 16 Comptes Rendus, 1882. XV. 35