Page:Encyclopædia Britannica, Ninth Edition, v. 15.djvu/288

Wiede- maun. Torsion and magnet ization. Relations betiveen Torsion and Magnetization. These were investigated by Matteucci, 1 and after liim by Becquerel and Wertlieim. 2 The whole subject was carefully studied by G. W. Wiedemann, 3 who has done more than any living physicist both in discovering new facts in this interesting field and in coordinating those formerly known. We extract from his Galvanisimis 4 the following series of parallel statements, which will serve the double purpose of making the i r eader acquainted with the principal facts, and of drawing his attention to the close analogy between the mechanical and magnetic properties of bodies, and to the almost perfect reciprocity of their experimental laws. 1. The permanent torsion of iron wires is diminished by magnetization in a proportion decreasing with increasing mag netization. 2. Kepetition of magnetization in the same direction diminishes permanent torsion very little farther ; but magnetization in the opposite direction causes a fresh and considerable diminu tion. 3. "Yhenthe permanent torsion of a wire has been removed as far as it can be by magnetizations within certain limits repeated alternately in opposite directions, it takes a maximum, of torsion Avhen magnetizedin one direction, a minimum when magnetized in the other direction. 4. A. permanently twisted wire partially untwisted loses less of rts twist when magnetized than an ordinary permanently twisted wire, It the untwisting has been considerable, feeble magnetization causes an increase of torsion, which rises to a maximum and then decreases as the magnetization is increased. The greater the untwisting the stronger the magnetization corre sponding to this maximum, and, when the untwisting is very great, the maximum may not be reached at all. 5. If a wire under the influence of a twisting stress is magnetized, the twist increases with weak but decreases again with strong magnetization. The first effect of magnetization is usually to increase the twist ; but, if the wire be jarred beforehand, the magnetization at once causes untwisting, which disappears when the magnetization ceases. C. If we magnetize an iron wire so that its free end has north polarity, and then pass a current from the fixed to the free end, or first pass the current and then magnetize, the free end of the wire as seen from the fixed end twists in the direction of the hands of a watch. The reversion of current or of magnetization reverses the twist ; reversion of both leaves it unaltered. [It would appear that when the magnetizingforce and thecurrent are both in action the twist tends to a maximum when either is increased, the other remaining constant] The alterations of the longitudinal and circular magnetization of I. The permanent magnetiza tion of steel bars is diminished by torsion in a proportion de creasing with increasing tor sion. II. Repetition of torsion in the same direction diminishes permanent magnetization very little farther ; but torsion in the opposite direction causes a fresh and considerable diminu tion. III. When the permanent magnetization of a bar has been removed as far as it can be by twisting within certain limits repeated alternately in oppo site directions, it takes a maxi mum of magnetization when twisted in one direction, a mini mum when twisted in the other direction. IV. A permanently magnetized bar partially demagnetized loses less of its magnetization when twisted than an ordinary per manently magnetized bar. If the demagnetization has been considerable, feeble twist causes an increase of magnetization, which rises to a maximum and then decreases as the twist is increased. The greater the de magnetization the greater the twist corresponding to this maxi mum, and, when the demagnet ization is very great, the maxi mum may not be reached at all. V. If a bar under the influence of a longitudinal magnetizing force is twisted, the magnetiza tion increases with small twists but decreases again with large twists. The first effect of twisting is usually to increase the magnet ization ; but, if the bar be jarred beforehand, the twist at once causes a decrease, which disap pears when the twist ceases. VI. If we twist the free end of a wire in the direction of the hands of a watch as seen from the fixed end, while a current from fixed end to free end either is passing through it or has passed through it, the wire be comes longitudinally magnetized so that its free end has north polarity. The reversion of cur rent or of twist reverses the mag netization; reversion of both leaves it unaltered. 1 Comptes Rendus, 1847. 3 Pogg. Ann., 1858, 1859, 1860. 2 Comptes Rendus, 1852. 4 Bd. ii. 492. iron wires may be shown by the induced currents thereby caused in a coil surrounding the wire or in the wire itself. For example, if an iron wire be circularly magnetized by passing a current through it, and then twisted in either direction, an induction current flows through the wire in the same direction as the original current ; and an opposite current is observed when the wire is untwisted again. This shows that twisting the wire diminishes the permanent circular magnetization, while untwisting partially restores it. 5 The relation between bending stress and magnetization has been studied by Guillemin, 6 Wertheim, 7 Ader, 8 and Kimball ; 9 but the results are not of sufficient interest to be cited here. The question has also been raised whether magnetization affects the elasticity of bodies, and has been answered by Wertheim and Wartmann in the negative. Both Kimball 9 and Piazzoli 10 find that the breaking ten sion of iron wires is increased by longitudinal magnetization ; the former puts the increase at 9 per cent, when the wire is saturated. This is the place to mention the so-called magnetic Magneti sounds " which accompany the magnetization and demag- sounds. netization of the strongly magnetic metals. It is now established beyond all doubt that these sounds have their origin partly at least in the mechanical strains accompany ing magnetization. In many cases direct magnetic or electromagnetic action, and even electrostatic and thermal actions, concur in producing them, and it is often diffi cult to say how much is duo to each of these several causes. This is especially to be observed where the sounds are produced by the passage of interrupted or undulatury currents through wires of the strongly magnetic metals. A full discussion of the matter belongs more properly to the subject of electric telephony ; but a few notes on the history and literature of the subject may be given here. Page 11 seems to have been one of the first to notice phenomena of the kind ; but Joule 12 appears to have first stated clearly that mag netic-mechanical strain was a specific cause. He says that the magnetic extension in the core of an electromagnet takes place so suddenly that the shock is sensible to the touch, and is accom panied by a musical note arising from vibration in the metal. Marrian, 13 Matteucci, 14 Beatson, and Wertheim 15 all took up the matter; and De la Kiv r e 16 published many investigations concerning it. In 1S61 Reiss published the invention of an electric telephone for the transmission of music and speech, which depended essen tially on the magnetic sounds produced by a varying current in an iron core. This instrument was the prototype of the telephone of Gray, and of the still more famous instrument of Bell, whose action, although often described as purely electromagnetic, is no doubt in part due to the magnetic strains. Among the more recent investi gations on this subject may be mentioned Ferguson, Proc. Roy. Soc. JEdin., 1878 and 1880 ; Ader, Comptes Rendus, 1879 ; Du Moncel, Ib.; Chrystal, Nature, vol. xxii., 1880; Hughes, Proc. Roy. Soc. Land., xxxi. and xxxii., 1881. General Remarks. Wiedemann has remarked with justice that most of the effects of strain upon magnetization and vice versa are complex. Apart from the possible admixture of direct magnetic action, we must distinguish (1) the mere disturbing effect of jarring : thus the first Effects application of a mechanical stress has the same effect as a complex, shock, i.e., it loosens the molecules of the body, as it were, and renders them more ready to follow any inductive magnetic force, while the first effect of magnetization upon a body under stress is precisely similar, and may in fact be imitated by mechanical jarring pure and simple; (2) after-effect, whether mechanical or magnetic, the conse quence of which is that the effect due to any mechanical stress or magnetizing force is affected by pre-existing stress and magnetization; (3) the proper effect of mechanical stress or magnetic force, which appears at once where one or the other is applied, and disappears when it is removed. 5 See Wiedemann and Villari, I.e. ; also Gore, Phil. Trans., 1874 ; H. and F. Strenitz, Wien. Ber., 1877 ; Hughes, Proc. Roy. Soc. Lond., 1881. 6 Comptes Rendus, 1846. 7 Ib., 1846, &c. 8 Ib., 1879. 9 Sill. Jour., 1879. 10 Wied. Beibl., 1880 ; see also Hoffmann, Ib. 11 Poc/ff. Ann., 1838. 12 Sturgeon, Ann. EL, 1842 ; Phil. Mag., 1847. 13 Phil. Mag., 1844. 14 Wied., Galv., ii. 515. l5 Ib.

16 Comptes Rendus, 1845 ; Phil. Trans., 1847, &c.