MAGNETISM 271 lieory mole- dar .agnets, exion etweeu iie ifferent he- omeua. In his excellent analysis of tlie phenomena, Wiedemann coordinates them throughout by means of an extension of Weber s theory of " molecular magnets " (Drehlare Mole cular may nete). This of course involves an attempt to pass beyond the mere results of experience ; and there can be no question that, on the whole, this theory explains the facts in a highly instructive and suggestive manner. The main defect in it is the multitude of assumptions and the want of clearness and definiteness in its conclusions. Thus it is sometimes not easy to see why exactly the opposite conclu sion should not be drawn and it appears hopeless to bring it to the test of a quantitative comparison with experiment. Without entering into the ultimate causes of magnetism, we might endeavour to reduce the phenomena to the smallest number of experimental facts. Thus, assuming merely the effects of longitudinal and transverse traction upon magnetization and the magnetic extension and com pression along and perpendicular to the lines of magnetiza tion, we might explain many of the results concerning the relation between torsion and magnetization. Let us take for example No. VI. of "Viedemann s parallel state ments. In fig. 44 let the upper end of the wire be the fixed end, and let P be a point in any of the thin coaxal cylindrical shells into which the wire may be supposed divided. First suppose the wire to be circularly magnetized by the action of a downward current, the resultant magnetic force at P being in the horizontal direction PB. If now the wire be twisted in the direction of the arrow T, it acquires two axes of greatest and least magnetic susceptibility Pe and P?-. The resultant magnetic force PB being resolved along these axes will induce more magnetism along Pe than along Pr ; hence the ceolotropy will cause the resultant magnetization to take the direction PB ; it will therefore have a posi tive vertical component downwards, which agrees with statement VI. In fact the twisting converts the circular lines of magnetization into right- handed helices. Kext let us suppose the wire untwisted to begin with, but magnetized both circularly and longitudinally, the components being PB and PA. The resultant magnetization will then have some direction such as Pe, but, by Joule s prin ciple, this will cause extension along Pe and compression along the perpendicular direction Pr ; consequently the wire will twist in the direction of the arrow T, which agrees with statement 6. Moreover, since magnetization along PB alone would simply cause the tube to expand along a horizontal section, and magnetization along PA alone would simply cause longitudinal extension, it is clear that when either A or B is given the twisting reaches a maximum and then diminishes when the other is in creased. x It does not seem unreasonable to expect that a general mechanical theory of this kind will yet be found to co ordinate all the facts ; although there are difficulties in its way at present. 2 The phenomena will then be reduced to two or three experimental facts at the utmost, which it will be the business of some ultimate dynamical theory of magnetism to explain. Effect of Temperature. Some information on this subject has been given incidentally above, p. 256. We collect here a few additional facts ; but a complete account of all that has been done could not be compressed within our available space, owing to the great diversity of opinion upon the subject. That the question is a very difficult one will appear at once, if we reflect that variations of temperature influence the density and molecular structure of magnetic bodies to a remarkable degree, and that thus secondary 1 According to the results of Villari and Thomson, if the magnetiza tion were beyond a certain critical value, Pe would become an axis of compression and P?- an axis of extension, in which case the wire would twist in the opposite direction. 2 See Thomson, Phil. Trans., 1879, p. 73. Not the least of these arise from gaps in our experimental knowledge ; e.g., regarding the effects of 2)ermanent set caused by traction and compression. T 44. influences arise in addition to the proper effect of tempera ture. That very high temperatures destroy both the magnetic Destruc- susceptibility and the power of retaining magnetism tion of altogether has been known since the infancy of magnetic a s net - science. Thus Gilbert found that a loadstone and apiece ^y of iron equally lost their power of affecting the magnetic high needle when heated very hot, and remarks that the tempera- magnetic property returns to the iron after it has cooled a turc - little, but that the magnetic virtue of the loadstone is altogether destroyed. 3 Similar results were obtained by Brugmans, Boyle, Cavallo, Barlow and Bonnycastle, Christie, Ritchie, Erman, Scoresby, Seebeck, and others. Faraday 4 found that a steel magnet lost its permanent magnetism rather suddenly at a temperature a little under the boiling point of almond oil ; it behaved like soft iron till it was raised to an orange-red heat, and then it lost its magnetic susceptibility and became indifferent. The temperature at which retentive power for permanent magnetism was lost appeared to vary in steel with the hardness and structure ; in fragments of loadstone it was very high : they retained their permanent magnetism until just below visible ignition in the dark, but, on the other hand, they lost their susceptibility at dull ignition, i.e., at a much lower temperature than iron. Nickel was found to lose its magnetic susceptibility at a much lower tem perature than iron, viz., about 330 to 340 C. 5 Cobalt is much more refractory, for it retains its susceptibility, according to Faraday, nearly up to the melting point of copper, i.e., to a white heat. The writer had occasion to verify these results in the course of some experiments on the magnetic sounds in wires of iron, nickel, and cobalt traversed by an interrupted current of electricity. The effect of extreme cold, produced in the ordinary Effect of way by means of solid carbonic acid and ether, was, accord- extreme ing to Trowbridge, 7 to diminish the moment of a steel coltl - magnet (magnetized at 20 C.) by about 60 per cent. The effect of moderate alteration of temperature varies Mode- greatly according to circumstances. We shall consider rate separately the effect upon the magnetic susceptibility and vam- upon the permanent magnetism ; but it must be noticed te mpera- that no such separation is possible in actual experiment, tare. The temporary magnetism of bars of cast iron, smithy Effect on iron, soft iron, soft steel, and hard steel magnetized by the 1|ia s- earth s vertical force was found by Scoresby 8 to be insensible netlc i at a white heat, but to be much greater at a dark red ^[^F heat than at the temperature of the air. The difference was most marked in the case of hard steel, no doubt partly because of the softening of the bar. Similar experiments were made by Barlow, Seebeck, and others. Kupfer 9 experimented on the subject using variations of temperature between and 100 C., and found the susceptibility of soft iron to increase with the temperature. Wiedemann s con- Wiede- clusion is that the first alteration of temperature, whether mann .s increase or decrease, increases the temporary magnetism of re iron or steel, whatever the temperature at starting. If the temperature be repeatedly altered and brought back to its initial value, the magnetization continues to increase, but after a time becomes more and more nearly constant at the initial temperature. After this state has been reached, an increase of temperature causes increase of magnetiza tion in very hard steel bars, a decrease of temperature a decrease of magnetization ; the behaviour of soft steel bars is exactly opposite. 3 De Magnete, lib. ii. cap. 3. 4 Exp. Res., vol. ii. p. 220, 1836. 6 According to Becquerel about 400 C., according to Pouillet about 350 C. 6 Nature, vol. xxii., 1880. 7 Sill. Jour., 1881. 8 Phil. Trans. Roy. Soc. Edin., vol. ix.
9 Wied., Qa.lv., ii. 521.