272 MAGNETISM Baur s Baur l and Wassmutli 2 have recently taken up the matter results. w ith all the advantages of modern experience. The former concludes from his experiments on iron by the ring method, at temperatures between and 150 C., that the magnetic susceptibility for a given magnetizing force increases with the temperature if the force be below a certain critical value (3 6 or so), but decreases as the temperature in creases if the force be above that value. 3 The smaller the magnetizing force the greater the influence of temperature on the magnetic susceptibility. The result of his experi ments at very high temperatures is that, for small magnetizing forces, the susceptibility at first increases rapidly as the temperature increases, reaches a maximum at red heat, and then falls suddenly to zero. For large forces, the susceptibility decreases gradually until red heat, and then falls suddenly to a very small value. According to him, if a bar be cooled from white heat the first traces of susceptibility are observed at a very bright red, -the brighter the greater the magnetizing force. He gives a variety of interesting results concerning the phenomenon of Gore, 4 all in accordance with what we have just stated. Diamag- In his earlier researches Faraday was unsuccessful in netism obtaining any evidence of the influence of temperature on, of flame. ^ Q susceptibility of weakly magnetic bodies, such as the chlorides of the magnetic metals or of diamagnetic bodies. 5 His earliest results were obtained with gases, and that too, strange to say, before the magnetic character of gases was Banca- fully investigated. It was Bancalari s discovery of the lari - extraordinary behaviour of flame between the poles of Fara- an electromagnet that led Faraday to resume his magnetic day s experiments on gases. Flames of all descriptions are ments" stron gly repelled from the axial line of a heterogeneous on magnetic field, so much so that it is impossible to induce heated the flame of a candle to go between the pointed poles of a gases. powerful electromagnet when they are placed at a short distance apart. The flame is blown aside, or even down wards, as if by a strong current of air issuing from between the poles. If a flat pointed flame is placed with its centre a little below the axial line, when the magnet is excited it drops down and spreads out below and around the axial line, assuming a fish-tail shape. It appears that the effect is not due to the solid matter in the flame but simply to the hot gases in it ; for the upper and cooler part of the stream of smoke from a freshly extinguished taper is scarcely affected, while the lower and hotter part is most powerfully acted upon, being blown aside and often split into two independent streams. A careful investigation led Faraday to the conclusion that oxygen, carbonic acid, and coal gas are rendered more diamagnetic, or, what is the same thing so far as the resultant differential action is concerned, less magnetic by heat, 6 and that this effect was much greater than could be accounted for by the mere rarefaction of the gas. He likewise obtained an increase of the susceptibility of oxygen by cooling it with ether and solid carbonic acid. Nitrogen appeared to be altogether Effect of indifferent. He found in a later series 7 of experiments heat on that the magnecrystallic property of bismuth was destroyed suscepti- a |. a temperature a little below its melting point, and that in weak * ne same thing happened to crystalline antimony a little mag- below red heat. In the thirtieth series of his experimental netics. researches he states that between 35 and 142 C. the susceptibility of a specimen of spathic iron ore perpendi cular to its magnetic axis decreased by 333 per cent, per degree centigrade of rise of temperature ; this agrees very closely with the formula which was found by Wiedemann to 1 Wied. Ann., xi., 1880. 2 Wien. Ber., 1880, 1881, 1882. 3 See the results of Faraday, Exp. lies., ser. xxx. 3424, &c. 4 Phil. Mag., 1869. 1870. 5 Exp. Res., 2359, 2397, 1845. 6 Exp. Res., vol. iii. p. 486. 7 Exp. Res., 2570 sq. , 1848. epresent very approximately the temperature effect for salt solutions, viz., k t = k (1 00325 ?). For the decrease n the magnecrystallic couple, or, which is the same thing, n the difference between the susceptibilities along and perpendicular to the magnetic axis, he found for the spathic ron ore 482 per cent, between and 138 C., and the percentage of f decrease was four times as great between - 14 and as between 129 and 143. The cor responding decrease in the case of crystalline bismuth Between 36 and 137 C. was 53 per cent. The experi ments of Pliicker and Matteucci led them to conclude that he susceptibility of diamagnetics diminishes with increase of temperature ; in the case of bismuth the decrease Between ordinary temperatures and its melting point is said to be about one-sixth or more. Canton seems to have been one of the first to study the Effect <
- ffect of moderate variations of temperature on the per- neat on
manent magnetism of iron and steel. The results of his j^nent and Hallstrb m s experiments went to show that permanent mag- magnetization decreases when the temperature rises, and netism. increases again when the temperature falls. In reality, however, as was shown by Kupffer, Biess and Moser, G-. Wiedemann, and others, the phenomenon is complicated ; for, if we repeatedly heat a magnet and allow it to cool to First its initial temperature, the magnetization lost at each heat- and P e ing is only partially recovered on cooling, and thus a ^ e ^" progressive_ loss goes on, until at last a constant state is reached, in which the magnetization lost on heating is completely recovered on cooling. In this respect, as well as in the effect on the magnetic susceptibility already discussed, there is an analogy between the effect of temperature and the effect of strain ; i.e., there is a first or permanent effect and a proper or temporary temperature effect. The permanent effect is that any alteration of temperature, be it increase or decrease, diminishes the permanent magnetization just as a shock or a jar would do, and probably for a similar reason. The proper or Proper temporary effect consists in a decrease of magnetization tem - with increase of temperature, which is completely recovered on decrease of temperature and vice versa. 8 If this be borne in mind, together with what has already been said above, it will not be difficult for the reader to see that the order and amount of the temperature variations, the hardness and form of the bar, and its magnetic history will all influence the temperature coefficient. To take one example, Wiedemann found that a bar magnetized at C. and then partially demagnetized by an opposite force, lost magnetism when heated ; if the demagnetization was not carried too far, it did not when cooled again to wholly recover what it had lost. If the demagnetization was carried a certain length, it recovered all that it had lost ; if farther still, more than it had lost. It was in fact found possible to demagnetize a bar, so as to render it apparently unmagnetic, and then to restore part of its original magnetism by merely heating and cooling it again. Similar phenomena were observed with a bar magnetized and de magnetized at 100, and then alternately cooled and heated. Unverdorben, 9 who arrived somewhat later at similar results, represents the matter by saying that the bar in this case has two magnetizations superposed, each having its own temperature coefficient. The following are a few additional references to sources of information concerning the present subject: Mauritius, Pocjg. Ann., 1863, and Phil. Mag., 1864 ; Jamiu and Gaugain, Comptcs Rcndus, 2>assim ; Fave, Ib., 1876 ; Poloni, Wicd. Beibl, 1878. 8 To give the reader an idea of the magnitude of this effect, we may mention that Whipple in determining the temperature corrections for magnetometer magnets at Kew, according to the formula found for the coefficient q values varying from 000762 to 000044, with a mean of 000161 ; and for q from 00000398 to 00000001, with a mean of 00000048 (Proc. Roy. Soc. Land., 1877). 9 Quoted in Lament, Ilandb. d. Mag., 82. An account of Lament s
own researches will be found in the same place.