254 M AGNETISM Demag netiza tion easier than magnet ization. Uni lateral property of de magnet ized bar. Repeated magnet ization and de- magnet ization. Tem porary and per manent magnet ism in hard and soft steel and in soft iron. produces a temporary magnetic moment of opposite sign, still leaves on ceasing to act a permanent magnetic moment of the same sign as before, although less in amount. 1 On increasing the demagnetizing current still farther the per manent moment is at last destroyed. In this process the permanent magnetism decreases faster than the demagnet izing current increases, so that the current required to destroy a given permanent magnetism is less than the current that originally produced it. 2 IV. When a fresh bar has been magnetized with any permanent moment, and then demagnetized by a current C , opposite to the magnetizing current C, a second application of - C , or of any weaker current in the same direction, will not produce a reverse permanent moment, although a current C in the same direction as C will magnetize the bar permanently in the original direction more or less strongly. It follows therefore that demagnetiz ing by an opposite magnetic force, although it may destroy the permanent magnetism of a body, does not render it magnetically indifferent, as heating to a white heat would do. The body remains in fact more easily magnetizable in one direction than in another. 3 V. In certain cases a fresh bar was magnetized by a current C, and then partly demagnetized ; it was then found that a current C was required to bring it back to its original permanent moment. VI. In another case a fresh bar was magnetized by a current C to permanent moment K, then reduced by a demagnetizing current C to permanent moment K , then by a direct current C" less than C brought to permanent moment K". It was then found that a current C was necessary to bring it back to permanent moment K ; and this held whether K was positive, zero, or negative. VII. When a bar is repeatedly magnetized and demag netized by currents of the same intensity, the permanent magnetic moments corresponding to a given force become, to begin with, a little greater than at first ; to begin with, they increase faster than the magnetizing force, though not so fast as at the first. The turning point, however, occurs for a weaker current than before. The magnetization obtained with the strongest current gradually decreases a little. The moments left by the demagnetizing current decrease less rapidly than before, so that a current at first capable of demagnetizing the bar altogether leaves after repeated magnetization and demag netization a slowly increasing residual moment. After a large number of repetitions of the operation of magnetiza tion by a current C and demagnetization by a current C , the bar finally reaches a constant state, so that each magnetization and demagnetization leaves a corresponding invariable permanent moment. When we pass beyond the limits C and - C , these phenomena are repeated in the same order as before. 4 VIII. All the above phenomena are most clearly seen in hard steel, less clearly in soft steel and iron. For small magnetizing forces the temporary moment in hard steel is less than in soft steel, and greatest of all in soft iron. The general rule is, the harder the material the less the tem porary and the greater the permanent moment for a given magnetizing force. IX. If, however, we consider the ratios of the temporary moments in soft steel and iron to the temporary moment in hard steel, all for the same force, then these ratios decrease gradually as the force increases ; so that the tem- 1 See Poggendorff, Pogg. Ann., 1852. 2 This result had also been arrived at by Abria, Ann. d. Chim. et d. Phys., 1844; and by Joule, Phil. Mag., 1847, Phil. Trans., 1855. 3 Similar conclusions were arrived at by Ritchie, Phil. Mag., 1833 ; Jacobi, Pogg. Ann., 1834 ; Marianini, Ann. Chim. et d. Phys., 1846. 4 On the same subject see Joule, Phil. Trans., 1856 ; also Von "VValtenliofen. Pnf/f/. Ann., 1864. porary moment in soft iron reaches its maximum sooner than in soft steel, and still sooner than in hard steel. 5 The earliest experiments from which definite values of K have been calculated are those of Weber. 6 A cylindrical bar, 10 02 cm. long and -36 cm. thick, was placed inside a spiral so long that the magnetizing force throughout the length of the bar could be assumed to be uniform. The moment of the bar was measured by the method of deflex ion, the action of the spiral on the deflected magnet being compensated by means of a part of its own circuit suitably arranged. The intensity of the current in the spiral was found in absolute measure by means of a tangent galvano meter. Assuming that the bar could be replaced by a very elongated ellipsoid, Kirchhoff calculated by means of the theory explained above (p. 249) the values of K for values of |) ranging from 29 6 to 248-4 (C.G.S. units), and found that it decreased steadily from 25 to 5 6. In the experiments of Von Quintus Icilius 7 bars were used which had been reduced by filing as nearly as possible to the form of ellipsoids of revolution. The magnetic moments were measured partly by the deflexion method, partly by the method of electromagnetic induction. In this last method a secondary spiral is placed upon the magnetizing spiral, and the induced current in it, caused by reversing the magnetizing current, is observed first when the ellipsoid is in the magnetizing spiral, secondly when it is not. When these currents are known in absolute measure, the moment of the ellipsoid can be calculated. The experimenter did not himself reduce his results so as to obtain K, but con tented himself with remarking that the ratio of the whole moment of the ellipsoid K to the strength of the undisturbed field p reached a maximum as |3 was increased, this maximum occurring for smaller values of |$ the more elongated the ellipsoid. The true meaning of his results was brought out by Stoletow, 8 who reduced them, and established the interesting fact that, as the magnetizing force 9 jp increases from very small values, K at first increases rapidly, then reaches a maximum, and afterwards decreases more slowly. For one ellipsoid K increased from 30 5, for p = -24, to a maximum 12O4, for p = 4 56, and then decreased to the value 39-4, for |$ = 30-07. In another, the initial value was 20-1 for H = -518, the maximum value 107 5 for f = 4 92, and the final value 2 86 for fj = 454-1. Thal^n, adopting a method indicated by Weber, 10 determined the value of K for small magnetizing forces. Long bars were placed in the axis of a cylindrical coil con siderably exceeding them in length. This coil was caused to rotate 180 about a horizontal axis, so that the magnetization induced by the earth s vertical force was reversed relatively to the coil. The current thus caused was meaured by means of the swing of a galvanometer in circuit with the coil; from this (see abo^e, p. 240) the moment of the induced magnetism was calculated ; and thence, assuming the bar to be replaceable by an ellipsoid, K was calculated. From three bars of the same metal each 400 4 mm. long, having diameters of 36 "4, 29 94, and 23 87 mm. respectively, the values of K deduced were 32-32, 31-80, and 32-64. For other specimens of iron he found values of K ranging from 27 24 to 44 23. 5 Similar results by Pliicker, Pogg. Ann., 1852 and 1855. 6 Electrodynamische Maasbestimmungen, Bd. iii. 26. 7 Pogg. Ann., cxxi., 1864. Similar results were obtained Oberbeck, Pogg. Ann., cxxxv., 1868. 8 Pogg. Ann., cxlvi. p. 443, 1872. 9 P here means the whole magnetizing force, arising partly from the inducing field and partly from the induced magnetism. Experi menter:: have needlessly complicated the already complex problem of ferro-magnetic induction by neglecting the all-important distinction between Ht) and 1 . 10 Abh. d. Gott. Gesellschaft, Bd. 6. Earlier values of K Qnintus Icilius. Thalen.
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