MAGNETISM of permanent or residual magnetism, which is thus in Auerbach s view a particular case of after-effect. To it we can apply the general rule given above, subject of course to like exceptions. It would be premature to pronounce any opinion as to the ultimate value of Auerbach s results ; but the elegance of his representation of the phenomena will scarcely be disputed. In the latter part of his paper he applies his views to explain the peculiarities in the curve of magnetiza tion with forces of ascending magnitude obtained when the after-effect is neglected, and to the cyclical process dis cussed by Warburg. 1 He also discusses the influence of Effect of the duration of the impulse of the magnetizing force and duration O f ^g sudden closing and opening of the current. His al ? e " conclusions agree in the main with those of Fromme : in ofim- particular he inclines to Fromme a view 2 that there is a pulse. specific magnetic effect produced in certain cases by the breaking of the current while the core is in the spiral. This effect in certain cases (with short thick cores) is so great that a permanent magnetization of opposite sign to the total induced magnetism remains. 3 This "anomalous magnetization " was first observed by Von Waltenhofen, 4 who also establishes the more general result, of which this is an extreme case, viz., that the residual magnetism of the core depends upon the rapidity with which the magnetizing force is reduced to zero. Auerbach lays down as a general principle that when the variation of the magnetizing force is slow and continuous the velocity of the transition does not influence the final magnetization ; but sudden transition causes the final magnetization to be less or greater than that obtained by gradual transition, according as the passage is from a greater to a less or from a less to a greater force. Addi- The reader who wishes to pursue the present subject farther tional should consult the works of the following experimenters : litera- Jamin, 5 who holds what he apparently regards as a new theory of ture. magnetization. It is in point of fact merely a modification of the theory of solenoids, somewhat restricted in its application to the phenomena of magnetic induction. His special point is that the lines of magnetization in a bar magnetized (say) by a magnetizing spiral only penetrate to a limited depth, which is greater the greater the current. The following experiments 6 are adduced in confir mation of his views. The steel tube of a Chassepot rifle was plugged at both ends by screwing into it bolts of the same metal. Inside was placed a cylindrical rod. It was found that, so long as the current in the spiral was not very great, the rod was not sensibly magnetized ; but, as the current increased, it became more and more affected, and by and by was as much permanently magnetized as if the enveloping tube had been absent. Again, the rod having been magnetized to saturation and inserted in the tube, a demagnetizing force was applied to the whole, and it was found possible to render the tube and core together seemingly neutral, or even oppositely magnetic, while the rod when taken out proved to be still power fully magnetized in the original direction. Again, a bar was magnetized by a powerful current, and then magnetized in the opposite direction by another current. The surface of the bar was then eaten away to a certain depth ; and it was found that the original magnetization reappeared. These experiments, although most interesting in themselves, do not appear to warrant the inter pretation which their author puts upon them. Jamin has made extensive researches on the magnetic distribution in bars and ribbons of steel, partly with a view to obtain empirical rules for the construction of powerful permanent magnets, in which he has been very successful. Gaugain, Comptcs Rendus, passim ; Ann, d. Chim. ct d. Phys., (5) xi. 1 Wied. Ann., xiii., 1381 ; r.f. Fromme, 76., xiii., 1881 ; also Himstedt, lb., xiv. , 1881. A similar phenomenon was observed by Meyer and Auerbach during their experiments on the gramme machine, Wied. Ann., v., 1878. a See an elaborate paper which we can only mention here, Wied. Ann., v., 1878. 3 Experiments on the same subject have been made by Righi. Comptes Rendus, 1880, or Wied. Beibl., iv., 1880 ; and by Bartoli and Ales- sandro, N. dm., 1880, or Wied. Beibl, iv., 1880. Cf. Fromme, Wied Ann., xiii., 1881. 4 Wien. Ber., 1863.
- Comptes Rendus, passim. . 6 Comptes Rendus, Ixxx., 1875.
Christiansen, "Researches on the Magnetic Distribution iu an Iron Bar, on one part of which is placed a Short Magnetizing Spiral," Wied. Beibl., i., 1877. Ruths, " Ueber den Magnetismus weicher Eisencylinder und verschieden barter Stahlsorten" (Dortmund, 1876), IVicd. Beibl., i., 1877. Whipple, "Induction Constants of Permanent Magnets of vari ous shapes, from the determination at Kew," Proc. Roy. Soc. Lond., 1877. Oberbeck, "Ueber die Fortpflanzung tier magnetischeu Induc tion ira weichen Eisen " (Halle, 1878), Wied. Beibl., ii., 1878. Kiilp, " Experimentaluntersuchungen iiber magnetische Coer- citivkraft," Carl. Rep., 1880. Baur, "Experiments with an Iron Ring on the Magnetization Function for very small Forces," Wied. Aim., xi. , 1880. Riecke, "On the Experimental Test of Poisson s Theory," Wied. Ann., xiii. p. 485, 1881. Siemens, a very interesting paper, "On the Effect of the Mag netization of Iron in any Direction upon its Permeability in the Perpendicular Direction,". Wied. Ann., xiv., 1881. Righi, Contributions to the Theory of the Magnetization of Steel," Mem. d. Ace. d. Bologna, 1880; Wied. Beibl., v., 1882. For a succinct account of several of the foregoing memoirs, see the "Nachtrage" to "Wiedemann s Galvanismus, and a paper by the same author in Pogyendorffs Annalen, clvii. p. 257, 1876. Influence of the Hardness and Structure, of Iron and Influonc Steel on Permanent Magnetism. Some information has oi nar( l- already been given incidentally on this subject, and n f ss a lengthy discussion would be out of place here. The t ure _ statements of the various authorities are very contra dictory. This is not to be wondered at ; for those best qualified to prepare the materials for experiment are generally deficient in the scientific knowledge requisite to enable them to form a sound judgment as to the result, while thoroughly trained scientific men have not as a rule acquired a command over the delicate manipulation of the forging and tempering of steel, an art which those who possess it usually find difficult to describe in words or reduce to rules. There is the further circumstance that many who have been successful in making good steel for magnetic or other purposes have found it for their interest not to publish the process by which success was attained. Fineness of grain and uniformity of temper are the Finenes greatest requisites in steel for permanent magnets. The f S latter in bars of any size is never attained in perfection, ^ ""* for the surface is always harder than the interior. The O f mischief which thereby arises may be understood by taking temper. the extreme case of a thin steel tube magnetized to satura tion, and then fitted with a perfectly soft iron core. It is clear that the core will act very much like the armature of a horse-shoe magnet ; the lines of force will run back through it, and the external action will be in a great measure destroyed. The different tempers of steel may be roughly classified as glass hard, straw colour, blue, and soft. The current statement is that the harder the steel the more difficult it is to magnetize, but the better it retains its magnetism. If this were so, provided sufficient magnetizing force to pro duce saturation were at command, the best temper for magnets would be glass hard. Lamont, however, whose experience was great, states that he found the loss after magnetization to be as great, and to continue as long, with glass hard as with blue tempered magnets. The same experimenter gives it as his opinion that great differences in the quality of magnets arise more from defects as to homogeneity, continuity, and uniformity of temper than from the quality of the steel in other respects ; he inclines, however, to a preference for English cast steel. Purity and homogeneity of structure are equally necessary in iron of high magnetic inductive susceptibility and small coercive force. Hammering, rolling, and drawing diminish the susceptibility and increase the coercive force. Rolling does so more in the direction of rolling than transversely, so that the iron becomes seolotropic. It is advisable in all
cases where high susceptibility is wished to anneal the