MAGNETISM 261 bjdy carefully after manufacture, by heating it in a wood fire and allowing it to cool very gradually ; this process is still more effective when the iron is covered all over before hand with half an inch or so of clay. The reader who wishes for further details on this subject should consult Lament s Handbuch das Magnctismus, chap. v. The follow ing references to the literature may be useful. Michell, Treatise of Artificial Magnets, 1750 ; Coulomb, Mem. dcl Acad., 1784 ; Barlow, Phil. Trans., 1822; Kater, Phil. Trails., 1821; Sabine, Phil. Trans., 1843; Hansteen, Fogg. Ann., 1825; Hacker, POIJIJ. Ann., 1848 ; Poggendorff, lb., 1850 ; Miiller, Ib., 1852; Matthiessen, Phil. Mag., 1858 ; Airy, Ib., 1863; Yon Walten- hofen, Pogg. Ann., 1864; Trove, Comptcs llcndus, 1869; A. L. Holz, Wied. Ann., v., 1878 ; Ruths, Wied. Bcibl. i. 1877 ; Chees- inan, Wied. Ann. 1882. Special Magnetic Character of Nickel and Cobalt. Besides the results of Rowland above quoted, we have on record experiments by the following physicists: Biot, Traite dc Phys., 1806 ; Gay Lussac, Ann. d. Chim. ct d. Phys., 1824 ; Lampadius, Schwcgger s Jour., 1814; E. Becquerel, Cov^tcs llcndus, 1845; Pllicker, Poc/g. Ann., 1854; Arndtsen, lb., 1858; Hankel, Wied. Ann., 1877; Becquerel, Ann. d. Chim. ct d. Phys., 1879 ; Gaiffe, Comptcs Eendus, 1881 ; Wild, Wied. Beibl, 1877. Experiments with Finely Divided Magnetic Metals and with Electrolytic Iron. These have been made by various physicists, mostly to test the theory of molecular magnets. The earliest of the experiments with finely divided iron was made by Coulomb, who mixed iron tilings with wax, and found that the magnetic moment was proportional to the mass of magnetic metal. Similar experi ments were made by the elder Becquerel, 1 his result being that the magnetic moment was proportional to the weight of magnetic sub stance, so long as the filings were not too densely distributed ; with increasing density the mixture acquires magnetic properties more like those of a continuous metallic mass. Several modern experi menters have gone into the matter with considerable care ; but their results are not sufficiently concordant, or of sufficient general interest, to justify us in dwelling at length upon them here. A few references to recent memoirs will suffice. Boernstein, Pogg. Ann., 1875 ; Toepler and Von Ettingshausen, Ib., 1877; Yon Waltenhofen, Wied. Ann-., 1879; Auerbach, Ib., 1880 ; Baur, lb., 1880. Experiments on electrolytically deposited iron have been made by Beez, Poyg. Ann., I860 ; Jacobi, lb., 1873 ; Beez, Ib., 1874 ; Holz, Ib. , 1875 ; Baur, Wied. Ann., 1880. Using a fine scratch on a varnished silver wire as electrode, Beez deposited a thread of iron between the poles of an electromagnet, and thus obtained a permanent magnet of extreme tenuity. It was found that the inductive susceptibility of this linear magnet was very small, and that considerable magnetizing force produced no increase of its permanent magnetism. Thus in one case the original magnetism was 360, the total magnetism under the induc ing force 370, the magnetism remaining after the force ceased to act 360. Broader, but equally thin, magnets deposited in a strong field in the same way gave more temporary magnetism than the linear magnets, but never more permanent magnetism than they possessed originally. Thicker plates exhibited greater temporary magnetism, and also an increase of the permanent magnetism acquired during deposition. With continued reversals of the magnetizing force electrolytic iron gave a continual decrease of the temporary magnet ism down to a certain limit (as does steel) ; but the negative per manent magnetism never approaches so near the positive after many reversals as in the case of steel. On the other hand, Jacobi found that iron reduced electrolytically from ferrous sulphate and sul phate of magnesia, even after tempering, took a considerable temporary moment, but retained very little permanent magnetism. IIolz found that the iron reduced from the solution of Jacobi and Klein was not sensibly hardened by heating and suddenly cooling, although its density was increased, and that its coercive force was diminished. On the other hand, it was found that hard tempering decreased the density of steel. He draws the conclusion that the coercive force is greater the farther apart the molecules. Baur s main result is that the maximum of magnetization with electrolytic iron occurs for much larger forces than with ordinary iron. These results are not wholly concordant ; but the discrepancies may be reasonably assigned to differences in the preparation of the metal. MAGNETIC PROPERTIES OF MATTER IN GENERAL. Among the earliest statements of the properties of the loadstone we find accounts of its action on other bodies ; but it is clear from their surroundings that these statements are purely fabulous. Many experimenters at a later date 1 Traite Complft du Magnetisme, chap. ii. p. 73. found indications of magnetic action In other inetals besides iron ; but with praiseworthy caution they ascribed them for the most part to the admixture of small quantities of iron. 2 There can be no doubt that the results of Cavallo 3 Early obtained with brass (especially hammered brass) were due observa- to impurity, for Benuet 4 failed to obtain any indications tions> of magnetism with pieces of brass made from pure zinc and copper, whereas he was immediately successful on adding small traces of iron to the metal. It very soon appeared, however, that an independent magnetic property must be ascribed to nickel and cobalt, and to these were by and by added with more or less certainty manganese and chromium. 5 Brugmans seems to have been the first to observe the repulsion by a magnet of a body not permanently magnet ized. He found that a piece of bismuth floating upon mercury in a small paper boat was repelled by both poles of a magnet. Lebaillif " confirmed the observation of Brugmans, and found that antimony possessed a like property. Saigey, 8 who experimented on the same subject, concluded that all bodies when suspended in air behave like bismuth, unless they contain traces of iron. Notwithstanding these results and others which we pass over, 9 the whole matter remained in obscurity till the repulsion of neutral bodies was rediscovered by Faraday in 1 845. He speedily unravelled the laws of the phenomenon, Faraday s showing how much depends on the nature of the body, redis- and how much upon the nature of the magnetic field. His co ^ e ? f 1 , observations enabled him in fact to comprehend under a mv esti- f ew general principles the action of all magnetic bodies gation. whether of the nature of iron or of the nature of bismuth. The earlier observers had fallen into difficulties by neglect ing the effects due to heterogeneity of field ; these were pointed out for the first time by Faraday, and since then order reigns where there was formerly confusion. The best arrangement for testing the behaviour of Experi- weakly magnetic bodies is to suspend either a small sphere mental of the substance or else a small cylinder in a heterogeneous arn " o e - magnetic field. This field is usually produced by placing f or t es t_ two pointed soft iron poles (fig. 38) on the arms of a power- ing ful electromagnet. The line joining these poles is called weakly the axial direction of the field ; directions perpendicular to " ia p ne this line are called equatorial. The magnetic force varies Fig. 38. along the axial line, being less in the middle than at the poles ; and it decreases everywhere from the axial line outwards. For some purposes poles of the shape shown in figure 39 are used ; here the line along the upper edges of the poles are lines of greatest force, whereas the line in the plane of the upper faces equidistant from the upper edges is a line of weakest force ; the force also decreases to the right of ab and to the left of cd. In suspending small spheres the best plan is to hang them from one end of an arm of wood db (fig. 40). At the other end of this arm is placed a counterpoise b, and the whole is suspended by a fibre of unspun silk u from a torsion head t, by means of which the arm db can be brought into 2 Cf. Lehmann, Nov. Comm. Petrop., 1766; Brugmans, Mag net ismus seu de Affinitatibus Magneticis Observations Academics, Leyden, 1778; Coulomb, Mem. de I Inst., 1812; Biot, Traitt de Physique, 1816, &c. 3 Phil. Trans., 1786 ; or Treatise on Magnetism, 1/87. 4 Phil. Trans., 1792. 5 Ritter, Gilb. Ann., 1800. 6 Loc ciL 7 p gg. Ann., 1827. 6 Bull. Univ. d. Sc., 1828. 9 See Von Feilitsch, Karstens Ency., Bd. xvi. ; Wiedemann s i
vanismus, Bd. ii. p. 5i6.