Page:Encyclopædia Britannica, Ninth Edition, v. 15.djvu/293

Rh MAGNETISM 275 power of 2061 Ib ; but magnets specially constructed for carrying power have surpassed this limit. As a specimen of scientific toys of this description may be mentioned the electromagnet of Roberts (fig. 47), which consists of a square block of iron deeply slotted with four parallel grooves into which three layers of copper wire cable are wound in zigzag fashion so that the current converts the flanges alternately into north and south poles ; the armature is a square block planed to fit the face of the magnet. The carrying power of a machine of this kind was 2949 Ib, i.e., more than 1 tons ! The forms of electromagnet used in the arts, e.g., in electric bells, fire alarms, telegraphs, telephones, electric light regu lators, dynamo machines, &c., are simply innumerable. It will be sufficient to allude. to those constructed for the purpose of pro- i ducing an intense magnetic field, uniform or non-uniform, over a larger or smaller area ; these find their practical application in the construction of dynamo-electric machines, but they are mainly interesting to purely scientific men on account of their use in the investigation of the properties of weakly magnetic bodies. Figure 40 shows the usual arrangement adopted for large laboratory magnets. In considering the greatest available strength of such magnets, it is necessary to bear in mind the fact Fig. 47. that magnetic saturation of iron is practically reached with mag netic forces much under the greatest that we can command. The strength of field in a narrow crevasse perpendicular to the lines of magnetization in saturated iron is less than 18,000 C.G.S. units; 1 and this is practically the utmost at present attainable, for any addition to the strength of the field, arising from direct action of the magnetizing helix, would not under ordinary circumstances affect the hundreds in this number. Further increase of magnetizing current after we have reached within a small percentage of the limit of saturation is a waste of power. Elias 2 of Haarlem seems to have been the first who applied the electric current directly with success in the manufacture of powerful permanent magnets. He used a short flat magnetizing coil which was pushed backwards and forwards along the bar, the ends of which were caused to abut against two pieces of iron, which becoming in ductively magnetized reacted on the bar, and also served to keep the magnetization at the ends more uniform. The famous Logeman magnets were constructed by this process. By far the most convenient way of magnetizing steel is to use an electromagnet. 3 The bar to be magnetized may be laid flat on the pole of the magnet before it is excited, and after excitation drawn slowly off. By repeating this process several times, with the north pole of the electromagnet for one half and the south pole for the other half, saturation can be very quickly obtained. Perhaps a better plan is to lay the bar with its ends on the two poles, and then excite the electromagnet. For reasons already sufficiently explained, it is advisable to hammer the bar with a mallet while the magnetizing force is in action, and to turn the current off and on several times in succession. 4 On account of the difficulty of tempering steel to any great depth from the surface, and for specific magnetic reasons as well, it has been customary in constructing powerful permanent magnets to build them up of thin laminse of steel, each of which is separately magnetized. Figure 48 represents an arrangement of this nature Fig. 48. adopted by Coulomb, and figure 49 a horse-shoe magnet constructed in the same way. It will be observed that the ends of the laminse are not exactly conterminous, the middle ones projecting more than the others ; this arrangement was adopted with the view of getting rid to some extent of the weakening effect which the induction of one lamina has upon the other. That such an effect exists and is very great was conclusively shown by Coulomb ; how far the modification 1 See above, p. 256. 2 Pogg. Ann., 1844 and 1846. 3 Frick, Pogg. Ann., 1849, was one of the earliest who practised this method. 4 It has been several times proposed to magnetize steel bars by heat ing them red hot, allowing them to cool to the proper temperature inder the magnetizing force, and then tempering while the force is still acting. Gilbert, Knight, Robison, Ham an, Gaugain, Aime, and Holz ( Wied. Ann., vii. , 1879) have all experimented with this method, but it does not appear to possess any advantage over the ordinary modern process, and need not be discussed here. Preser vation of magnets. in question cures it is another matter ; much no doubt depends oil the purpose for which the magnet is required ; but it is scarcely worth while to discuss the subject here. We may call attention to a farther point in the construction of Coulomb s magnet, viz. , that the ends of the laminae are embedded in two soft iron terminals N and S ; there can be no doubt that, for some purposes at least, this is an advantageous arrangement. Among the famous modern makers of permanent magnets Hacker of Nuremberg, Logeman and Wetteren of Haar lem, 5 Willward, and Jamin deserve to be specially mentioned. 6 In the preservation of permanent magnets it is essential to avoid extreme changes of tem perature and shocks. When the magnet is laid aside it should be made part of a closed magnetic circuit ; in the case of a horse-shoe magnet this is attained by simply laying a piece of soft iron, called the keeper, across the poles ; bar magnets should be kept in parallel pairs, north pole to south pole and south pole to north pole, with two pieces of soft iron between the poles. When this is done the induced magnetism. reacts on the magnets and diminishes the demagnetizing force ; the action of shocks then ceases to destroy the permanent magnet ism, and may even increase it. ULTIMATE THEORIES OF MAGNETIC PHENOMENA. If we pass over the stream theory, which, although Stream partially developed by Euler, has never taken root in theory. modern physical science, the first great theory that we find proposed with a view to the explanation of magnetism is the two-fluid theory of Coulomb and Poisson. This is not an ultimate theory in the modern sense, inasmuch as it is not dynamical ; but it was, doubtless, looked upon as ultimate in the days when the imponderable fluids had a recognized role in the physical sciences. In the two-fluid Two- theory the imaginary positive and negative attractive agents (called magnetism in the empirical theory developed above) are regarded as imponderable fluids ; but the essential point in the definite form of the theory due to Poisson is that he regards a body susceptible to magnetic induction as made up of an infinite number of particles of infinite permeability immersed in an impermeable medium. After pointing out that, if the particles were of elongated form, and arranged so that the axes of elongation had one pre ponderating direction, or if they were arranged so that the linear density in different directions varied, the result would be seolotropy, he assumes that they are spheres uniformily distributed in the impermeable medium so that the volume of the magnetic particles in unit volume of the substance is the fraction L The problem of magnetic induction under the influence of a uniform force is then the same as the problem of electric induction for an infinite number of perfectly conducting spheres uniformly distributed in a non-conducting medium. He finds for the permeability Maxwell has pointed out one fundamental objection to this theory, viz., that the value of k calculated from the formula just given by means of observed values of -a in the case of iron is greater than it would be even if the magnetic spheres were packed in the closest possible manner. Another objection is that the theory affords no explanation of the variability of k with different forces. We might of course modify the hypothesis, as was done by Pliicker, by supposing that a resistance depending on the magnitude of the force opposes the separation of the fluids in the magnetic molecules, and that in certain cases a frictional resistance tends to prevent their reunion. We might in this way explain magnetic saturation and permanent magnetism ; but the theory thus bur dened has no more scientific value than the purely empiric theory, and, moreover, affords no clue to the phenomena of diamagnetism. 5 Advised by Elias and Van der Willigen ; see Nature, vol. xix. p. 552, 1879. 6 Further details as to the advantages and disadvantages of various forms of magnets will be found in Wiedemann s Galvanismus, and Lament s Ifandbuch des Magnetismus. See also a recent paper by W. Holz, Wied. Ann., 1880, on hollow cylindrical magnets, and another by Gray on the moments attainable with hard steel bars, Phil. Mag., 1878.