Page:Encyclopædia Britannica, Ninth Edition, v. 15.djvu/271

253 Lenz Joule. K de creases for large forces. Mailer. Maxi mum of magnet ization. Koosen s experi ment. Wiede mann s re searches. view from which to criticize the experimental results now to be cited. In the researches of Lenz and Jacobi : the magnetic moment of the core was measured by the induction current in a secondary coil placed upon the magnetizing spiral. A considerable portion of their work was directed to proving principles which we here take for granted, e.g., that the magnetizing force is independent of the thickness of the wire of the magnetizing spiral, of the radius of its wind ings, and so on. They concluded from these experiments that the magnetization is proportional to the magnetizing force ; i.e., K is constant for a given quality, &c., of metal. The experiments of Joule, 2 which were made independently about the same time, led in general to a similar result. His method consisted in measuring by means of a balance the attraction P between two electromagnets actuated by the same current C. If the magnetization of the core were strictly proportional to the magnetizing force, i.e., to the current, then P would be proportional to C 2 , and P/C 2 would be constant. In most cases this was so ; but in two cases, where the cores of the electromagnets were very thin and the windings more than usually numerous, the ratio P/C 2 was found to decrease as the current increased. This shows that the magnetization tends to a maximum value as the current increases, in other words, that, for very large values of |p, K decreases. Miiller, 3 using the method of deflexions, arrived at a similar conclusion. His cores were 56 cm. long and from ( J mm. to 44 mm. thick, his magnetizing spirals from 48 2 cm. to 53 2 cm. long ; his results are therefore impure, and the empirical formula by means of which he represents them of comparatively little importance ; but the approach to a maximum of magnetization (saturation) is quite clearly demonstrated. He found, in accordance with theory, that if we increase the external magnetizing force (^ ) saturation is more quickly reached in thin than in thick bars. Somewhat similar experiments were made by Von Waltenhofen, 4 who deduces 5 from some of his own experiments with very thin cores, and from the experiments of Miiller, Weber, and Dub, 1678 to 2125 mm. mg. sec. units of magnetic moment per mg. of iron as the maximum of magnetization. This would give from 1317 to 1668 C.G.S. units for the maximum magnetic intensity in iron. These numbers, derived from more or less impure results, are merely rough approximations, but thej r agree very well with those derived at a later date by methods less open to theoretical objections. The approach to saturation may be very neatly demon strated as follows. 6 The same current is sent through a galvanometer and through the coil of an electromagnet with a thin core. The electromagnet is so placed that its action on the needle of the galvanometer just compensates the action of the galvanometer coil for a particular strength of current ; the needle then points to zero. If now the current be increased, since the increase of magnetization does not keep up with the increase of the current, the action of the coil prevails, and the needle deviates accordingly. The most extensive and important of the earlier researches into the general nature of magnetic induc tion are those of Wiedemann. 7 An epitome 8 of his results, with references to contemporary or preceding researches in the same direction, will put the reader in possession of almost all the more important general facts 1 Pogg. Ann., xlvii., 1839. 2 Sturgeon s Annals of Electricity, vol. iv., 1839; Phil. Mag., ser. 4, vol. ii. 3 Pogg. Ann., Ixxix., 1850. 4 Siizber. d. Wien. Akad., 1865. 5 Pogg. Ann., cxxxvii., 1869. 6 Koosen, Pogg. Ann., 1852 ; also Dub, Ib., 1853. 7 Pogg. Ann., c., 1857 ; Ib., cvi., 1859 ; Ib., cxvii., 1862. 8 Abridged from the author s own work, Galvanismus, Bd. ii. 309 sq. known until the quantitative experiments of Stoletow, Rowland, and their followers gave a complete account of the general characteristics of the function K. In these experiments the method of deflexion was used. Experi- The magnetizing spiral was placed magnetic east and west, raental and in the continuation of its axis was hung a magnetic steel mirror in a thick copper box to damp its oscillations. The deflexions of this mirror, read as usual with a scale and telescope when the core was not in, gave a measure of the current ; and the increase of the deflexion on introduc ing the core gave a measure of the magnetic moment of the core. The cores were cylinders 22 cm. long., 1-35 cm. thick, and the length of the spiral was only 24 cm., so that perfectly pure results could not be obtained. To compensate to some extent for the shortness of the spiral, the bars were gently drawn to and fro several times before being placed in the final position for which the reading was taken. In order to measure the permanent magnetism the core was removed, the current broken, the core returned to its former position, and a reading again taken. The conclusions arrived at were as follows. I. When a steel or iron bar is magnetized for" the first Maxi- time by a current C, the temporary moment K produced nium of during the action of the current at first increases faster ^-^ than the current, tfien more slowly, and finally tends to a an ,i maximum, as shown by Joule and Miiller. The period of turning quicker increase is more marked in long than in short point, bars ; it shows itself even on remagnetizing bars that have been several times magnetized and demagnetized. As C increases, the maximum of K is reached sooner in thin and long bars than in short and thick bars. Between the period of increase of K/ C and its period of decrease there is no period of any considerable length for which it is con stant. This last fact may be shown by means of the experiment of Koosen described above ; viz., if the com pensation be made for very small currents, when the current is increased, at first the electromagnet prevails, and the needle goes to one side of zero, then the current in the coil prevails, and the needle returns towards zero, and finally deviates on the other side. The point at which the ratio K/C has its maximum for any particular electromagnet is called by Wiedemann the " turning point " (Wendepunkt). The turning point relates to the body as a whole, and the value of the external magnetizing force jp for which it occurs depends both on the form of the body and on the nature of the metal. It has therefore no very definite physical meaning. It must be carefully distinguished from the "saturation point." Any element of a body is said to be magnetized to saturation when no increase of the magnetic force can increase its magnetization any farther. It may happen, however, that some parts of a body are magnetized to saturation while others are not. With regard to the turning point, Dub IJ has shown that with similar and similarly wound cores the turning point occurs for the same value of the current. This is of course in agreement with an obvious corollary of the general theory of magnetic induction. 10 II. In a freshly 11 magnetized bar the permanent moment which remains after the action of the current has ceased at first increases quicker than the producing current ; but for stronger currents a turning point is reached ; and then the moment increases more slowly than the current, and approaches a maximum. III. In attempting to destroy the permanent magnetism of a bar by means of a demagnetizing current, it may happen that a current, which, during its action, already 9 Pogg. Ann., 1868. 10 See Thomson, quoted by Joule, Phil. Trans., vol. cxlvi., if 11 That is, after being heated white hot to destroy all pre-existinj;

magnetism.