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XI. Studies on Magnetism. By E. BOUTY, Professor of

Physics at the Lycée of Rheims*.
P to the present time there does not exist a complete theory

of the magnet. Notwithstanding the relative simplicity of the phenomena presented by soft iron, one could not expect to explain these apart; and the study of steel magnets is still too little advanced to supply the elements of a satisfactory physical theory.

Such being the situation, I thought that an experimental and close investigation of the phenomena presented by steel magnets (e.g. those accompanying their production, union, or separation) would not be devoid of interest. The present is a first attempt in this direction. The questions which form the subject of it, though hitherto very little studied, would yet offer numerous numerical verifications for any accurate theory of magnetism ; and this would suffice to render highly important researches of the sort we have undertaken.

Most of the investigations the subject of which has been magnetization by currents refer to soft iron. Lenz and Jacobit, Joule, Müller g, Wiedemann || especially, and more recently Quintus Icilius , Stoletow **, and Rowland +t preoccupied themselves with determining the magnetic moments, temporary or perma

* Translated from a copy, communicated by the Author, of a Thesis presented to the Faculty of Sciences, Paris, 1874.

† Pogg. Ann. vol. xlvii. (1839). # Phil. Mag. S. 3. vol. ii. (1839). § Pogg. Ann. vols. lxxix. & lxxxii. (1850, 1851). i Ibid. vols. c., cvi. & cxvii. (1857-1862). Ibid. vol. cxxi. (1864).

** Phil. Mag. January 1873. ft Ibid. August 1873. Phil. Mag. S. 4. Vol. 49. No. 323. Feb. 1875.

G

nent, developed by a current of given intensity in a bar placed in the axis of a spiral excited by the current. Several of these physicists treat also, subsidiarily, the same question for steel. As regards the accessory phenomena accompanying magnetization, they are so numerous and varied that they constitute an inexhaustible mine which still, notwithstanding numerous labours, has scarcely been touched. We will cite only those memoirs which have the closest connexion with the subject of the present investigation.

Quetelet * studied the magnetism produced in a bar of steel by friction with a magnet. He established that the magnetism increases, up to a certain limit, with the number of the frictions, according to precise laws, to which we will return by-and-by. Hermann + and Scholzs, under the direction of Frankenheim, proved an analogous augmentation when a bar of steel is brought near the pole (free or covered with paper) of an electromagnet, or when a steel bar is several times introduced into a spiral traversed by a current.

Coulombll, and afterwards Lamonts, in their numerous studies on all branches of magnetism, have enriehed the science with observations on the influence of the temper of steel upon its moment of saturation, and on the phenomena which accompany the union or separation of superposed magnetized plates. Villari **, and long previously Abriatt, made some experiments on the brief duration of the phenomenon of magnetization.

The temporary magnetization of steel, observed for the first time by Musschenbroek and Epinus, has been the subject of interesting memoirs by Poggendorff 11 and Wiedemanns.

But the most complete investigation we possess on steel magnets is found in the recent labours of M. Jamin | ||. These researcbes, which it is not our duty to estimate here, open to physicists a path in some sort quite new, and in which we should be happy to have made one step.

1. METHODS OF MEASUREMENT. The determination of the magnetic moment of a magnet is most frequently effected by one of the two methods indicated

* Ann. de Chim. et de Phys. Ser. 4. vol. liii.

De naturali magnetismo in Chalybem inducendo quanto momento sit tempus. Vratisl., 1865.

| Quanti sit momenti tempus in magnetismo inducendo, certa quadam fluidi galvanici intensitate adhibita. Vratisl., 1863. s Pogg. Ann. vol. cxxiii. || Mémoires de l'Académie, passim. Magnetismus.

** Pogg. Ann. 1873. tt Annales de Chimie et de Physique, Ser. 3. vol. i. 11 Pogg. Ann. vol. xlv.

IS Galvanismus, vol. i. Hill Comptes Rendus de l'Acad. des Sciences, 1873–74.

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and applied by Coulomb—the torsion method, and that of oscillations. The first is applied especially to bars; the second extends also to small needles, but loses much of its precision when the magnets are of very small dimensions.

Divers physicists, among others Wiedemann *, have calculated the magnetic moments of a magnetized needle from the deviation produced by it upon a very short magnetized needle furnished with a mirror and movable about a vertical axis. This process culiarly suitable wben the magnetic moments of one and the same needle variously magnetized are to be compared, provided that the magnetic moments to be compared are not too feeble.

In this case the needle A, whose magnetic moment we wish to compare, is fixed in a horizontal groove, the direction of which is sensibly perpendicular to the magnetic meridian. The distance from the centre of the needle A to the centre of needle B should be sufficiently great relatively to the length of A to avoid a notable alteration of the results by a slight displacement of the poles of A in conscquence of a more or less considerable magnetization. The azimuth of the line of the centres of A and B can be chosen arbitrarily; and if the needle A receive the same position in all the experiments, the deviations observed will be proportional to the moment it possesses. If the needle is regular, it

may be returned to its groove with its ends reversed, and the mean of the deviations be taken.

I have frequently employed this process of measurement in the study of magnetization by currents; but mostly, as in the study of the breaking of the current, I have had to measure the ratio of the magnetic

moments of needles of very small dimensions, and different in length and diameter. I have then made use of a special arrangement, which I will describe in detail,

Principle of the method.-Suppose a rigid support, movable about a vertical axis. Fix

upon this support (1st) a horizontal needle the magnetic moment M of which is known, (2nd) the needle whose magnetic moment x we

Fig. 1.
wish to determine. The two needles
are placed one above the other in such
wise that their axes form a right
angle, and at a sufficient distance
for their reciprocal action not to alter
the distribution of the magnetism in
either of them.

The system thus formed takes,
under the influence of the earth's
magnetism, a determinate position
of equilibrium, such that the mag-

* Galvanismus.

N

M

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a

M

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netic axis of the needle M (fig. 1) makes, with the plane of the magnetic meridian N S, an angle a determined by the equation x= M tana.

(1) If the moment x is sufficiently small in proportion to M, the angle a can be determined by Poggendorff's optical method*. For that purpose the support of the needles carries a small vertical silvered mirror, in which, with the aid of a telescope, the image of a horizontal graduated scale is observed, which is placed beneath and very near the objective, and perpendicular to the optic axis of the telescope.

The moment M of the directing needle may be made as small as we please. It follows that it is possible by this method to measure the magnetic moment of very small needles, comparable in their dimensions to particles of file-dust. I have been able to effect measurements relative to needles of 2 millims. length and 0.2 millim. diameter.

When we wish only to compare with one another the magnetic moments x, al of several minute needles, it is not necessary to know the moment M of the directing needle; we have, indeed, designating by d, a' the deviations they produce :

x tan a

;

tan a

tan a

= =

a

and, on account of the minuteness of the angles a and a',

x
tan a

a
tan 2a
n!

(2)

tan la n' where n and n' are the two readings made upon the scale, the zero of which is supposed to be in the plane of the magnetic meridian.

Apparatus.--The support of the needles is merely a small stick, thin and light, of sealing-wax, hard and not readily fusible. The directing needle A B (fig. 2) is attached to the lower extremity of the wax; and a very narrow glass tube T passes through the latter towards its upper part, in such a way that its axis is horizontal and perpendicular to the axis of the directing needle. The system is suspended by a nipper or hook of copper to a single cocoon-thread, and enclosed in a wooden box blackened on the inside, and closed in front by a plane glass. The cocoon-thread passes through an aperture in the top of the box, and is grasped by a brass nipper. And a disk of red copper is placed under and very near the directing needle, the oscillations of which it deadens.

*“Methode der Spiegelablesung" (Pogg. Ann. vol.vii. 1826), employed by Gauss (Göttingische gelehrte Anz. 1833, Nos. 205–207), to whom the invention of it is most frequently attributed.

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