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At least the subject would deserve to be taken up, and it would be interesting, as much in theory as in practice, to determine, better than has been done, the best form that should be given to compass needles.

It would remain to us to examine to what degree the existence of currents in a direction from east to west below the surface is possible; to what cause they may be attributed; if they are reconcilable with the other phenomena of terrestrial physics: finally, if they may be perceived directly. These are so many interesting questions, of which we shall treat in the Fifth Part, the subject of which is terrestrial electricity or magnetism. We shall then see which hypothesis of the nature of this magnetism is most calculated to account both for its action upon electric currents and upon the magnetised needle; and if this double action may be readily included in the same explanation.

CHAP. III.

ON MAGNETISATION BY DYNAMIC ELECTRICITY.

WE have already seen, in the preceding Chapter, that immediately after Oersted's discovery, M. Arago showed that an electric current attracts iron filings and produces magnetisation just as a magnet would do. He always found that, in order to impress a more decided magnetism upon a steel needle, it was necessary to employ a conductor bent into a helix, within the axis of which he placed the needle, instead of using the rectilinear current. The influence of this form, when given to a conductor in electro-magnetic phenomena, is altogether in harmony with Ampère's theoretical ideas. M. Arago equally succeeded in magnetising a needle, whether employing the current of the pile or the discharge of an electrical machine; and, which is still better, that of a Leyden jar. Davy, on his part, had shown that a needle may be magnetised by placing it transversely over a wire traversed by a voltaic current, or by an electric discharge. No long time elapsed before it was demonstrated that the employment of electric currents, which is excellent for magnetising soft iron, whose coercitive force is almost null, is insufficient to impress powerful magnetism upon steel needles, whose coercitive force cannot be completely overcome, especially when they are tempered, except by the discharge of one or more Leyden jars.

We will proceed, therefore, to study in succession the magnetisation of steel, produced essentially by electric discharges; that of soft iron, produced especially by electric currents; and the various phenomena, especially the molecular, that give evidence of the magnetisation developed by dynamic electricity.

Magnetisation of Steel.

When a steel needle is magnetised by passing the discharge

or a current through the wire of a helix, the precaution must be taken of placing the needle in a glass tube, around which the wire is coiled, in order to prevent the electricity passing through the needle, by the contact of the steel with the wire, instead of making the tour of the spirals. If we examine the position of the north and south poles in a needle thus magnetised, we find, by admitting Ampère's theory, that the discharge has determined currents in the steel pursuing the same direction as that which has produced the magnetisation ; which ought to be the case in this theory, because magnetisation consists in giving a uniform direction to the electric currents, pre-existing around the particles of steel or iron. This effect is produced in the most direct and most advantageous manner by the action of exterior currents, arranged as the molecular currents should be in the magnetic body, were it magnetised, and which oblige these latter to place themselves in the same direction as they have themselves, and consequently in a position parallel to each other.

Independently of the direction according to which the discharge traverses, the direction in which the helix is wound naturally influences the direction of the currents, and consequently the position of the magnetic poles. In a righthanded helix, namely, one in which the wire is wound to the right, the south pole of the needle is always at the extremity through which either the discharge or the current enters; or, which comes to the same thing, at the extremity that is in communication with the positive electricity. In the left-handed helix, namely, that in which the wire is wound towards the left, the north pole is at the extremity through which the positive electricity enters. This also is a rigorous consequence of Ampère's theory, as is demonstrated by the directio of the

Fig. 123.

current (Fig. 123.). M. Arago, having wound a long wire around the same tube so as to make in succession several

helices contrary to each other, and having placed therein a long steel needle, and after having made a powerful discharge or an energetic current traverse the system of helices, found that the needle was magnetised, but that it presented a consecutive point at the junction of each helix. With two contrary helices, we obtain the same pole at each of the extremities of the needle, and a single contrary pole in the middle; that is to say, three poles: with three helices, we obtain contrary poles at the two extremities, and two consecutive points between each pair; and so on. M. Arago also recognised that, if the helix is long in respect to its diameter, and if the spirals are very near together, the position of the needle in the interior of the tube has no influence over the degree of magnetism that it acquires, provided that it is always placed parallel to the axis. The magnetisation is very feeble if the needle is placed exteriorly to the helix, even when care is taken to place it as near as possible, and in a position parallel to the axis.

M. Nobili endeavoured to employ a flat spiral for magnetisation instead of a helix, such as Arago had employed, or a straight wire, such as Davy had used. He placed the steel needles between the spirals, insulated from each other, and perpendicularly to their plane; he passed an electric discharge through all the wire of the spiral. The needles situated near the centre were magnetised in a contrary direction from that in which those were magnetised that were at a greater distance off; and there was a point, at a certain distance from the centre, where the magnetisation was null. We should remark that, in this experiment, each needle is placed between two currents, which, moving in the same direction, ought to give it a different magnetisation; for in order that the action of the currents should conspire, it would be necessary that the current situated on one side of the needle should move in a contrary direction from the current situated on the other side. The definitive magnetisation, therefore, depends upon the relative intensity of the two currents whose individual action is opposed. We shall see further on why near the centre the one prevails, and why near the edge of the spiral

it is the other. We must first analyse this class of effects in less complicated cases. This analysis was first made by M. Savary, and afterwards completed by M. Abria. We will put forth the remarkable results to which these two philosophers successively arrived.

The first of these results is, that the intensity and even the direction of the magnetisation produced upon a steel needle by a discharge transmitted through a rectilinear wire, depends upon the distance of the needle from this wire; in such sort that the intensity, far from diminishing constantly with the distance, augments with it from a certain point, where it is at its minimum; and that, on the same side of the wire for a given direction of discharge, the same magnetic poles are found placed at one extremity or at the other, according to the distance of the needle or the conductor. Thus, near the wire, the poles are placed conformably to theory; but at a greater distance, and by the mere fact of this augmentation of distance, they are found placed in a contrary direction. The wire employed by M. Savary for transmitting this discharge was of platinum, was about two yards (two metres) in length and th in. in diameter. The steel needles, which were all as much alike as possible and strongly tempered, were about

an inch in length (·5895 in.), and th in. (·0098 in.) in diameter. He judged of the intensity of the magnetism that they had acquired by the number of oscillations they made under the influence of terrestrial magnetism. The electric discharge was produced from a battery of 22 sq. ft. of surface. In order to prevent the needles being mutually influenced, they were not placed vertically above each other; but care was taken, while still placing them at different heights above the wire, of separating them in the horizontal direction; which was easily done, in consequence of the length of the discharging wire.

The following table points out for each needle, beside its vertical distance above the wire, the duration of 60 oscillations and the direction of the magnetism that it has acquired by the effect of the discharge. The positive direction is that which corresponds to the direction of the current, conformably

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