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Thus, two needles of two centimetres in length, magnetized, the one without envelope, the other in a small cylinder of pewter of two millimetres radius, by a single discharge, employed for the first 8', 30", the second 43′′ only made sixty oscillations.

Three needles, of fifteen millimetres in length, and Omil., 4 in diameter, the one in a cylinder of copper of five millimetres radius, the second in a similar cylinder of brass, the third without envelope, received in the same helix, quantities of magnetism having for their measure the numbers + 2′, 35′′; + 45′′; — l′ 52′′, durations of sixty oscillation for each needle. The sign which precedes the latter value, indicates that the needle without envelope was magnetized in a contrary direction to the others. The discharge was very strong, and the cylinder of copper had almost destroyed the action which the pewter on the contrary had considerably raised.

By comparing two metallic tubes of equal thickness and length, but of different radius, and consequently of different masses, we find that the largest, that whose mass is the most considerable, exercises the greatest action. If two tubes have the same diameter, the same thicknesses, and unequal lengths, the shortest is that whose action has the most influence. Of two helices, on the contrary, the longer is the most powerful. I suppose the length of the tubes sufficiently large in relation to that of the needles.

Here is an example. I compound two cylinders of pewter of three millimetres in thickness, one of sixty-five millimetres in length, the other 100 millimetres. Their action, with a rather weak discharge, were in the proportion of about three to one.

There must exist then between the thickness and the length of a metallic cylinder a certain relation, with which the influence of this cylinder is the greatest possible, under the action of a given discharge.

If a metallic tube is at the same time very long, and of au interior sufficiently great, the needles parallel at its axis receive quantities of magnetism sensibly equal in all the space enveloped by it, at least a short distance from its extremities.

We may change the nature of the isolating substance which separates the helix from the metallic envelope, and give to this envelope in the interior of the helix any position whatever, provided that the axis of their figure is always parallel, without changing the action of the metal on the needle which it encloses. On comparing these two experiments with experiments quite similar to those of M. Arago, on the direct magnetization of needles without envelopes we see that the electric movement acts in the same manner on any metal whatever, and on steel, which alone preserves its magnetism.

When we multiply concentrically around the needle alternating layers of conducting and non-conducting materials, the first actions do not appear sensibly modified by the fact of their insulation. There is no doubt, on the contrary, that their action is not very much enfeebled by the sections perpendicular to the axis of the figure, or in which these planes pass by this axis. In fact, envelopes of great

thickness, of fine copper filings, or even of iron, equal in weight to the cylinders of those metals which completely destroy the influence of a given discharge, scarcely at all modify this influence. We see then the analogy which exists between these results with the beautiful experiments of M. Arago, on the rotation of plates.

If in a metallic cylinder, for example, in a tube of glass, filled with mercury, needles parallel to its axis are placed at different distances from the surface, from the circumference to the centre, and we then compare them to a needle submitted without envelope in the same helix to the same discharge, we at first remark a gradual increase of intensity, a maximum, and then a diminution, which extends to the centre. If the discharge is weak enough, or the envelope sufficiently thick, there is a certain radius on which the sum of the actions of the metal is null. It is even very probable that, with electric discharges much stronger and metallic thicknesses proportionally greater, we should find several concentric surfaces where the action is null.

I here give some measures of magnetic intensities, obtained at different distances from the metallic surface, in a tube of glass of 10 mill. interior radius, filled with mercury :

Distance of the exterior

surface of the liquid... 1 5; 2m. 0; 2m 8; 40; 60; 12. 0. Duration of 40 oscilla

tions............... 28" 9; 28′′ 8; 29′′ 1; 30" 8; 49′′ 2; 1′ 22′′ 7. A needle magnetized with the same discharge, out of the influence of the mercury, made the same number of oscillations in l′ 49′′ 3: the needles were 2cent. long, 0m. 5 diameter.

The only metals that I have tried in very thick cylinders, iron, copper, pewter, and mercury, act with a decreasing energy. As to non-metallic conductors, such as nitric acid, sulphuric acid, and water, if their energy is not null, the experiments I have attempted were not sufficiently delicate to enable me to recognize it in a certain

manner.

We have just seen how the metals, submitted to the influence of discharges in the interior of helices, modify the magnetism. It will be easy to conclude from that the manner in which these metallic plates, under the influence of discharges transmitted by a rectilinear wire conductor, act on the steel needles disposed transversely to this wire; however, as this action presents two distinct cases, I now proceed to expose them separately. I suppose the needles to be in contact with the metallic surfaces, and the discharges inferior to those which, by their direct action, produce in steel, following the distance of the wire, opposite magnetic states.

1st. A large plate interposed between the conductor and the needles for very feeble discharges, considerably enfeebles the

It is almost indifferent whether the needle be or be not isolated from the conductor; that is to say, if it is sufficiently isolated by the defect of absolute contact, which can only be established by means of a strong pressure.

magnetism, and augments it with stronger discharges. Thus, for a similar discharge, a thin plate and a thick one may produce contrary effects, and there is a certain thickness where the effect is null.

2nd. The needles put on the plate, between this place the wire; with very feeble discharges it augments their magnetism, and more so as its thickness is increased. There is a certain discharge with which a thick plate augments it and a thin plate diminishes it. With stronger discharges they are both enfeebled, the latter especially, and it finishes by giving to the needles a contrary magnetism to that which the current itself developes.

In general the two faces of the same plate exercise contrary actions.

When the discharge is strong enough to magnetize the needles by its direct action, in contrary directions following their distance from the wire, the magnetism, under the influence of the metals, results from several causes, each of which is submitted to these laws periodically different. The different parts of the metallic plates also pass, as well as the needles, through a series of opposite states, and in each of these states they act as a magnet possessed of an analogous polarity.

In searching to compare the different plates of different metals, of similar form and equal thickness, it was remarked that soon not only that the relations of their actions varied, but that the order of the series that was wished to be formed by it was found to be reversed. The red copper in thin plates acted less than the brass; in still thinner plates it finished by acting more. I here give some numerical values. The direction of the magnetism is always designated by the signs and . Three needles at two millimetres beneath the conductor, first, magnetised on a plate of glass, the duration of sixty oscillations: +1', 25"; second, on a plate of pewter,

-1', 28′′; third, on a similar plate of copper, - 1′ 56′′. With a

stronger discharge and two thinner plates of copper-1', 4′′; pewter -1', 34". The difference between the action of tin and that of pewter is not greater than the difference between the actions of pewter and copper. I find the three following values with three similar needles; on the glass + 1', 34′′; on the tin, 1′ 6′′ the pewter, 1', 36′′.

; on

Silver acts almost like copper, gold has a much greater action. The action of metallic envelopes is a means of studying what takes place in the different parts of the steel needles themselves during magnetization. It is, in fact, in the same manner as other metallic envelopes, that the exterior laminæ of the needles act in the interior parts, and this influence may differ materially from the magnetic action which they exercise at a later period, as particles magnetised in a permanent manner. The experiment I have reported on magnetism, produced at different distances from the surface, in a tube of glass filled with mercury, is an example of this class of researches. It would be necessary, to render it complete, to distribute the needles

throughout the whole mass and up to its extremities. The reaction of these needles on the metal which envelopes them, at least in a first approximation, may be neglected.

In proportion as with equal charges the tension diminishes, as is the case when we augment the extent of the electrized surfaces, the influence of metals during the discharge becomes more feeble. This influence will doubtless be sufficiently small under the action of a current of electricity, circulating without interruption and without sparks from the conductor to the cushions, in the frictional machines powerful enough to produce them, as M. Rodolfi has announced permanent magnetism in steel needles. This same current however may offer, by its action on the metals, characters which distinguish it from voltaic currents, as it is already distinguished by the greater extent of conductors of a very small diameter which it may traverse. I have not yet, for want of a convenient apparatus, verified these conjectures.

On Magnetism produced by Voltaic Currents.

The phenomena I have just exposed are reproduced and measured with facility. This is not the case with those I am about to describe. A great number of causes make them vary every moment. And again I have only been able to draw from researches still very incomplete, a small number of general results.

All the points of a conducting wire, equal and homogenous, traversed by a voltaic current, exercise equal actions.* If it is rectilinear, it magnetizes equally in all its lengths, at least for a length which is not very great and at some distance from its extremities, but it magnetizes very little if the pile is not very strong. I prefer rolling it from space to space in such a manner as to form several small helices of some turns each of it, similar and separated by any portions of figure and extent whatever. Similar needles are equally magnetized in each of these helices.

The magnetization by voltaic currents, is entirely developed in a very short time; it is sensible instantaneously, at least with very small needles. A feeble pile may act during a long time, if its intensity is not augmented, even on a needle not tempered, without changing in an appreciable manner the degree of magnetism which it gives to the needle at the moment of establishing the communication. This is true, even when the degree is very far from a state of saturation.

In general, in whatever manner the pile is varied, if the current does not change its direction during the time a needle is submitted to its action, the needle will be magnetized as if the pile had always had the strongest of these different intensities. The magnetism produced by a pile, then, may furnish very different indications from those which we draw from the finished deviations of an already magnetized needle.

M. Becquerel has already proved this fact from another class of observations.

Independently of the slow variations of the continuous current which traverses a pile, the tension at the instant the communication is established, as at the moment at which it is broken, produces a visible spark, or at least a transmission of electricity analogous to the discharges procured from the frictional machines. The same effect may be repeated, at intervals more or less apart, every time that the communications are only imperfectly established, as by the contact of wires of copper and mercury, especially if the surfaces are moistened with a saline mixture.

We ought, then, to observe again, in the magnetization by the pile, the phenomena observed in the passage of an electric discharge, modified by the action of the continuous current and the smallness of the tension.

*

In proportion as the tension is augmented, and the conductibility diminished, that part of the effect of discharge is greater, the action of the current continues with less influence. Thus, with an apparatus of 20 pairs, and a very bad conducting liquid, I have obtained in a most certain manner, since the needles several times presented the same results, a direction of magnetism contrary to that which gave the same pile the most lively excitement. Some needles, which passed a little beyond the extremity of the small helices were then magnetized in the same direction, and more strongly than the needles placed at the centre of the same helices. The contrary took place when the pile was more energetic. I have not however sufficiently observed this phenomenon to point out the cause of it, if this is not the possible cause. The dry piles ought to produce it easily.

It is especially in the action of other metals besides steel and iron, that the influence of small tensions is sensible. If two needles are placed in the same helix, the one without any envelope, the other encircled by a thick cylinder of copper of five millimetres radius, for example, an energetic current magnetizes them almost equally, and more so as the conductibility is greater. A feeble current gives them degrees of magnetism still more different; the communication of which we may renew and interrupt successively a great number of times, as the conductibility is more imperfect, and the tension stronger.

Thus with an apparatus of ten pairs, feebly excited, I find that by multiplying the immersions of wire in mercury, a very small needle may be magnetized in such a manner as to make sixty oscillations in 36"; a similar needle placed in a cylinder of copper, the same number of oscillations in 1', 2". The plates having been lowered into the liquid, and afterwards withdrawn, without having touched either the communication between the poles established

The conducting wire was soldered to the extreme plates, and the needles placed before the immersion of the plates were not removed until it was withdrawn from the liquid.

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