« PreviousContinue »
332. M. Ampere also announced the fact of sions are entirely different from the ordinary tne attraction and repulsion of two wires con- electrical attractions ard repulsions. uecting the poies of a baitery; ard showed, that 335. M. Arago stated to the Royal Academy the magnetic needle, which had previously been of Sciences that he had ascertained the atiracused to prove the magnetic attractions and re tion of iron filings by the connecting wire of the pulsions of the wire, could be replaced by ano- battery exactly as by a magnet.
This fact ther connecting wire like the first. This disco- proved not only that the wire had the power very seemed to liberate the phenomena of mag- of acting on those bodies already magnetised, netism from any peculiar power resident in the but that it was itself capable of developing magmagnet, and to prove its production by electri- netism in iron that had not previously been city alone. When by Oersted’s discovery it had magnetised. When the wire in connexion with been shown that a wire connecting the poles of the poles of the battery was dipped into a heap a l'oltaic battery would act on a magnet, attract- of filings, it become covered with it, increasing ing and repelling it, just as another magnet would its diameter to the size of a goose-quill; the de, it was fair to assume that the wire possessed instant the communication was broken at the powers of the magnet it supplied ; and when either pole, the filings dropped off; and when the second magnet was replaced by another con-. it was re-established, they were re-attracted. necting wire, as in Ampere's experiment, and This attraction took place with wires of brass, the powers and actions still remained as before, silver, platina, &c., and was so strong as to act it was perfectly correct to consider these powers on the filings when the wire was brought near and actions as magnetical ; so that it became them without actual contact. It was shown not evident that magnetism could be exerted inde- to belong to any perinanent magnetism in the pendent of magnets, as they are usually called, wire or tilings by the inactivity of both when and of any of the means of excitation usually the connexion was not made with the battery; employed, but wholly hy electricity, and in any and it was proved not to be electrical attraction good electrically conducting medium.
by the connecting wire having no power over 333. The phenomena with two conductors filings of copper, or brass, or over saw-dust. situate between the poles of the battery are as When soft iron was fused, the magnetism given follows:- When they are parallel to each other, was only momentary; but on repeating the exand the same ends of them are similarly related periment, with some modification, M. Arago to the battery; viz. when the supposed currents succeeded completely in magnetising a sewingexisting in them are in the same direction, then needle permanently. they attract each other; but if the opposite ends 336. The theory which M. Ampere had formed, be connected with the battery, so that the cur to account for the magnetic phenomena of magrents conceived to exist in them are in opposite nets by electrical powers only, assumed that directions, they repel each other. If also, the magnets were only masses of matter, around one being fixed the other moveable, the cur- the axes of which electrical currents were moving rents be sent, or the connexions be made, in in closed curves. This theory led him, when inopposite directions, then the moveable one will formed by M. Arago of his experiments, to exturn round until they are in the same direction. pect a much greater effect if the connecting wire The contrast between these attractions and re were put into the form of a spiral, and the piece pulsiors, and those usually called electrical, are to be magnetised were placed in its axis. Acvery striking. The one take place only when the cording to the theory, in a needle or magnet, circuit is completed: the other only when it is pointing to the north, the currents were in the incomplete. The attractions take place between upper part from east to west. In consequence the similar ends of the wires, and the repulsions of these expectations, M.M. Ampere and Arago between the dissimilar ends; but the electrical made experiments with spirals or helices, and the attractions take place between dissimilar ends, results are mentioned in M. Arago's paper, on and the repulsions between similar ends. These the communication of magnetism to iron filings take place in vacuo, but those do not. When in the fifteenth volume of the Annales de the magnetic attraction brings the two wires Chimie. together, they remain in contact; but when 337. On twisting a wire round a rod, it may electrical attraction brings two bodies together, be made to pass either in one direction or the they separate after the contact.
other, giving rise to two distinct but symmetri334. "These experiments were varied in several cal helices, which have been named by botanists ways by M. Ampere; and the apparatus with dextrorsum and sinistrorsum. Though their which they were made appears, from the plates diameters be equal, and the spirals which comand description published, to be very delicate, pose them have equal inclinations, yet they can ingenious, and effectual. The general results never be superposed; for, however they are drawn up by M. Ampere himself from them turned about, their direction is the same. The are, (1.) That two electrical currents attract when dextrorsum, or, as we may call it, the right helix, they move parallel to each other, and in the same proceeds from the right hand downwards direction; and repel when they move parallel to towards the left above the axis; the tendrils of each other in a contrary direction. (2.) That when many plants exhibit instances of it, and it is the metallic wires, traversed by these currents almost exclusively used in the arts: the siniscan only turn in parallel planes, each of the trorsum, or left helix, proceeds from the left currents tends to direct the other into a situation hand downwards towards the right above the in which it shall be parallel, and in the same axis. direction. (3.) That these attractions and repul 338. Having made some of these helices, one
was connected by its extremities with the poles scription of the effects with spiral wires can be ; of a l'oltaic battery, and then a needle wrapped viz. considering it as a matter of experiment only, in paper placed within it; after remaining there and not of theory. a few minntes, it was taken out, and found to 343. In consequence of the view which M. De strongly magnetised; and the effect of a he- Ampere had taken of the nature of magnetism lix above that of a straight connecting wire was as dependent simply upon currents of electrifound to be very great.
city, it became an important object with him 339. Then, with regard to the position of the to ascertain the action of the earth upon such poles in the magnetised needle, it was found currents excited by the Voltaic battery; for, that, whenever a right helix was used, that end from his theory, he expected that it would be of the needle towards the negative end of the equally efficient in directing these currents as battery pointed to the north, and that towards in directing those supposed to exist in the magthe positive end toward the south; but that with netic needle. After some trials, he succeeded a left helix, that end of the needle towards the in overcoming the obstacles to delicate suspenpositive, pointed north ; and the other end south. sion, contact, &c., and constructed an apparatas
340. In order to establish this point, the con in which a part of the wire connecting the two Decting wire was sometimes formed into one poles of a battery was rendered so light and mobelix, sometimes into two or three, which was bile as to move immediately; the connexion readily done by twisting it round a glass tube, was completed with the pole, and took a direction or rod, first in one direction, then in another; which, with regard to the earth, was always conand when needles previously enclosed in glass stant, and in accordance with M. Ampere's theory. tubes were then placed in these helices, the An account of these experiments, with the appamagnetic poles they received were always in ac ratus used in then, was read to the Royal Acacordance with the statement just given. In one demy. The first consisted of a wire bent su as case, also, where the connecting wire had been to form almost a complete circle of about sixteen formed into three consecutive helices, the middle inches in diameter; the two extremities were one being of course different to the other two, a made to approach, and were placed one just besingle piece of steel wire sufficiently long to pass neath the other; and being aitached to two steel through all three of them being enclosed in a points, were connected by them with two little baglass tube was placed within them. On being sins of platina containing mercury, fixed so as to again removed and examined, it was found to receive them; only one of the points touched the have six poles ; first, a north pole, a little further bottom of the cup in which it was placed; so that on a south pole, then another south pole, a north the friction was scarcely any, and the mercury sepole, another north pole, and at the further end cured a good contact. The cups were connected a south pole.
with other wires that passed off to the Voltaic 341. M. Boisgeraud read a paper to the Royal battery; so that it was easy to make this moveable Academy of Sciences, containing the detail of circle connect either one way or the other between Dumerous experiments, most of which, however, the poles ; and being enclosed in a glass case, are variations of Oersted's first experiments. Ile any movement it might receive was readily obremarked, that connecting wires, or arcs, placed served without danger of its resulting from any any where in the battery, would affect the needle, other cause than the electric action. a result that follows as a consequence from Oer 344. When the extremities of this apparatus sted's and Ampere's experiments. He notices were connected with the poles of a battery, the the difference of intensity in the effects produced circle immediately moved, and after some oscilwhen bad electrical conductors were employed lations placed itself in a plane perpendicular to to complete the circuit, a difference which the magnetic meridian of the earth ; and, on every Oersted himself had pointed out in the case of repetition of the experiments, the same effect water. M. Boisgeraud, however, proposed to took place. The direction in which it moved ascertain the conducting power of different sub depended upon the way in which the connexion stances by placing them in one of the arcs, cells, had been made with the battery; and if it be of divisions of the battery, and bringing the assumed that there is a current passing through magnetic needle, or Ampere's galvanometer, the wire, from the positive to the negative end, towards another are, viz. to the wire, or other the curve so arranged itself that that current connecting body, used to complete the circuit in always passed downwards on the eastern side, the battery. With regard to the positions, M. and upwards on the west. This circle moved Bois geraud's notices of the needle and wire, they round a perpendicular, and, therefore, only reare all confirmatory of Oersted's statements. presented the direction of the magne‘ic needle:
342. M. Ampere read another memoir on the in order to represent the dip, a wire was formed phenomena of the Voltaic pile, and on the me into a parallelogram, and being fixed to a glass thod he intended to pursue in calculating the axis was suspended by fine points, and connected action of two electrical currents. Jle also as before, so as to move round a horizontal showed the mutual action of two rectilinear elec axis, then this axis being placed perpendicular trical currents ; viz. of two straight portions of to the magnetic meridian, and the wires being the connecting wires; for it appears that the connected with the poles of a battery, the paralphenomena of attraction, repulsion, &c., were lelogram immediately moved towards a position first observed with spiral wires. These actions, in the plane perpendicular to the dipping-needle; however, are exactly similar; and the view al- 'when the communication was broken, it returned Tearly given of them, as it relates to straight towards its first position; and when renewed, it wirs, is consequently more simple than the de- resumed the second, evidently indicating the
magnetic influence of the earth over it. In cor a magnet was made by torming a helix, and sequence of the difficulty of placing the centre making the wires at the two extremities return of gravity in the centre of suspension, and keep- through the centre of the helix half way, and ing it there, this conductor did not take its posi- then pass out upwards and downwards, so as to tion exactly in a plane perpendicular to the form a perpendicular axis on which the whole dipping needle, but approached towards it till might move. The extremity of a battery being in equilibrium between the magnetic and the connected with these two ends of the wire, the gravitating power of the earth.
helix became magnetical, and was attracted and 345. M.M. Biot and Savart read a memoir to repelled by a magnet precisely as a real magnet the Academy of Sciences, the object of which would have been. was to determine the law by which a connecting 347. M. Biot's examination of the effects of wire acted on magnetised bodies. Small rectan- magnetism, as impressed by electricity in motion, gular plates, or cylindrical wires, of tempered is too excellent to be passed unnoticed in our steel, were made magnetical by the double touch, chronological account of discoveries in this and being then suspended by silk-worm threads science. were placed in differed positions with, and at 348. “When the electric current, evolved from different distances from, the wire connecting the a Voltaic battery, is transmitted through any poles of the battery. The terrestrial magnetism metallic bodies whatsoever, it gives them instanwas sometimes combined with that of the wire, taneously a magnetic virtue; they become then sometimes opposed to it, and soinetimes neu- capable of attracting soft and unmagnetised iron, tralised by the vicinity of another magnet. The This curious fact was discovered by M. Oersted. different positions of equilibrium, and the num- If we expose to these metallic bodies, a magnetic ber of oscillations of the needles, were then ob- needle, they attract one of its poles, and repel the served, and data gained, by which M.M. Biot other, but only relative to the parts of their surand Savart were conducted to the following re- face to which the needle is presented. Needles sult, which expresses the action exerted by a of silver or copper are not affected, but merely molecule of austral or boreal magnetism, placed those susceptible of being magnetised. These at any distance from a very fine and indefinite effects subsist only under the influence of the cylindrical wire, rendered magnetic by the Voltaic electrical current. If we suspend the circulation current. Let a line pass from this molecule of the electricity, by breaking off the communiperpendicularly to the axis of the wire, the force cations established between the opposite poles of which draws the molecule is perpendicular to the Voltaic apparatus, or even if we retard conthis line and to the axis of the wire; its intensity siderably its velocity, by joining its poles with is reciprocal to the distance. The nature of the bad conductors, the magnetic power instantly action is the same as that of a magnetised peedle ceases, and the bodies which had received it placed on the surface of the wire in a direction return to their usual state of indifference. determinate and constant in its relation to the 349. This simple sketch already displays direction of the Voltaic current; so that a mo- many new properties. All the processes hitherto lecule of boreal magnetism, and a molecule of employed to magnetise bodies had produced an austral magnetism, would be drawn in different effect on only three pure metals, iron, nickel, and directions, though constantly according to the cobalt, and on some of their compounds, steel preceding expression.
for example, which is merely a carburet of iron. 346. M. Ampere noticed an effect produced Till now it was never possible to render silver, by the connecting wire bent into a helix. This copper, or the rest of the metals magnetic. But may be easily understood from considering that the electric current gives all of them this prothe direction of the magnetic power is always perty; it bestows it transiently by its presence; perpendicular to the conducting wire. When, and, as we shall presently see, it diffuses it therefore, the conducting wire is parallel to the through the whole mass, in a manner equally axis of the helix, the power is perpendicular to singular, and which has no reseinblance to what that axis; when the wire forms a circle round is produced, when we develope magnetism by the axis, in a plane perpendicular to it, the power our ordinary processes, which consist in longituis in the direction of the axis; but when, as in dinal friction with magnetic bars. the helix, it passes round the axis in a direction 350. “To produce these novel phenomena in intermediate between parallelism and perpendi- the simplest manner, we must, with M. Oersted, cularity, the direction of the power is of course in- establish a communication between the two exclined accordingly. In this case the power may tremities of the Voltaic apparatus, by a simple be considered as composed of two portions, one metallic wire, which may be easily directed and perpendicular to the axis, the other parallel to bent in all directions. We place afterwards, in it. As M. Ampere considered magnets to be the neighbourhood of the battery, a very sensible assemblages of currents perpendicular to their magnetic needle, horizontally suspended. As axes, he wished, in his imitation of them, to do soon as this is settled in the direction due to the away with that effect due to the extension of the magnetic force of the terrestrial globe, we take a wire in the direction of the axis of the helix, flexible portion of the conducting, or conjunctive and succeeded in this by making the wire at one wire, as M. Oersted calls it, and having stretched end return through the helix so as not to touch it parallel to the needle, we bring it gently near it in any part; for, in this position, its magnetic it, either from above, from below, from the right, effects being contrary to those belonging to the or from the left. We shall see an immediate length of the helix, and also near to them, they deviation of the needle; but, what is not the least neutralised or hid each other. An imitation of remarkable circumstance, the direction of this
deviation differs according to the side by which skilfully developed the consequences which the conjunctive wire approaches it. Duly to flowed from this property. His researches, comprehend this astonishing phenomenon, and to which preceded those of the other French philos fix iis peculiarities with precision, let us suppose sophers, have considerably occupied the Acathat the conjunctive wire is extended horizontally demy; hut as the order of exposition, occasioned from north to south, in the very direction of the by the mutual dependence of the phenomena, magnetic direction in which the needle reposed, binders me from beginning with them, I have and let the north extremity be attached to the endeavoured to compensate for this inversion by copper pole of the trough, the other being fixed rendering justice at once, to labors which have to the zinc pole. Imagine, also, that the person anticipated and facilitated others. who makes the experiment looks northward, 353. • In the above experiments which M. and consequently towards the copper or nega- Oersted had made, the conjunctive wire is pretise pole. In this position of things, when the sented to steel needles, previously magnetised. wire is placed above the needle, the north pole It may be asked, if the action then exercised is of the magnet moves towards the west; when the proper to the conjunctive wire, as the action of wire is placed underneath, the north pole moves a bar of steel tempered and magnetised is protowards the east; and, if we carry the wire to the per to this bar, or it the action is communicated right or the left, the needle has no longer any to the wire by the presence of the magnetic lateral deviation, but it loses its horizontality. needle, as we see soft iron, which exercises no If the wire be placed to the right hand, the north maguetic power of itself, acquire transiently pole rises; to the left, its north pole dips; and this power in the presence of magnets? To in thus transporting the conjunctive wire all decide this question it was necessary to examine around the needle, in directions parallel to one whether a body, not magnetic in itself, but capaanother, we merely present it to the needle, by ble of becoming so by influence, soft iron for exthe different sides of its circular contour, without ample, would experience a sensible action at the affecting in the least the proper tendency of the approach of a conjunctive wire, traversed by the needie towards the terrestrial magnetic poles. Voltaic current. This was effected by M. Arago, Since then the deviations observed in these suc- who showed that filings of iron are attracted by cessive positions are first of all directed from these wires ; a simple but important fact, which right to left, when the wire is above the needle; defines clearly one of the characters of the force then from above downwards, when the wire is by which the phenomenon is produced. to the left; from the left to the right, when the 354. "The first thing which we must determine, wire is beneath; and, finally, from below upwards is the law according to which the force enanatwhen it is to the right hand, we must necessarily ing from the conjunctive wire decreases at difconclude from these effects, that the wire de- ferent distances from its axis. This enquiry has ranges the needle, by a force emanating from been the object of experiments which I made itself
, a force directed transversely to the length along with M. Savart, already known to the of its axis, and always parallel to the portion of Academy by his ingenious discoveries in acousits circular contour, to which the needle is oppo- tics. We took a small needle of magnetised site. M. Oersted drew this inference from his steel in the form of a parallelogram, and, to first observations.
ensure its perfect mobility, we suspended it 351. . Now this revolutive character of the under a glass bell, by a single fibre of the silkforce, and revolutive according to a determinate worm, and gave it at the same time a horizontal direction, in a medium which like silver, copper, direction. Then, in order that it might be enof other pure metal, seems perfectly identical in tirely at liberty to obey the force emanating all as parts
, is a phenomenon very remarkable, from the conjunctive wire, we screened it from of which we had heretofore only one singular ex- the action of the magnetism of the earth, by ample in the deviations which certain bodies placing a magnetised bar at such a distance, and impress on the planes of polarisation of the lumi- in such a direction, that it exactly balanced this nous rays. The first fact of the magnetism, action. Our needle was thereby placed in the transiently impressed upon the conjunctive wire same freedom of movement as if there did not by the Voltaie current, might have offered itself exist any terrestrial globe, or as if we had been to a vulgar observer. I do not know whether able to transport ourselves with it to a great dissotne traces of this property may not have been tance in space. We now presented it to a conpreviously perceived and indicated; but to have junctive cylindrical wire of copper, stretched in recornised this peculiar character of the force, a vertical direction, and to which we had given and to have delineated it, agreeably to its pheno- such a length, that its extremities necessarily mena, without hesitation and uncertainty, is the bent in order to attach them to the poles of the praise which truly belongs to M. Oersted, and electric apparatus, should have, in consequence which constitutes a condition entirely new in the of their distance, so feeble an action on the needle movement of electricity.
that it might be neglected with impunity. This 352. “ As soon as this beautiful discovery was disposition represented therefore the effect of an known in France, England, and Germany, it indefinite vertical wire, acting on a horizontal excited the most lively sensation among men of and independent magnetic needle. As soon as stience. One of our colleagues, in particular, the communication of the Voltaic current was M. Ampere, ardently verified it in all its cir- completed, the needle turned transversely to the cumstances. Seizing with sagacity the revolu- axis of the wire, conformably to the revolutire tive character of the force impressed on the con- character indicated by M. Oersted ; and it set jencive wire, he directed it with judgment, and itself to oscillate around this direction, as a
clock pendulum, moved from the perpendicular, ble breadth, whose surfaces are composed of oscillates round the vertical by the effect of gra- parallel right lines, we find that all these bodies vitation. We counted with an excellent seconds act on the magnetic needle, as bundles of wires watch of M. Breguet, the time in which a certain parallel to their length would do ; which proves number of these oscillations, twenty for example, that the power developed in them by the elecwere performed; and by repeating this observa- trical current is exerted freely through their very tion, when the wire and needle were at different substance, and is not weakened by their interposidistances, we inferred the decreasing intensity of tion, as the radiation of heat through hot bodies the force, precisely as we determined, by the is enfeebled and intercepted by the interposition oscillations of the same pendulum, the variations of these very bodies. of gravity at different latitudes. We thus found 357. “Instead of leaving the needle in the that the force exercised by the wire was trans- preceding experiment at liberty to move, fix it verse to its length, and revolution, as M. Oersted invariably, but render the conjunctive wire movehad observed; but we discovered besides that it able, by suspending it on two points; then it will decreased in a ratio exactly proportional to the be the latter which will move towards the needle, distance. However, the force which we thus or recede from it. In fact, it is a general law of observed was in reality a compound result; for mechanics, that re-action is always equal to action. on dividing in imagination the whole length of If the wire attract or repel the needle in certain the conjunctive wire, into an infinity of seg- circumstances, the needle ought in the same cirments of a very small altitude, we perceive that cumstances to attract or repel the wire. This each segment ought to act on the needle, with experiment belongs to M. Ampere. different energy according to its distance and 358. “Now, let us operate no longer with a direction. Now these elementary forces are pre- magnetic needle on the wire placed in its posicisely the simple result which it is important to tion of mobility, but let us expose it to the mag. know; for the total force, exercised by the whole netic action of the terrestrial globe, which is wire, is merely the sum of their actions. Calcu- known to be perfectly similar to that of a comlation enables us to pass from this resultant mon magnet whose poles are very distant. This to the simple action. This has been done by M. force will likewise make the conjunctive wire Laplace. He has deduced, from our observations, move according to the same laws, at least if it be that the individual law of the elementary forces, sufficiently freely suspended, and it will impress exercised by each section of the conjunctive on it a determinate direction relative to the plane wire, was the inverse ratio of the square of the of the meridian, just as it would direct any other distance; that is, precisely the same that we know magnetic body. This result was realised by to exist in ordinary magnetic actions. This ana- M. Ampere. lysis showed that, in order to complete the know 359. · Finally, instead of presenting a conledge of the force, it remained to determine if junctive wire to a magnet, present two conjuncthe action of each section of the wire was the tive wires to one another, in parallel positions. same, at an equal distance, in all directions; or Then if the revolutive direction of the force be if it was inore energetic in a certain direction the same for the two wires, they will both conthan in others. I have assured myself by delicate cur in giving one direction to a magnetic needle experiments that the last is the case.
placed between them; but, if the direction of 355. “What we now know of the law of the the revolutive movement be opposite in each, forces, is sufficient for explaining and connect- they will tend to turn the needle in opposite ing together a multitude of results, of which I directions. These are simple consequences of now proceed to indicate briefly some of the most the law of the forces. Now, on trying these two curious. For example, let us conceive as we arrangements, M. Ampere has found that in the have done above, an indefinite conjunctive wire, first the two wires come together, and that in the stretched horizontally froin south to north. Let second they mutually repel each other. Thence us present laterally to it a magnetic needle, of a we must make two inferences; first, that the cylindrical shape, and suspended so that it can wires exert on each other actions perfectly analo take no movement but in the horizontal direc- gous to those which they exercise on magnetic tion. For greater simplicity, let us withdraw it needles; and next that the distribution of these from the influence of the terrestrial magnetism, forces in each of their particles is analogous as to by neutralising this influence with the action of direction, with what it is in magnetic needles a magnet suitably placed. This being done, themselves. These two new conditions relative when the needle rests at the same height as the to the nature of the force, render this experiment wire, so as to point exactly to its axis, it is very important. neither attracted nor repelled; but, if we raise it 360. In the different arrangements which we above the wire, it presents one of its poles to it, have just described, the conjunctive wires and and makes an effort of approximation. If, on the the magnets attract or repel principally by their contrary, we sink it below, the needle turns most contiguous parts; for, with regard to the about, to present its other pole, and is then rest, their distance rapidly diminishes the attracted anew. But, if we constrain it to present action. llence it is evident that we should augthe same pole as at first, the needle is repelled, ment the energy of the effects, if we approximale and the effects are precisely inverse on the right together the different parts of the conjunctive and on the left hand of the wire.
wire, preserving to them, however, the same 356. “ If, instead of transmitting the electrical general line of action. M. Ampere has also current across a simple wire, we make it pass verified this position, by coiling the conjunctive through tubes, plates, or other bodies of a sensi- wire in the form of a flattened spiral, on the