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wire to dip in mercury; which will not only carry off the stream of electricity in various directions, but also allow of perfect freedom of

notion.

501. Let us now investigate the action of currents moving in the circumference of a circle, which may be examined by means of a conducting wire bent into such a form, that its extreinities come very near each other, but are prevented from touching by being covered with silk, or other insulating substances. The force acting on such a wire, while it is transmitting electricity by the influence of a rectilineal current in the vicinity, will tend to bring it into such a position, as that its plane shall coincide with that of the rectilineal current, and so that the direction of the currents in the adjacent portions may be the same. In this position, these adjacent portions attract each other, while a repulsion is exerted between the straight current and that in the remoter part of the circle, which moves in the opposite direction. These two forces are, therefore, opposed to each other; but the attractive force prevails, on account of the greater vicinity of the attracting than of the repelling portions. That portion of the original force which made the plane turn upon itself, so as to bring it into parallelism with the straight current, must be regarded as the directive force. It is here composed of two forces, the one attractive, the other repulsive, but which, acting on opposite sides of the axis of rotation, concur in their effect. Thus while the approximative force is the difference of the two forces, the directive force is equal to their sum.

502. It will be easily understood, that forces of a similar kind are the result of the action of the circular current on the straight wire; and that the latter is urged to assume a position in the same plane with the former, and so that the ad jacent currents may be in similar directions.

503. The action of a circular current upon a small portion of a straight current at right angles, or otherwise inclined to the plane of the former, and lying wholly on one side of it, will be somewhat modified. If the direction of the straight current, when prolonged, pass near the centre, the forces which act upon it will be nearly balanced, and no re-action will result; if it be near to the circumference the action of the adjacent portion will predominate, and we shall obtain results analogous to those which we have traced with regard to straight wires. Revolving motions will result either in the straight wire or in the circular one, according to their positions, and according to the direction of the fixed points which may limit their movements. The direction of the motion is determined by the circumstances of the approach or recession of the current to or from the same point, as we have before explained.

504. Having considered the action of a circular on a rectilineal current, we may now study the reciprocal action of two circular currents. When the centre of the one lies in the plane of the other circle, a directive force will operate, tending to bring its whole circumference into that plane, and to assume a new position; and when, in this position, the resultant of all the

forces which are in operation will be an attrac tion or a repulsion, according as the currents in the adjacent portions of the circumferences are moving in the same, or in opposite directions.

505. If the two circular currents be situated opposite to each other, so that the centre of the one be in a line perpendicular to the plane of the other, and passing through its centre, similar phenomena will take place with respect to the directive and approximative forces; which will produce, in the first place, a tendency to parallelism, and then either attraction or repulsion. For each position of the centres intermediate to the two former, we shall find a particular position of equilibrium in planes inclined at a certain angle, whose intersection is exterior to the circles themselves. This position of equilibrium is determinate, and excludes the possibility of any continued rotatory or revolving motion.

506. It is an important preliminary to the study of Ampère's theory to obtain correct ideas of the action of the two circular currents upon one another, because such currents are supposed, in his hypothesis, to be the elements of all magnetic action. This magnetic action may be regarded as the resultant of the forces exerted by every part of the circular currents; and as constituting two forces emanating from its centre, and being of an opposite species on each side of the plane of the circle. If we suppose the circle to occupy a vertical plane passing nearly through the eye of the observer, placed without the circle, and the current of positive electricity to be passing downwards on the side next to the spectator, and upwards on the remoter side, then the force exerted on the side to the right of the plane, considered as a magnetic force, will correspond to the northern polarity, or to a polarity belonging to that end of the compass which turns to the north. The force extending to the left, will, of course, correspond to the southern or opposite polarity. When two circular currents are brought together on the sides where similar polarities reside, they repel each other, because the two currents are then moving in opposite directions in each; but, when the dissimilar polarities are presented to each other, attraction takes place, because the currents are then similarly directed.

507. The intensity of all these forces is much increased when the powers of several circles are combined, which may be obtained by bending the conducting wire so as to compose a spiral; the successive coils of which will conspire together in producing the respective polarities on each side: spirals thus constituted act exactly as magnets whose poles might be supposed to be situated in the centre of each disc.

508. But the im tation of magnetic bodies is rendered still more complete, when the turns of the wire are made, not in the same plane, but on the surface of a cylinder, so as to form a helix instead of a spiral. If the wire, after having formed a helix, be bent back so as to return in a straight course in the interior of the cylinder, with the usual precautions against contact, we obtain a very perfect accordance with the theory. already examined.

509. We come now to the subject of terrestrial magnetism. If we assume that the action of the solar rays on successive parts of the torrid zone, from east to west, produces currents of positive electricity in that direction, and which may be regarded as collectively circulating in what may be called the magnetic equator; attended, as they must always be, by counter-currents of negative electricity in the same equator, but in the opposite direction; and also that these currents have the same properties and modes of action with all other electrical currents,-then there must result, as a necessary consequence, a twofold polarity, apparently belonging to the earth, and directed to the pols of this magnetic equator.

510. It is to be observed, that as that polarity, which is situated near the north pole of the earth, results from a current moving in a direction similar to that of the hands of a watch, it will have the properties of a southern polarity, in the sense in which we have invariably used the term; that is, it will attract the north pole of a magnet, and repel the south pole; while actions the reverse of this will take place in the southern hemisphere. It is unnecessary to remark, how exactly this theory accords with all the known facts relative to the action of the earth on magnets. The directive power which acts on magnets on the surface of the earth, is the result, not of any real influence proceeding from that part of the earth to which their poles point, but of the action of the currents at the magnetic equator, and the tendency of the currents in the magnet itself to turn it, so that they shall attain the position of equilibrium we have already adverted to, in considering the mutual action of two circular currents. This position is precisely the plane which is perpendicular to the line of magnetic direction; that is, to the axis of the dipping needle: for as the electric currents in the needle are at right angles to its axis, it follows that when they arrange themselves in conformity with the equatorial currents of the earth, that are circulating east and west, that axis and the whole needle will point to the north and south --as we find they actually do.

511. The nature of this influence is more clearly discernible when it is exhibited in its simplest form, on a single circular current, which, as we have seen, may be regarded as the element of a magnet. A conducting-wire bent into the form of a circle, when free to move, always assumes, by the electro-magnetic action of the earth, a position in a plane descending to the south, intersecting the horizon in a line passing east and west, and inclined to it at an angle which is the complement to the dip; that is, in a plane which is perpendicular to the magnetic meridian. Its northern and southern polarities are equally real with that of a magnetic needle; but appertain to an imaginary axis passing through the centre of the circle and perpendicular to its plane. The direction of the currents on its south side, or that nearest the equator, is similar to those in the earth's equator-that is, from east to west (the positive current being always understood as defining the direction).

All these circumstances are sufficiently illustrated in the theory we have been endeavouring to explain, and of which we shall furnish experimental data in referring to the apparatus con trived by Mr. Barlow, and employed in the theatre of the London Institution.

512. The same phenomena are observed, if the course of a moveable wire be that of a parallelogram; or, indeed, any plane figure which returns into itself, as well as if it were a circle. By varying these forms, we are enabled to observe and distinguish the effects of the earth's influence on wires which are parallel to the direc tion of the dip, and on such as are at right angles.

513. The action of the earth on spiral conducting wires is precisely similar in kind to that of single circles, but it is more powerful in degree. Helices are, in like manner, found to obey the terrestrial influence, just as magnets do when placed in similar circumstances as to freedom of motion, provided the electrical currents which they convey are of sufficient intensity. Continued progressive, or even rotatory motion may be obtained by the same influence, in conductors whose motions are limited to certain planes, either in parallel directions, or round an axis. So that, in fact, every experiment that has been tried, and a great variety has been devised by the ingenuity of numerous experimentalists, has served but to confirm the correctness of Ampère's views of the theory of magnetism. It is easy to distinguish whether the motion of any part of a Voltaic circuit is the effect of the influence of the earth, or merely of the other portions of the same circuit, by reversing the communications with the ends of the pile or battery employed: in the former case, the direction of the motion is immediately reversed by this change; and in the latter case, the action continues the same as before.

514. Ampère is far from supposing that the successive action of the solar rays on the equatorial regions of the earth is the sole cause of the electric currents that circulate in them. Internal changes, taking place in the earth itself, must also concur in producing them; for it would otherwise be impossible to account for the observed variations in their effects. The diurnal variations may, however, fairly enough be attributed to the alternate changes of temperature occurring in different parts of the torrid, and even of the temperate zones.

515. The phenomena of magnetic induction, whether effected by currents of electricity passing through a conducting body, or by a magnet in which such currents are assumed to exist, are also in perfect conformity with this hypothesis. A conducting wire tends to the determination of currents in the same direction as those which it conveys itself, in all the magnetisable bodies in its vicinity: these currents continue to circulate with more or less permanency, after the removal of the current which originally determined them. In soft iron they soon disappear: in steel they continue to maintain themselves, and give rise to permanent magnets. The polarities thus induced will have transverse directions with respect to that of the current to which they owe their

origin, for the reasons we have already so fully explained.

516. Numerous facts have induced Ampère to conclude, that the circulation of electric currents peculiar to magnets takes place round each particle of the magnetic body; he has also adopted the opinion that these currents pre-existed, in the bodies susceptible of magnetism, before this property was imparted to them; but, as they were moving in every variety of direction, they neutralised each other, and could produce no external effect. It is only when a determinate direction has been given to them, either by another magnet or by a Voltaic current, that they become capable of exerting any magnetic

action.

517. By a very curious experiment, Ampère has proved, that a powerful electric current has a tendency to excite similar currents in neighbouring bodies, not generally susceptible of magnetism. A copper wire of considerable length was rolled round a cylinder, so as to form a coil, all the turns of which were separated from each other by silk riband. Within this spiral coil a ring of brass was freely suspended by a fine metallic thread, passing through a small glass tube, which was placed between the threads of the copper coil. The circumference of the ring, in every part, was thus brought very near to the copper wire, through which a powerful Voltaic current was made to pass. Under these circumstances the brass wire was attracted or repelled by a magnet, in the same way as it would have been had it formed part of the same Voltaic circuit. The action, indeed, was but feeble, and Ampère, in his first trials, fa led in his endeavours to render it sensible; but, on persevering in the attempt, his success, at last, was complete and unequivocal.

518. A simple circular current, or what will act still more powerfully, a spiral coil, when presented to a magnet exhibits phenomena precsely analogous to those afforded by the ends of magnets; acting as north poles on one side, and as south poles on the other. If the currents in these spirals be reversed, the polarities on each side are in consequence immediately reversed; what was before a north, now becomes a south pole; and vice versa.

519. But the simple circular conducting wire or ring exhibits, in consequence of the vacant space in its centre, phenomena which neither the spiral coil, of which the turns occupy the whole disc of the circle, nor any magnetised iron can produce. M. De la Rive contrived, upon this principle, a very pretty and instructive experiment, which is noticed in another part of this article. A floating conducting ring being placed so as to encircle a magnet, but in such a way as that the currents in each did not accord, was repelled along the magnet till it reached its end; when it spontaneously turned half round, and was then attracted by the magnet, again encircled it, and proceeded to settle itself round the middle of its length, where it remained in equilibrio.

520. A still more perfect accordance with magnetic phenomena is presented by the heliacal arrangements which we have denominated

Voltaic magnets. These possess regular poles at both ends; the one being north, the other south: which poles are immediately changed into the opposite kinds, by merely reversing the course of the current. They obey the action of magnets which are presented to them, are attracted and repelled, and assume determinate positions with respect to the magnet, just as if they were ordinary magnets: of which, indeed, they possess all the essential properties, and for which they may be substituted in almost every form of experiment.

521. The phenomena of revolving motions, effected either in magnets or in wires, by their mutual action, as first discovered by Mr. Faraday, and as afterwards extended by Ampère, Barlow, Savary and others, and which have been regarded by most philosophers as indicative of the rotatory tendency being an ultimate fact, will be found, on attentive examination, to be not only in strict accordance with, but to be direct consequences of Ampère's theory. Instead of constituting objections to that theory, as was at one time supposed, they have proved, in fact, to be amongst the strongest confirmations of its truth. It would extend this article to too great a length were we to engage in the detail of the circumstances of each experiment, so as to follow all the particular applications of the theory, and trace their agreement with the observed results; but the general principles on which they are to be accounted for have already been sufficiently explained. It is also to be remarked, that Ampère ascertained, by suitable variation in the experiments, that these rotatory movements, although strictly deducible from his own theory of the constitution of magnets, where the action of the portion of conducting wire was alone taken into account, were generally, in a much greater degree, the effect of electric currents taking place in the mercury, into which the extremities of the wire were immersed, and the re-action of which on the wire produced a considerable repelling force. He found it, indeed, as impracticable in mechanism, as it was impossible in theory, to produce rotation without employing fluid conductors in some part of the Voltaic circuit.

522. To take one of the simplest cases of clectro-magnetic action, let us suppose a vertical conducting wire, in which the positive current is descending, presented to a magnetic bar suspended by its centre, so as to move freely in a horizontal plane, and which has assumed its usual position in the magnetic meridian by the influence of the earth. In this position, all the currents contained in the magnet are ascending on its western, and descending on its eastern side. The former will therefore be repelled, and the latter attracted by the wire, and the magnet will so arrange itself that the middle of its attracting side shall be opposite to the wire.

523. When two magnets, on the other hand, are presented to each other, end to end, it will depend upon the direction of the currents be ng similar or dissimilar at the adjacent ends, whether attraction or repulsion will take place; the former happens when the north and south poles are opposite to each other: the latter when simi.

lar poles front each other. The first case may be illustrated by two watches laid the one above the other, so that the dial of the one may be in contact with the back of the other; the hands will then, in both watches, be moving in the same direction. The second case is represented by their being laid face to face, when it will be seen that the motion of the hands are now in opposite directions.

524. But the attractive or repulsive forces are not merely produced by currents at the ends of the mangets; they are the result of the action of all the currents from one end to the other of each magnet. We must regard the total action as composed of the attraction or repulsion of one whole side of the one to one whole side of the other; and of a similar attraction or repulsion between the two other sides: while the contrary action is excited between those respective sides which may be differently grouped. Thus, calling the east and west sides of a magnet the sides which face those points, when its axis is in its natural position in the magnetic meridian, the east side of the one will attract the east side of the other, and repel the west side: the west side will, in like manner, attract the west and repel the east. The tendency of this action is to bring the two eastern sides parallel, and as near to each other as possible: when this position has been attained, the north pole of each magnet will be adjoining to the south pole of the other, and the attractive action will be at its maximum. The same must be understood, mutatis mutandis, of the repulsive action, which is greatest when the east side of the one is parallel and adjoining to the west side of the other; in which case the two poles of the same name in each magnet are adjoining to each other.

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525. The attentive consideration of these combinations of forces will explain a difficulty which at first might be apt to startle us. When the north end of one magnet is directly opposed to the south end of another, the adjacent currents run in similar directions, and there is, therefore no difficulty in understanding how attraction takes place but if the one magnet be moved a little to one side, and brought in a parallel direction, till the two ad acent ends have just past one another, we then find that such a coincidence of adjacent currents no longer takes place on the contrary the eastern side of the one, where the current is descending, is close to the western side of the other, having an ascending current. Repulsion therefore, as it would seem, should now take the place of attraction: whereas we find that, under these circumstances, the two poles still exert a powerful attraction. The reason, however, will appear when the actions of all the other currents, besides those that are immediately adjacent, are taken into account. It will then be found that the repélling positions belonging to the two magnets are, in consequence of the great obliquity of their actions, much less powerful than the attractive portions, which act at a greater angle.

526. Mr. Buxton has published a new theory of electro-magnetism in a work of Mr. Partington's, to which it may be advisable briefly to call the reader's attention. He says, 'I have at all times

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been particularly averse to the basis upon which the present theory of electro-magnetism founded, where there is supposed to exist a strong analogy, if not a comple identity, between the electric and magnetic fluids; and also to the many unnatural modifications which have been resorted to in order to support the above position. I allude to the various routes which the electric fluid has been supposed to take in its transmission through a conductor connected with the two poles of a galvanic battery, in order to account for the deflection of a magnetised needle exposed to its influence, while it is a well-known fact that electricity invariably takes the most direct and shortest route in its transmission through any conducting substance.

527. Naturam expellas furcâ tamen usque recurret,' says Horace; and in this instance it is verified. For even with the assistance of these assumptions, and many others, which are not only irreconcileable with the known and established laws of nature, but even incongruous to each other, we are unable to account for many of the most interesting phenomena of the science. It would be wrong to reject any hypothesis without giving some grounds of object on, and I shall therefore endeavour to show, that though these two properties may, in an unqualified sense, be made to agree and to possess similar attributes, yet neither of these argues a physical identity, or even analogy. It is true, they may be made to agree, but this only in a bare definition, inasmuch as they may both be defined to be properties of natural bodies; but this must not be taken as a general definition, as it would lead to the presumption of a co-existence of these two properties in all bodies, whilst, in fact, the metallic bodies are those only wherein they are found in combination: the non-metallic bodies, though evidently possessing the electric principle, seem destitute of the magnetic. Hence we draw our primary distinction, that electricity is a general physical property of all bodies, and that magnetism is a particular property of metals.

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528. If electricity and magnetism were identical, their phenomena should be similar, and should be exerted under similar circumstances.

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530. Electricity appears to attract all bodies, but repels itself. Thus, when a body in its natural state of electricity is brought within the influence of another body which contains a quantity of the electric principle redundant to its natural capacity, the first body will abstract the fluid from the second, and be attracted by it until they both become similarly electrised, when they will be repelled.

531. From these premises, we draw the conclusion that the attractive and repulsive qualities of electricity are dependent upon the different relative proportions of this principle in each containing body, with respect to its capacity, and to the

different relative proportions contained in both hodies with respect to one another.

532. ́ But in magnetism these qualities are exerted under a complete dissimilarity of circumstances; for, on two magnetic bodies being brought within the sphere of each other's action, we find that the different parts of the several bodies act differently upon each other; hence, that part of the magnetised body which we denominate the north pole, attracts that part of a similar body denominated the south pole, and, è converso, any two of these similar and corresponding parts in different bodies repel each other. From this we may infer, that the attractions and repulsions exerted by magnetic bodies are dependent upon different modes of action, being exerted in different situations in similar budies, and not from any difference in the relative proportions of the magnetic principle existing in the different bodies, as is the case in electricity. 533. The dissimilar modes of action employed by electricity and magnetism, in exerting their attractive and repulsive qualities, may be elucidated by the following experiment: If a piece of steel, highly magnetised, be brought within a short distance of another piece of steel in its natural state, the latter will be powerfully attracted; and, until being drawn into close contact with the former, they will both exhibit a similarity of magnetic phenomena, and remain in that situation until disturbed by some extrinsic agent. Whereas, in order that these properties of attraction and repulsion exerted by magnetism should coincide in their modes of action with electricity, a mutual attraction should at first take place between the two bodies until both become equally magnetic, when they should be mutually repelled.

534. So far I have merely mentioned those properties of electricity which, from a too hasty observation, might appear also to obtain generally in magnetism; but I might adduce many properties in each of these which are not found in the other. This might, however, lead to a disquisition which would form an octavo volume, and I must therefore refrain from trespassing on your valuable pages.

535. Though the renouncement of the principle of identity of electricity and magnetism is a sufficient denial of the present theory, it will be found that, even with the admission of this principle, it would be incapable of accounting for many of its most brilliant phenomena. Among these are the interesting experiments of the attraction of two parallel galvanised wires, when the route of the electric fluid is similar; and their repulsion, when the electric fluid in each passes in a contrary direction. Another peculiarity which theorists have been unable either to reconcile or smother, is the fact, that if a piece of soft iron be placed in a tube of glass, and hermetically sealed, the whole being enclosed in a spiral conducting wire, on the galvanic stream passing through the spiral, the enclosed piece of iron will become strongly magnetic; if we admit this as the immediate effect of electricity, we must deny the impermeability of glass by the electric fluid, and with it the theory of the Leyden jar, and in fact endanger the whole science of electricity.

536. The admission of an analogy existing between electricity and magnetism, seems the effect of a too superficial observation and hasty conclusion, and the notion seems first to have arisen from the circumstance that lightning has been known sometimes to destroy the polarity of magnetised needles, at others to have magnetised pieces of steel which had not before any sensible magnetic properties. After the observation of these facts, and when the identity of lightning and electricity was afterwards established, it was presumed that common electricity would necessarily produce the same effects: this is found to be the case, and steel may be magnetised either by the Voltaic pile, or by the electric machine.

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537. But, though electricity is thus capable of exciting magnetism in iron and other metals, still this property is not confined to electricity, for the same effect is produced by filing, drilling, twisting, and several other mechanical means. Electricity ought, therefore, merely to be considered as one of the various means which nature employs to excite the latent magnetic properties of metals.

538. Having so far premised, I shall now endeavour to show that the phenomena of electromagnetism are merely some of the most simple operations of nature, and that they involve no difficulties but what are easily explained by her most simple laws.

539. If we admit of magnetism being an essential property of metals, as no magnetic phenomena are sensible to our observation, except under particular circumstances, we must necessarily infer that this property exists in a latent state, which must arise from the natural disposition of the parts of the metal being unsuitable to the circulation of, the magnetic fluid; a necessary precedent to any magnetic phenomena.

540. That some metals are more easily magnetised than others is a well-known fact, and this frequently obtains in similar metals which have been differently operated upon; this must be ascribed to the above cause, viz.:-to the different organisation of different metals. Thus the latent magnetic principle in steel may not only be brought into action by means of electricity, but the same may be effected by a blow, twisting, &c., and by being rubbed upon an already excited magnet; whilst, to excite the latent magnetic principle inherent in gold, silver, brass, and other metals, the transmission of an electric stream across the metal seems invariably necessary: this, by disturbing the mutual cohesion of the parts of the metal, and by opening a free passage for its own circulation, at the same time, effects a similar arrangement for the circulation of the heretofore latent magnetic fluid.

541. If the electric force thus employed be but small, and the parts of the metal through which it has been transmitted not sufficiently removed without their mutual sphere of attraction, the magnetism excited will be but temporary; for, immediately upon the discontinuance of the electric stream, the parts of the metal by their natural attraction will resume their original arrangement, and the magnetic principle will again become quiescent. If, however, the elec

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