ELECTROMAGNETISM

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ELECTROMAGNETISM ; the name applied to a veiy interesting class of facts, principally developed by professor Oersted, of Copenhagen, in the year 1819. The power of lightning in destroying and reversing the polarity of a magnet, and of communicating magnetic influence to iron previously not magnetic, had long been observed, and had led to the supposition that similar effects might be produced by the common electrical or galvanic appara compass needle, wh< under the wire, the viates from the ma, declines towards the of the conjunctive \ the negative electric end of the voltaic a[ of declination depend of the electricity, and needle. Ifwechang conjunctive wire out ridian towards the < change in the abov except that of its a mo be disposed horizontc die, the effects take manner ; i. e. the pole which is placed the junctive wire, which electricity of the batti the east. When the stretched alongside c same horizontal pla declination, either t( but it causes it mere; tical line, so that th< negative influence o wire, dips when the side, and rises when i we stretch the conj above or beneath tin perpendicular to the it remains at rest, un near the pole of th case it rises when the by the west part of when it takes place When we dispose tlu a vertical line oppo needle, and make of the wire receive the negative end ( pole of the needle east; but if we place point betwixt the pol was the nature of the first discovery in electromagnetism. It was no sooner announced, than tlie experiments were repeated and varied by philosophers in all parts of the world; and a multitude of new facts were soon brought to light through the labors of MM. Ampere, Arago and Biot, in France, and sir H. Davy and Mr. Faraday, in England. Two very important facts were ascertained by Ampere and Davy,that the conjunctive wire becomes itself a magnet, and that magnetic properties might be communicated to a steel needle not previously possessing them, by placing it in the electric current. The former of these facts is proved by throwing some iron filings on paper, and bringing them under the wire, when they will immediately adhere to it, forming a tuft around it ten or twelve o times the diameter of the wire: on breaking the connexion with tlie batteiy, however, they immediately fall off, proving that the magnetic effect depends entirely on the passage of the electricity through the wire. The degree of force of this magnetic property thus communicated to the uniting wire was imagined, by sir H. Davy, to be proportional to the quantity of electricity transmitted through it. Hence the finer the wire, the more powerfully magnetic was it rendered; and hence, also, a battery of very large plates, such as is used for producing intense heat and light, was found to give the strongest magnetism to the wire connecting its poles. Accordingly we find that the calorimotor of doctor flare (see Galvanism), a galvanic arrangement, in which the plates are nearly two feet square, exhibits the strongest magnetic effects, and this notwithstanding the powerful heating effects that accompany its action; the heat excited not diminishing or interfering with the magnetism, but apparently increasing it; for a fine platina wirt?, so intensely ignited as to be near tlie point of fusion, is observed to attract larger quantities of iron filings than when at a lower temperature. To communicate magnetic properties to steel needles, which before did not exhibit them, it is necessary merely to place them in contact with, or near to the conjunctive wire. The position in which they are to be placed, with regard to the wire, is important, as the permanence of their magnetic quality depends upon it. If they are ]'laced parallel with it, they lose their "Magnetism when the connexion with the battery is broken, which shows that their magnetism arose only from their forming part of the electric circuit, like the con necting wire itself. But if they are placed across the wire, they become permanently magnetized, and retain tlieir power equally with needles prepared in the ordinary way. The polarity is different, however, according as the needle is placed above or below the wire. When a needle is placed under the uniting wire, the positive end of the battery being on the right hand of the operator, the end of the needlenext to him becomes the north pole, and the other end the south pole. On the contrary, when a needle is held above tlie wire, the reverse of this takes place ; the end next to the observer becomes the south, and the other the north pole. Even the same opposition is observed when needles are placed in a perpendicular position, on different sides of tlie wire: in those on one side, all the lower ends are found to be north poles, while, in those on the opposite side of the wire, the upper ends are all north poles, and the lower extremities all south poles. Direct contact of the steel needles with the conjunctive wire is not necessary, for they become instantly magnetic when brought near it, even though thick plates of glass are interposed. As was remarked with regard to the connecting wire, galvanic batteries, consisting of large plates, are most powerful in communicating the magnetic influence. When the conjunctive wires of two distinct galvanic arrangements are made to approach each other, we observe magnetic attractions and repulsions. Two wires of copper, silver, or any other metal, connecting the extremities of two galvanic troughs, being placed parallel to each other, and suspended so as to move freely, immediately attract and repel each other, according as the directions of the currents of electricity flowing through them, are the same or different. When both the negative or both the positive extremities of the troughs are turned to the same quarter, so that the electric current passes , along each wire in the same direction, the two wires attract each other; but when the position of one of tlie troughs is reversed, so that the electric currents in the two wires flow in opposite directions, the wires repel each other. Upon this experiment is founded the most plausible theory of magnetism, viz., that it arises from the attractions and repulsions of currents of electricity, constantly circulating round every magnet. This is conceived to explain the reason why the magnetic needle places itself tit right angles to a wire conducting electricity, namely, that the electric current passing along the wire nviy coin mde with that circulating round the magnet. The magnetic effects produced by galvanic arrangements are obtained also by electricity evolved from the common machine, and still more from this power concentrated in the Leyden jar; the magnetism communicated agreeing in every respect as to the permanence of the polarity, the variations when the needle is placed above or below the wire, &.c, with that produced by the voltaic pile. Magnetism is communicated to needles in a different manner from that of placing them across the conjunctive wire. The wire is formed into a hollow screw, or helix, by rolling it round a solid rod, and the needle to be magnetized, wrapped in a paper, or put into a glass tube, is placed in the centre of it, and the communication with the galvanic battery established. This arrangement (according to the theory of M. Ampere) conveys the electric current by the spiral convolutions, round and round the needle, and communicates to it, or developes in it, the electric circulation constituting magnetism. By this contrivance, it is found that a maximum effect is obtained in a shorter time than by any 3ther method. The position of the north and south pole varies according as either end of the helix is connected with the positive or the negative pole, which shows that the electric current flows along the uniting wire from the positive or zinc extremity to the negative or copper end of the pile. The electricity of a common machine produces the same effect. Having alluded to the principal facts relating to electromagnetic phenomena, the ingenious theory of M. Ampere, by which they are explained more extensively and with more precision than by any other hitherto advanced, deserves to be stated. It is the more deserving of attention, as having led its author to the discovery of some of the most remarkable tacts detailed al>ove; and, if future researches shall continue to increase its probability, it will no doubt be regarded as one of the finest instances of correct induction, supported by minute experiment, which the history of any science can exhibit. The first principle of this theory has been already "tatedv;that two .currents of electricity attract when they move parallel to each other and in the same direction, and refuel when they move parallel to each other iticontrary directions. This fact is directly the reverse of the usually observed phenomena of electricity; fbr it is well known that bodies in the same state of electricity repel each other, ami in oppo site states attract. Hence M. Ampere infers, that these results are not produced by electricity in its known and common s^ate of tension, but are dependent on properties belonging to electricity, previously unsuspected, and peculiar to it when in motion, or flowing in currents. Electricity, when accumulated, has the power of causing certain effects, particularly attractions and repulsions, which are familiar to us, and are called electrical; but when moving in currents, it exerts new powers, and these constitute magnetism. Reviewing the various experiments which have been enumerated, we find, that the connecting wires of two batteries attract and repel each other, according to the directions of the electric currents flowing through them; that the magnetic needle is, exactly in the same manner, attracted and repelled by a connecting wire, according to the direction of the current of electricity moving through the wire; that the position of the needle may be varied, in almost any degree, by changing the position of the connecting wire; that whenever the electric circuit is broken, this influence on the needle ceases, and is renewed whenever the communication between the poles of the battery is restored ; that the connecting wire, of whatever metal it may consist, becomes a perfect magnet, as long as the current flows along it, so as to attract iron filings and small steel needles, without attracting copper filings,, or any other metal but iron; that steel needles may be converted into permanent magnets, by simply placing them across the connecting wire ; that the electric currents having this magnetizing power are not, like accumulated electricity, confined by glass, or other nonconductors, but pass through all bodies with facility, as magnetism was before known to do; that the magnetizing power is exerted by electricity, whether procured by a galvanic apparatus, or a common machine ; that powerful magnets may be formed, by conducting electric currents round steel wires, as in the helix, and that the position of the north and south poles of these magnets depends upon the direction in which the currents are made to move round them. These, and a great number of other facts, it is conceived, clearly demonstrate the perfect resemblance, or rather identity, of electricity and magnetism. Magnetic phenomena are thus, in fact, a series of electrical phenomena ; and magnetism may, with propriety, form a branch of electricity, under the head of Electrical Cur* rents. Though this intimate relation of identity be admitted, it is not so obvious how, by it, the properties of the common magnet are explained. Currents of electricity, according to the theory, are essential to the production of magnetic phenomena ; but these are not obvious in a com% rnon magqet. M. Ampere has suggested their existence, however, and has so arranged them theoretically, as to account for a great proportion of magnetic appearances. A magnet he conceives to be an assemblage of as many electric currents, moving round it in planes perpendicular to its axis, as there may be imagined lines, which, without cutting one another, form closed curves round it. Magnetization, he says, is an operation by which there is given to the particles of steel (which, of the more common metals, appears to be the only one capable of being permanently impressed with this power) an electromotive energy, which causes a circulation of these currents to be continued round them. The excitation and continuance of this electromotive action is rendered less improbable, when we consider the electric power developed in the tourmaline and boracite by heat alone, and when we find, as in the electrical columns of De Luc and Zamboni, that electricity may be generated for years without ceasing or diminishing, by a small and simple apparatus. Such, then, is the constitution of a magnet. It is a mass of iron or steel, round the axis of which electric currents are constantly circulating, and these currents attract all other electric currents flowing in the same direction, and repel all others which are moving in an opposite direction. From these attractions and repulsions another effect follows, that the currents of one magnet have always a tendency to move any other magnet near it, till the currents in the second shall coincide in direction with those of the first. It is from this cause, as will presently be explained, that the magnetic needle always turns to the meridian, and that the needle in Oersted's experiments became at right angles to the connecting wire. One important circumstance is always to be kej>t in view, that the electric currents flow round every magnet in the same direction in reference to its poles. If, for instance, we place a magnet with its north pole pointing to the north, in the usual position of the magnetic needle, the current of electricity flows round it from west to east; or, on the eastern side of the magnet, it is moving downwards, and on the western side upwards; on the upper side, from west to east, and on the lower side, from east to west. This, it is found, is a uniform law. On these principles the phenomena of magnetism are easily accounted for. Thus, to take one of the most obvious arid well known facts, that of two magnets attracting when their opposite poles are approached to one another, as the north, of one to the south pole of the other. Let us suppose a magnet in the position, which has just been stated, with its north pole directed to the north; and let a second magnet be placed beyond it, and in a line with it, with its north pole also pointed to the north. Then, it is obvious that the south pole of the second magnet will be next to the north pole of the first; and from their position it follows, that the electric currents must be flowing in the same direction, or, in both of them, from, west to east: hence, as currents moving in the same direction attract, these opposite poles, if within a certain distance, ought to attract each other, which, accordingly, will be found to be the case. Now, let the second magnet be reversed ; let its south pole be directed to the north, and its north pole approached to the north pole of the first magnet; the electric currents will flow round the magnet in the same manner as before; but in reference to the first magnet and to the meridian, their direction will be reversed : their direction will now be from east to west, upwards on the eastern side, and downwards on the western; consequently, the currents in the two magnets, being now opposite, will repel, or the two north poles will repel each other.In the experiments of professor Oersted, it was found, as has been stated, that when the extraneous influence of the magnetism of the earth was counterbalanced, the tendency of a magnetic needle always was to place itself at right angles to the wire connecting the poles of the galvanic battery. The reason of this is easily explained upon the present hypothesis. In the needle, the currents flow round its axis from end to end; but in the connecting wire there is no circulation round the axis, but a constant stream from one end, namely, the negative, to the other, the positive extremity : hence, fbtr the current along the wire to coincide with the current across and round the magnet, it is necessary that the latter shall stand across the former; and as it appears, tliat, from the attractions and repulsions which these electric currents exert, they are able to move one or both of the magnetic bodies (according as they are light and mobile), till they coincide, the needle moves if the wire is fixed till it stands at right angles to the wire; and if the magnet is fixed, and the wire movable, the reverse happens. The other phenomena, of die needle turning to the west when placed l>elow the wire, to the east when placed above it, &c, may with facility he explained in.the same manner by the principles, that currents flowing in the same direction attract; and that in eveiy magnet they move in a constant current, which is, when the north pole is turned to the north, from west to east, or upwards on the west side, and downwards on the east side. The developement of permanent magnetism in steel needles when placed across the wire, while it is only temporary when they are fastened parallel with it, depends on the same cause : in the latter case, it arises merely from the transmission of electricity from end to end, while, in the former, the electromotive energy of the particles is developed and called into action, which, when set in motion, seems to have the power of continuing itself. These electric currents have the power, which accumulated electricity has not, of penetrating all substances, as was before known respecting magnetism. This is probably owing to their low state of tension ; and, in conformity with this, .large plates, which evolve electricity in but a slight intensity, produce magnetic effects most distinctly. The agency of igalvanism, and that of common electricity, are equally capable of giving rise to magnetism when flowing iu currents, which adds another to the proof that these are the same power. To complete the view *>f Ampere's doctrine, it remains only to explain the influence of the earth on the jnagnet, by which the needle is kept always in one position, nearly coinciding with the meridian. He asserts, that currents of electricity, analogous to those .which circulate round every magnet, are nmicfontlir flnnrinra %,.1 "1,^ r>.l~1," "r influence of the glob< ly of a copper win with the two extreii] each other. It waj move with the great points were immerse ry, with which the battery were conned munication was estal a current of electricii circle, it immediate!) after some oscillation at right angles to the west, or so that the e downwards on the < wards on the westei been stated, is exa( which the currents ir (supposing it placed the north). The cir< regarded as a sectioi] magnet, or asrepr currents flowing rou ber of these circles yond another, the i like the end of the pole, and the neare However the experi] circle always placed if the galvanic curr< the connecting wires opposite direction, th a semicircle, and still and so that the eh always flow downvi side, and upwards < Here, then, are distil] ism, particularly tlu one of the axes poi north, which can be combined influence moving round the very much in, favoj and there appears to wanting, to connect""Aan important addition will be made to the objects of chemical science. The department of magnetic phenomena, which, while included under natural philosophy, has been regarded as obscure, almost beyond the hope of elucidation, will be transferred to form a branch of the former science, and, divested in a great measure of its mystery and difficulty, will come in for consideration among the other agencies of the electric fluid. Additional probability will also be given to the opinion, that not only electricity and magnetism, but caloric and light along with them, are merely, modifications of one another, or of one common agent.In conclusion, with regard to the cause of the electric currents inferred to be constantly circulating round the globe, it is as yet in obscurity. They are supposed to move at right angles to the magnetic meridian, or nearly parallel with the equator, on the eastern side of the earth moving from us, and on the western side flowing towards us. These currents may be compared to that which flows from the negative pole of a voltaic battery in action, to the positive pole, and, by the medium of the uniting wire, round again to the negative pole. It is conjectured, that the arrangement of the materials of the globe may be such as to constitute a battery, existing like a girdle round the earth, which, though composed of comparatively weak elements, may be sufficiently extensive to produce the effects of terrestrial magnetism. Its irregularity, and the changes which it may accidentally or periodically. suffer, may explain the phenomenon of the variation of the compass; or the general action roducing the currents of electricity may e affected by different causes, as the earth's motions, currents of the atmosphere, evaporation, or the solar heat. It is supposed that much of the variation, depends on the progress of oxidation in the continental regions of the globe. What is called the diurnal variation may be conceived to be produced by the diurnal change of temperature in the superficial layers of the earth, which possess electromotive energy.** To those who are desirous of extending their knowledge respecting electromagnetism, the folio wing sources of information may be recommended: The original memoirs of Oersted {Ann. of Phil, xiii, and N. S. ii); Arago (Ann. de <?himie, and Rteueil cTObservations ElectroUffnamiqucs); Ampere (Ann. de Chim. et Phys. xv. 69)j Sir H. Davy (Phil. Trans. 1821) 3 Faraday (Quart. Journ. xii, 47,416); Barlow, and others ; also Manual of ElectroDynamics,' by J. F. Demonferrand, with notes by professor