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plane of whose contours he acts with magnets, as on the side of a single wire.

361. Among the arrangements which he has thus formed, one of the most remarkable consists in winding the conjunctive wire around a cylinder of wood or glass, forming an elongated spiral. Then the force emanating from each point of the thread, being always directed transversely to its length, becomes in each element of the spiral perpendicular to the place of the coils, and consequently parallel to the length of the spiral itself. Farther, on account of the revolutive character of the force, all the inner points of the different rings exercise, in the interior of the spiral, forces which are equal, and operate in the same direction: whilst in their action exteriorly, the forces emanating from the different points of each ring, oppose and weaken each other gently by their obliquity. Thus the result of all these actions ought to be much more energetic within the spiral than outside of it; a consequence which actually happens. If we place in the interior of a spiral, unmagnetic steel needles, they will acquire in a few instants a permanent and very perceptible longitudinal magnetism; whereas, if we place them without the spiral, they suffer no change. This experiment is due to M. Arago. Sir H. Davy, without being acquainted with it, has since succeeded in magnetising small steel needles, by rubbing them transversely on a single conjunctive wire, or even without contact, by placing them at some distance from it. This process does not differ from the preceding, except in using the force of only one wire, a force which the spiral multiplies. 362.Since the electricity developed by the friction of our ordinary machines, differs in no other respect from that evolved from the Voltaic apparatus, than that the former is retained and fixed, while the latter is in motion; we find that, whenever we cause the electricity of our machines to flow in a continuous current, it has produced absolutely the same effects as the Voltaic battery. The similarity, or rather the identity, of these two forms of electricity, is manifested likewise in the production of the electro-magnetic phenomena. This has been shown by M. Arago, who transmitted along the spirals of M. Ampere, no longer the Voltaic current, but a succession of very small sparks, drawn from a common electrical machine. Small steel needles, placed in the interior of these spirals, were thus magnetised in a few instants, and the direction of their magnetic polarity was found to be determined in reference to the surfaces charged with the resinous or vitreous electricity, precisely as happens with the copper and zinc poles of the Voltaic apparatus.'

by supposing that the wire itself became magnetic during the passage of the electricity through it, and direct experiments, which were immediately made, proved that this was the case. He threw some iron filings on a paper, and brought them near the communicating wire, when immediately they were attracted by the wire, and adhered to it in considerable quantities, forming a mass round it ten or twelve times the thickness of the wire: on breaking the communication they instantly fell off, proving that the magnetic effect depended entirely on the passage of the electricity through the wire. The same experiment was tried on different parts of the wire, which was seven or eight feet in length, and about the twentieth of an inch in diameter, and the iron filings were found every where attracted by it; and, making the communication with wires between different parts of the battery, iron filings were attracted, and the magnetic needle affected in every part of the circuit.

364. It was easy to imagine that such magnetic effects could not be exhibited by the electrified wire without being capable of permanent communication to steel. Sir Humphry fastened several steel needles, in different directions, by fine silver wire to a wire of the same metal, of about the thirtieth of an inch in thickness, and eleven inches long, some parallel, others transverse, above and below in different directions; and placed them in the electrical circuit of a battery of thirty pairs of plates, of nine inches by five, and tried their magnetism by means of iron filings: they were all magnetic; those which were parallel to the wire attracted filings in the same way as the wire itself, but those in transverse directions exhibited each two poles, which, being examined by the test of delicate magnets, it was found that all the needles that were placed under the wire (the positive end of the battery being east) had their north poles on the south side of the wire, and their south poles on the north side; and that those placed over had their south poles turned to the south, and their north poles turned to the north; and this was the case whatever was the inclination of the needles to the horizon. On breaking the connexion, all the steel needles that were on the wire in a transverse direction retained their magnetism, which was as powerful as ever, while those which were parallel to the silver wire appeared to lose it at the same time as the wire itself.

365. He attached small longitudinal portions of wires of platinum, silver, tin, iron, and steel, in transverse directions, to a wire of platinum that was placed in the circuit of the same battery. The steel and the iron wire immediately acquired poles in the same manner as in the last experiment; the other wires seemed to have no effect, except in acting merely as parts of the electrical circuit; the steel retained its magnetism as powerfully after the circuit was broken as before; the iron wire immediately lost a part of its polarity, and in a very short time the whole of it.

363. We must now revert to the discoveries that were rapidly proceeding in this country. Sir Humphry Davy found, in repeating the experiments of M. Oersted with a Voltaic apparatus of 100 pairs of plates of four inches, that the south pole of a common magnetic needle placed under the communicating wire of platinum was strongly attracted by the wire, and remained 366. The battery was placed in different diin contact with it, so as entirely to alter the directions as to the poles of the earth; but the rection of the needle, and to overcome the mag- effect was uniformly the same. All needles netism of the earth. This he could only explain placed transversely under the communicating

wires, the positive end being on the right hand, had their north poles turned toward the face of the operator, and those above the wire their south poles; and on turning the wire round to the other side of the battery, it being in a longitudinal direction, and marking the side of the wire, the same side was always found to possess the same magnetism; so that in all arrangements of needles transversely round the wire, all the needles above had north and south poles opposite to those below, and those arranged vertically on one side, opposite to those arranged vertically on the other side.

367. It was found that contact of the steel needles was not necessary, and that the effect was produced instantaneously by the mere juxta position of the needle in a transverse direction, and that through very thick plates of glass: and a needle that had been placed in a transverse direction to the wire merely for an instant, was found as powerful a magnet as one that had been long in communication with it.

368. Sir Humphry placed some silver wire of one-twentieth of an inch, and some of one-fiftieth, in different parts of the Voltaic circuit when it was completed, and shook some steel filings on a glass plate above them: the steel filings arranged themselves in right lines, always at right angles to the axis of the wire; the effect was observed, though feebly, at the distance of a quarter of an inch above the thin wire, and the arrangement in liues was nearly to the same length on each side of the wire.

369. He ascertained, by several experiments, that the effect was proportional to the quantity of electricity passing through a given space, without any relation to the metal transmitting it: thus, the finer the wires the stronger their magnetism.

370. A zinc plate of a foot long, and six inches wide, arranged with a copper plate on each side, was connected by a very fine wire of platinum; and the plates were plunged an inch deep in diluted nitric acid. The wire did not sensibly attract fine steel filings. When they were plunged two inches, the effect was sensible; and it increased with the quantity of immersion. Two arrangements of this kind acted more powerfully than one; but when the two were combined, so as to make the zinc and copper plates but parts of one combination, the effect was very much greater. This was shown still more distinctly in the following experiment:---Sixty zinc plates with double copper plates were arranged in alternate order, and the quantity of iron filings which a wire of a determinate thickness took up observed: the wire remaining the same, they were arranged so as to make a series of thirty; the magnetic effect appeared more than twice as great; that is, the wire raised more than double the quantity of iron filings.

371. The magnetism produced by Voltaic electricity seems (the wire transmitting it remaining the same) exactly in the same ratio as the heat; and however great the heat of a wire, its magnetic powers were not impaired. This was distinctly shown in transmitting the electricity of twelve batteries of ten plates each of zinc, with double copper arranged as three, through fine

platinum wire, which, when so intensely ignited as to be near the point of fusion, exhibited the strongest magnetic effects, and attracted large quantities of iron filings, and even small steel needles from a considerable distance.

372. As the discharge of a considerable quantity of electricity through a wire seemed necessary to produce magnetism, it appeared probable that a wire electrified by the common machine would not occasion a sensible effect; and this was found to be the case, on placing very small needles across a fine wire, connected with a prime conductor of a powerful machine and the earth. But, as a momentary exposure in a powerful electrical circuit was sufficient to give permanent polarity to steel, it appeared equally obvious, that needles placed transversely to a wire at the time that the electricity of a common Leyden battery was discharged through it, ought to become magnetic, and this was actually the case, and according to precisely the same laws as in the Voltaic circuit; the needle under the wire, the positive conductor being on the right hand, offering its north pole to the face of the operator, and the needle above exhibiting the opposite polarity.

373. So powerful was the magnetism produced by the discharge of an electrical battery of seventeen square feet, highly charged, through a silver wire of one-twentieth of an inch, that it rendered bars of steel two inches long, and from one-twentieth to one-tenth in thickness, so magnetic, as to enable them to attract small pieces of steel wire or needles; and the effect was communicated to a distance of five inches above, or below, or laterally from the wire, through water, or thick plates of glass, or metal electrically insulated.

374. The facility with which experiments were made with the common Leyden battery, enabled Sir Humphry Davy to ascertain several circumstances which were easy to imagine, such as that a tube filled with sulphuric acid, of onefourth of an inch in diameter, did not transmit sufficient electricity to render steel magnetic; that a needle placed transverse to the explosion through air, was less magnetised than when the electricity was passed through wire; that steel bars exhibited no polarity (at least at their extremities) when the discharge was made through them, as part of the circuit, or when they were placed parallel to the discharging wire; that two bars of steel fastened together, and having the discharging-wire placed through their common centre of gravity, showed little or no signs of magnetism after the discharge, till they were separated, when they exhibited their north and south poles opposite to each other, according to the law of position.

375. These experiments distinctly showed, that magnetism was produced whenever concentrated electricity passed through space; but the precise circumstances, or law of its production, were not obvious from them. When a magnet is made to act on steel filings, these filings arrange themselves in curves round the poles, but diverge in right lines; and in their adherence to each other form right lines, appearing as spicula. In the attraction of the filings round the

wire in the Voltaic circuit, on the contrary, they form one coherent mass, which would probably be perfectly cylindrical, were it not for the influence of gravity. In first considering the subject, it appeared to Sir Humphry, that there must be as many double poles as there could be imagined points of contact round the wire; but when he found the north and south poles of a needle uniformly attracted by the same quarters of the wire, it appeared to him that there must be four principal poles corresponding to these four quarters. Dr. Wollaston has, however, pointed out that there is nothing definite in the poles; that the phenomena may be explained, by supposing a kind of revolution of magnetism round the wire, depending for its direction upon the position of the negative and positive sides of the electrical apparatus.

376. To gain some light upon this matter, and to ascertain correctly the relations of the north and south poles of steel, magnetised by electricity to the positive and negative state, Sir Humphry Davy placed short steel needles round a circle made on pasteboard, of about two inches and a half in diameter, bringing them near each other, though not in contact; and fastening them to the pasteboard by thread, so that they formed the sides of a hexagon inscribed within the circle. A wire was fixed in the centre of this circle, so that the circle was parallel to the horizon, and an electric shock was passed through the wire, its upper part being connected with the positive side of a battery, and its lower part with the negative. After the shock all the wires were found magnetic, and each had two poles; the south pole being opposite to the north pole of the wire next to it, and vice versa; and when the north pole of a needle was touched with a wire, and that wire moved round the circle to the south pole of the same needle, its motion was opposite to that of the apparent motion of the sun.

377. A similar experiment was tried with six needles arranged in the same manner, with only this difference, that the wire positively electrified was below. In this case the results were precisely the same, except that the poles were reversed; and any body, moved in the circle from the north to the south pole of the same needle, had its direction from east to west.

378. A number of needles were arranged as polygons in different circles round the same piece of pasteboard, and made magnetic by electricity; and it was found that in all of them, whatever was the direction of the pasteboard, whether horizontal or perpendicular, or inclined to the horizon, and whatever was the direction of the wire with respect to the magnetic meridian, the same law prevailed; for instance, when the positive wire was east, and a body was moved round the circle from the north to the south poles of the same wire, its motion (beginning with the lower part of the circle) was from north to south, or with the upper part from south to north; and when the needles were arranged round a cylinder of pasteboard so as to cross the wire, and a pencil-mark drawn in the direction of the poles, it formed a spiral.

€79. It was erfectly evident from these ex

periments, that as many polar arrangements may be formed as chords can be drawn in circles surrounding the wire.

380. Supposing powerful electricity to be passed through two, three, four, or more wires, forming part of the same circuit parallel to each other in the same plane, or in different planes, it could hardly be doubted that each wire, and the space around it would become magnetic in the same manner as a single wire, though in a less degree; and this was found to be actually the case. When four wires of fine platinum were made to complete a powerful Voltaic circuit, each wire exhibited its magnetism in the same manner, and steel filings on the sides of the wires opposite attracted each other.

381. As the filings on the opposite sides of the wire attracted each other in consequence of their being in opposite magnetic states, it was evident, that if the similar sides could be brought in contact, steel filings upon them would repel each other. This was very easily tried with two Voltaic batteries, arranged parallel to each other, so that the positive end of one was opposite to the negative end of the other. steel filings upon two wires of platinum joining the extremities strongly repelled each other. When the batteries were arranged in the same order, viz. positive opposite to positive, they attracted each other; and wires of platinum (without filings) and fine steel wire (still more strongly) exhibited similar phenomena of attraction and repulsion under the same circumstances.

382. As bodies magnetised by electricity put a needle in motion, it was natural to infer that a magnet would put bodies magnetised by electricity in motion, and this was found to be the case. Some pieces of wire of platinum, silver, and copper were placed separately upon two knife edges of platinum, connected with two ends of a powerful Voltaic battery, and a magnet presented to them; they were all made to roll along the knife edges, being attracted when the north pole of the magnet was presented, the positive side of the battery being on the right hand, and repelled when it was on the left hand, and vice versa, changing the pole of the magnet. Some folds of gold leaf were placed across the same apparatus, and the north pole of a powerful magnet held opposite to them; the folds approached the magnet, but did not adhere to it. On the south pole being presented, they receded from it.

383. Sir Humphry Davy, in continuing his researches on the magnetic phenomena produced by electricity, found that these phenomena were precisely the same, whether the electricity was small in quantity, and passing through good conductors of considerable magnitude; or, whether the conductors were so imperfect as to convey only a small quantity of electricity; and in both cases they were neither attractive of each other, nor of iron filings, and not affected by the magnet, and the only proof of their being magnetic was their occasioning a certain small deviation of the magnetised needle.

384. Thus, a large piece of charcoal placed in the circuit of a very powerful battery, being a very bad conductor compared with the metals

would not affect the compass needle at all, unless it had a very large contact with the metallic part of the circuit; and if a small wire was made to touch it in the circuit, only in a few points, that wire did not gain the power of attracting iron filings; though, when it was made to touch a surface of platinum foil coiled round the end of the charcoal, a slight effect of this kind was produced. And in a similar manner fused hydrate of potassa, one of the best of the imperfect conductors, could never be made to exert any attractive force on iron filings, nor could the smallest filaments of cotton, moistened by a solution of hydrate of potassa, placed in the circuit, be made to move by the magnet; nor did steel needles floating on cork on an electrised solution of this kind, placed in the Voltaic circuit, gain any polarity; and the only proof of the magnetic powers of electricity passing through such a fluid, was afforded by its effect upon the magnetised needle, when the metallic surfaces, plunged in the fluid, were of consider able extent. That the mobility of the parts of duids did not interfere with their magnetic powers, as developed by electricity, was proved by electrifying mercury and Newton's metal fused in small tubes. These tubes, placed in a proper Voltaic circuit, attracted iron filings, and gave magnetic powers to needles; nor did any agitation of the mercury or metal within, either in consequence of mechanical motion or heat, alter or suspend their polarity.

385. Imperfect conducting fluids do not give polarity to steel when electricity is passed through them; but electricity passed through air produces this effect. Reasoning on this phenomenon, and on the extreme mobility of the particles of air, Sir Humphry Davy concluded, as M. Drago had likewise done from other considerations, that the Voltaic current in air would be affected by the magnet.

386. Mr. Pepys charged the great battery of the London Institution, consisting of 2000 double plates of zinc and copper, with a mixture of 1168 parts of water, 108 parts of nitrous acid, and twenty-five parts of sulphuric acid; the poles were connected by charcoal, so as to make an arc, or column of electrical light, varying in length from one to four inches, according to the state of rarefaction of the atmosphere in which it was produced; and a powerful magnet being presented to this arc, or column, having its pole at a very acute angle to it, the arc, or column, was attracted or repelled with a rotatory metion, or made to revolve by placing the poles in different positions, being repelled when the negative pole was on the right hand, by the north pole of the magnet, and attracted by the south pole, and vice versa.

387. It was proved by several experiments that the motion depended entirely upon the magnetism, and not upon the electrical inductive powers of the magnet; for masses of soft iron, or of other metals, produced no effect.

388. The electrical arc, or column of flame, was more casily affected by the magnet, and its motion was more rapid when it passed through dense than through rarefied air, and in this case,

the conducting medium or chain of aeriform particles was much shorter.

389. We must now again revert to the continental philosophers. M. Von Buch, of Utrecht, while engaged in repeating the experiments of Oersted and others, obtained results according with them, except in one instance of difference with Oersted. M. Oersted says, that if the uniting wire be placed perpendicularly to the plane of the magnetic meridian, whether above or below it, the needle remains at rest, unless it be very near the pole; in that case, the pole is elevated when the entrance is from the west side of the wire, and depressed when from the east.' M. Von Buch points out that this state of rest does not continue in two of the four positions of the wire. When the connecting wire is beneath the centre of the needle, and the positive current is from east to west, the needle remains unmoved. When the current is from west to east, it performs half a revolution. On the contrary, the connecting wire being above the current from east to west, makes the needle turn half way round; while that from west to east leaves the needle unmoved. M. Von Buch conceives the difference of his results and M. Oersted's to depend upon the superior power of his apparatus; and indeed it is sufficiently evident that the incompleteness of Mr. Oersted's results depended upon the weakness of his pile. The attractions and repulsions, or the elevations and depressions, he speaks of when the wire was brought near the poles proves the existence of that action which, in M. Von Buch's experiments, was strong enough to turn the needle round; and, if the position of the wire and needle in these experiments be compared with the positions deduced from M. Oersted's experiments, it will be found that, in two of the cases, those pointed out by M. Von Buch, it was necessary a half revolution of the needle should take place to bring it into a state of equilibrium with the wire in those positions.

390. M. Von Buch, also, appears to have ascertained the effect of common electricity in producing magnetism without a previous knowledge of what had been done by others in that way, and succeeded in producing the effect by a smaller power than had before been used for that purpose. He found that a strong discharge was not necessary, nor even a Leyden phial; but, fixing a helix between the prime conductor of a machine and another insulated conductor, placing a steel needle in it, and then drawing sparks from the latter conductor, the needle became magnetic. One single turn of a machine, with two discs, eighteen inches in diameter, was sufficient to make the needle evidently magnetic.

391. Ir. Italy, many experiments relating to magnetism by electricity had been made, and which, though new at the time to those who made them, had been previously made by others. A series was made by M.M. Gazzeri, Ridolfi, and Antinori, at Florence. The results are as follows: needles placed in helices connected with the poles of the battery received their full magnetisation in one minute. Needles on the outside of the helices would receive no magne

tism, unless there was one or more also within, and then they became magnets with their poles in opposite directions to the poles of the inner magnet. The helix was changed into a square form, by having its wire wrapped round a parallelopiped; the magnetising effect remained the same. A needle aud a long wire of platina were wrapped in a sheet of tin-foil, and that part which contained the needle introduced into a spiral of copper wire; the circuit was then made by the platina wire without the copper spiral; being in connexion with either pole, the needle became magnetised. A spiral of copper wire, with a needle in it, was placed on the surface of a basin of mercury, and the mercury then made part of the circuit: the needle became feebly magnetic. Sparks from a common machine, taken through a helix containing a steel needle, made the needle magnetic. These philosophers appear to have found that the connecting wire placed in other parts of the battery than from end to end would not magnetise needles. There was however, probably, some mistake in this.

392. M. la Borne, in repeating Arago's experiments, varied the use of the helix, by making it of iron, and putting it round the straight wire, through which an electrical discharge was made. The helix in this case became the needle to be maguetised, and it was found to be a strong magnet, the poles being in the positions so often referred to. Such a magnet is flexible and elastic, and may be doubled, lengthened, or shortened: on bringing the two poles together, its action on a magnetic needle was much diminished.

393. M. Berzelius described an experiment, which consisted in placing a thin leaf of tin, eight inches long and two inches wide, parallel to, and in the plane of, the meridian, and in that position connecting it with the elements of a Voltaic circle. A magnetic needle brought near the lower edge of this plate was thrown 20° from the magnetic meridian. On moving it slowly upwards, it took its natural position, when level with the middle of the plate, except that it was raised at one end, and depressed at the other; and, when near the upper edge, it moved 20° from the magnetic meridian in the opposite direction to what it did below. When the needle was moved up and down on the opposite side of the plate, the same deviation and effects took place, but in opposite directions. A small portion of the upper edge of the leaf was cut, and turned upwards, forming a projection above the edge. The needle, brought within equal distance of this projection and the edge, was more affected by the former than the latter.

394. Then, using a square plate of tin, and forming the connexion at opposite angles, it was found on examination that the intervening angles acted more powerfully on the needle than any other parts-a circumstance which proves,' M. Berzelius says, 'that the magnetic polarity of the current goes to opposite extremities, as happens with electric polarity, and in artificial magnets.' 395. The tin band, or leaf, placed in a horizontal plane, and in the magnetic meridian, acted on the needle just as a wire would have done. The greatest deviation of the needle was immediately under or above the middle of the leaf,

and the edges acted as in the former position. The positions assumed by the needles in these experiments was exactly what would be expected. The experiments received all their interest from the way in which their maker applied them to support his particular opinion, and apart from that had not much new in them. M. Berzelius thought that a round wire, when made the conductor, presented a more complicated case than when a square one, or a parallelopiped, was used. 396. In 1823 a paper was published by Sir Humphry Davy on the subject of electro-magnetism, which is of too important a character to admit of abridgment. He says, 'Immediately after Mr. Faraday had published his ingenious experiments on electro-magnetic rotation, I was induced to try the action of a magnet on mercury connected in the electrical circuit, hoping that, in this case, as there was no mechanical suspension of the conductor, the appearances would be exhibited in their most simple form; and I found that when two wires were placed in a basin of mercury perpendicular to the surface, and in the Voltaic circuit of a battery with large plates, and the pole of a powerful magnet held either above or below the wires, the mercury immediately began to revolve round the wire as an axis, according to the common circumstances of electromagnetic rotation, and with a velocity exceedingly increased when the opposite poles of two magnets were used, one above, the other below.

397. 'Masses of mercury, of several inches in diameter, were set in motion, and made to revolve in this manner, whenever the pole of the magnet was held near the perpendicular of the wire; but, when the pole was held above the mercury between the two wires, the circular motion ceased, and currents took place in the mercury in opposite directions, one to the right, and the other to the left, of the magnet. These circumstances, and various others which it would be too tedious to detail, induced me to believe that the passage of the electricity through the mercury, produced motions independent of the action of the magnet; and that the appearances which I have described were owing to a composition of forces.'

398. Sir Humphry says, 'I endeavoured to ascertain the existence of these motions in the mercury, by covering its surface with weak acids; and diffusing over it finely divided matter, such as the seeds of lycopodium, white oxide of mercury, &c.; but without any distinct result. It then occurred to me, that from the position of the wires, currents, if they existed, must occur chiefly in the lower, and not the upper surface of the mercury: and I consequently inverted the form of the experiment. I had two copper wires, of about one-sixth of an inch in diameter, the extremities of which were flat and carefully polished, passed through two holes, three inches apart, in the bottom of a glass basin, and perpendicular to it; they were cemented into the basin, and made non-conductors by sealing-wax, except at their polished ends; the basin was then filled with mercury, which stood about a tenth or twelfth of an inch above the wires. The wires were now placed in a powerful Voltaic circuit. The moment the contacts were made, the phe

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