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immersed in it. Then the contact being made at C (which, by means of the wire DC, communicates with the quicksilver) the wire gz will immediately assume a rapid rotatory motion, much greater than in the former case, the resistance being very considerably diminished by the mode of suspension. The direction of the motion, according to the arrangement in the figure, being from left to right, to a person coinciding in position with the magnet. It may, however, be reversed by reversing the magnet, or by changing the contact, as in the preceding cases. 428. This experiment is also due to Mr. Faraday, and its explanation is the same as the last; for since when the magnet is free, it will, as we have seen, revolve about the wire from right to left, it follows that, when the magnet is fixed and the wire free, the latter will revolve in an opposite direction (the action and re-action between the wire and the magnet being reciprocal), which is still, however, towards the left of a person supposed now as coinciding in position with the magnet, and his head to the north.

429. The resistance being very inconsiderable in this experiment, it may be exhibited in a more simple manner. For instance, instead of piercing the foot of the cup, as in the figure referred to, it will be sufficient to use a tea-saucer, or any other shallow vessel, and to bring a strong magnet as near to it as possible under the table; when the motion will take place precisely in the same manner as above.

430. By this means also we may establish a most important fact, viz. that it is indifferent, as to the result of the experiment, what may be the position of the magnet; that is to say, if we keep the extremity of it as nearly as possible under the centre of the vessel, we may hold it either vertical or horizontal, or incline it in any angle, and at any azimuth, without greatly changing the rate of the rotation; it being always understood that the magnet should be of considerable length, in order that its other pole may not affect the motion of the wire.

431. The machine for the exhibition of these motions, according to Mr. Faraday's construction, is shown at fig. 1, plate III. ABCD is a stand of wood, EF a brass pillar, FG a fore art or projecting piece of brass, through the extremity of which passes the wire LHK; at L there is a sort of ball and socket joint; the socket being in the upper part, and the ball fitting it, on the small wire Lm. Both the socket and ball are amalgamated, and a piece of silk fixed to the ball, or head of the wire, passes through a hole drilled in the wire LHI, and by which the smaller wire is suspended, thereby preserving the contact, and leaving to the latter a perfect freedom of motion: ab is a glass cup having a hole through its foot, into which is in serted a copper tube, soldered to a copper disc just the s ze of the bottom of the glass, and which disc is cemented to the foot of the latter.

432. The wire Z z is also so.dered to another copper disc, upon which the glass rests; and by which the contact is carried on from Z to the quicksilver in the cup, and thence to the wire L; lastly, a small magnet ns is inserted into VOL. VIII.

the copper tube, passing through the stem of the glass above mentioned.

433. The foot of the cup cd is pierced, and discs of copper applied as in the cup ab; but the wire passing through the foot is solid, and to it is fixed, by a short string, the small magnet ns, which is thus free to revolve about the descending wire HK; quicksilver, as in the preceding cases, being poured into the cup, till the wire II K is slightly immersed in it at K. The contact with the battery being now made at Z and C', the motions will take place as described in the last two experiments; viz. that the magnet ns in the one cup will revolve about the wire K, while the wire Lm will at the same time be revolving about the other magnet n s.

434. If the cup cd be placed where the cup ab is represented, then the magnet and wire be.ng both free, they will revolve about each other, and thus produce a pleasing variety in the experiment.

435. A section of this machine is shown at fig. 2. 436. Mr. Faraday also describes another apparatus, which requires a less galvanic action than the former to produce the rotation. This is shown at fig. 3; it consists of a piece of glass tube, the bottom part of which is closed by a cork, and through it is passed a small piece of soft iron wire, so as to project above and below the cork. A little mercury is then poured in, to form a channel between the iron wire and the glass tube. The upper orifice is also closed by a cork, through which a piece of platinum wire passes, being terminated within by a loop; another piece of wire hangs from this by a loop, and its lower end, which dips a very little way into the mercury, being amalgamated, it is preserved from adhering either to the iron wire or the glass. Things being thus arranged, a very minute galvanic power being applied by contact with the lower and upper end of the apparatus, and the pole of a strong magnet being applied to the external end of the lower iron wire, the moveable wire within begins rapidly to rotate round the temporary magnet thus formed; and which rotation may be inverted either by changing the contact or by inverting the magnet. Mr. Faraday states that this instrument is so sensible that a rotation has been produced in it by two plates, each only one inch square.

437. To exhibit the rotation of a magnet on its axis by the effect of a galvanic wire, let ABDE, fig. 4, represent a cup of glass or wood, NS a magnet, having at its lower extremity a fine steel point, inserted in the agate a; b c is a thin slip of brass or ivory, hav ng a hole through which the magnet passes freely, and by means of which it is kept perpendicular: at the upper extremity N of the magnet is a thin cylinder, as a piece of quill, forming a cup or reservoir z to receive a small quantity of quicksilver; and into this is inserted the wire Z, amalgamated at its lower point; and Cc is a stout wire passing through the side of the cup into the quicksilver. Then, the contact being made at C and Z, the magnet will begin to revolve on its axis, with a very astonishing velocity, and continue in motion while the power of the battery lasts.

438. This pleasing experiment is due to M.

K

Ampere, who employs only a piece of platinum attached to the magnet, to produce, by its superior gravity, a vertical position of the latter in the mercury; the upper wire being then inserted into the quicksilver in the cylinder z, and the other wire into the cup C, the motion is produced exactly as above described: the greatest freedom of motion is, however, given by the apparatus shown in the figure. The explanation of this rotation is very obvious according to the hypothesis we have adopted, for the tangential force of the wire, acting upon the magnetic particles on the surface of the magnet, must necessarily produce the rotation in question.

439. To exhibit the rotation of a galvanic wire on its axis by the action of a magnet, let NS, fig. 5, be a magnet, represented as broken in the figure, but which is fixed, in the experiment, in a foot, in order to keep it vertical, and let abcd be a light hollow copper or brass cylinder, having a steel point passing downwards into the agate cup f, fixed to the upper end of the magnet, and let e be a small tube or quill fixed on the wire passing through the top of the cylinder, holding a little quicksilver, and receiving into it the descending conducting wire Z. AB is a piece of wood turned to fit on the cylindrical magnet NS, which has a hollow groove on its upper surface to receive a quantity of quicksilver, into which the lower edge of the cylinder ad is slightly immersed, the surface being covered with weak dilute nitric acid. AC is a wire passing into the quicksilver. It is obvious that thus (the contact being made at Z and C) the galvanic circuit is carried from Z through the cylinder abcd, thence to the quicksilver, and hence again through the wire A E, to the other extremity of the battery, whereby the cylinder abcd is made to become a part of the conducting wire; and it will be found to revolve on its axis with a great velocity, fully equal to that of the magnet in the last experiment; the direction of the motion, with the arrangement shown in the figure, being from left to right, to a person coinciding in position with the magnet.

440. To exhibit a quicksilver vortex by means of a galvanic wire and magnet, it is only necessary to take any shallow non-conducting vessel, and put into it a quantity of pure mercury, into which is to be inserted the conducting wires Z, C, proceeding respectively from the zinc and copper sides of the battery. And if now the north end of a strong magnet be brought under the vessel, the quicksilver round the wire C will begin to revolve about the same, forming a beautiful vortex; the direction of the motion being from left to right. If the magnet be removed under the other wire, the same kind of motion will be produced, but its direction will be reversed; and the same change of motion will take place, of course, in each case, by changing the end of the magnet.

441. The explanation here is precisely the same as in the last experiment; the moveable part of the conductor in this case owing its mobility to its fluid nature, whereas in the former it is due to the peculiar mode of suspension. This very elegant experiment was, as we have stated in another place, first made by Sir H. Davy.

442. To exhibit the rotation of the galvanie wire, independently of the galvanic battery, we must employ the apparatus exhibited in fig. 6, where ABCD is a small copper vessel about two inches and a half high, and the same in diameter; abcd is another small cylinder of copper, of the same height, soldered to the former vessel at its lower end de, a hole being left in the bottom of the former to receive it. The cylinder abcd is therefore open, and will admit a cylindrical magnet to be passed up; and it will, at the same time, hold a quantity of dilute acid within the space AD, dabc, BC: 22' is a zinc cylinder, very light, of rather less altitude than the copper one. To the cylinders ab and 22 are soldered two copper wires, as shown in the figure, the upper one having a steel point proceeding from E downwards, and resting in a small metal hole at F; and consequently the cylinder will be free to move upon its point of suspension at F.

443. The apparatus being thus arranged, and the acid placed in the cell as above described, insert through the interior cylinder the north end of a strong cylindrical magnet, and balance the whole apparatus upon it, when immediately the zinc cylinder will begin to revolve with a greater or less velocity, according to the strength of the acid, the freedom of motion, and the power of the magnet. Mr. Barlow has frequently produced, with this simple apparatus, a motion, amounting to 120 rotations per minute. The only difference between this and the other rotations we have described is, that the galvanic power is here produced by the apparatus itself, instead of having recourse to the battery.

444. For it is obvious that the wire from Z Z to E, may be considered as a conductor proceeding from the zinc; and the wire from a b to F, as one from the copper side of the battery; and consequently, the same effect is to be expected here as in the preceding cases. It is unnecessary to add, that, with the north end of the magnet upwards, the motion is from left to right, and the contrary with the magnet reversed. This experiment is due to M. Ampere.

445. A very pleasing addition has been made to this apparatus by Mr. J. Marsh. It consists in having a second point descending from F, which is made to rest in an agate cup, fixed on the top of the magnet, fig. 7, and upon which the whole machine is balanced, having a perfect freedom of motion; and, to preserve this balance. the magnet is placed vertically in a foot. The machine being now charged with acid, a compound motion takes place, the zinc cylinder revolving in one direction, and the copper vessel in another, producing thus a very pleasing effect; the latter, however, is by no means so rapid as the other, in consequence of the weight of the acid; and, in fact, that of the whole machine being supported on the lower point.

446. To show the effect of a horse-shoe magnet on a freely suspended galvanic wire, let Zs, fig. 8, denote a part of the galvanic wire, freely suspended by the chain connexion at o, proceeding from the zinc eud of a battery, its lower extremity being amalgamated, and slightly immersed in a reservoir of pure mercury, having a

connexion at C with the other extremity of the battery. NS is a horse-shoe magnet, posited as shown in the figure.

447. The contact being now made at C and L, the hanging part of the wire, oz, will be thrown out of the mercury into the position o z'; the contact being thus broken, it falls by its own gravity into the mercury; by which means, the contact being renewed, it is again projected, and so on with an extraordinary rapidity; and if the position of the magnet be reversed, or the contact be changed, the direction of the motion will be changed also, but the effect will be the same. 448. This singular motion may be still explained by the hypothesis that has been advanced; for the wire having a tendency to pass round the north end of the magnet to the right hand, and round the south end to the left hand, is urged by equal forces directly in a line with the open space of the magnet, the equality of the two forces preventing the rotatory motion about either, but both conspiring to give to the wire the rectilinear motion which has been described. This experiment is also due to Mr. J. Marsh.

449. If we wish to exhibit a wheel and axle rotation by means of a horse-shoe magnet, the machine represented in fig. 9 will produce this motion. A B is a rectangular piece of hard wood; CD an upright wooden pillar; DE a piece of stout brass or copper wire; and ab a somewhat smaller wire, soldered upon it at E, on the lower side of which the wheel W, of thin copper, turns freely; hf is a small reservoir for mercury, sunk in the wood; and g i a narrow channel running into it. HH is a strong horse-shoe magnet. Mercury being now poured into the reservoir fg, till the tips of the wheel are slightly immersed in it, and the surface covered with weak dilute nitric acid, let the connexion with the battery be made at i and D, and the wheel W will immediately begin to rotate with great velocity. If the contact be changed, or if the magnet be inverted, the motion of the wheel will be reversed; but, in general, the best effect is produced when the wheel revolves inwards. The suspension of the wheel is shown in fig. 10. This is a necessary consequence of the motion described in the last experiment, by which it was suggested, and is explained on the same principles.

450. The machine for exhibiting a compound wheel and axle rotation, with two horse-shoe magnets, is shown in fig. 11. ABGD is a rectangular piece of board, having two grooves about half an inch deep, cut in it parallel to its length. Cp, Zq, are two wires, having cups for connexion at Z and C, and each passing into its respective groove, ab, cd, filled with mercury, into which are slightly immersed the points of the wheels W W'; these being fixed on an axle W W', and resting upon the two supports mn, rs, brought to a fine edge at n and s, in order to reduce the friction as much as possible, and to give the greater freedom of motion. NS are two horse-shoe magnets, posited as in the figure, with the like poles interior and exterior of the wheels.

451. The apparatus being thus prepared, and the contact made at Z and C, the wheels will be

gin to rotate, and in a very short time will acquire a velocity exceeding, very considerably, any of the motions hitherto described.

452. It is unnecessary to say, that by changing the contact, or by inverting the magnets, the direction of the rotation will be also changed. The usual precaution of covering the surface of the mercury with weak dilute nitric acid, will increase the rapidity of rotation; but it is not actually necessary in this case.

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453. To exhibit the terrestrial directive quality of a galvanic wire, let AB, fig. 12, represent a piece of wood fixed to any convenient support, through which pass the two wires G, E, and where they remain fixed. At their upper and lower extremities are soldered the small metal cups, a, b, c, d. DHIK, &c., is a part of the conducting wire, bent into the form shown in the figure, having small steel points soldered upon it at c and d. These points are inserted into the cups c, d, the upper one only resting on the base of its cup, the other being merely brought into contact with d, by a little quicksilver placed in it for that purpose, by which means the rectangle has a great freedom of motion given to it, the only solid contact being on the point c. Mercury is also poured into the other cups, for the sake of a more perfect and certain communication than that afforded by the mere juxta-position of the wires.

454. The apparatus being thus prepared, the two wires proceeding from the copper and zinc sides of the battery, are inserted into the cups a, b, and thus the connexion is established; first by means of the wire G with the cup c, thence by means of the contact of the points with the cup and mercury; it is carried forward from c through the rectangle, to the cup d, whence it proceeds to the cup a.

455. We have already seen that of this connecting wire, the part from e to d has a perfect freedom of motion upon the point at c, and will therefore obey any exciting force. This force, in the experiment in question, is the magnetic influence of the earth; and in consequence of which the rectangle, immediately the contact is made, places its plane perpendicular to the plane of the magnetic meridian; and to which position it will always return, after a few vibrations, if it be drawn out of it by the hand or otherwise.

456. This arrangement of the moveable conductors is perfectly consistent with our hypothesis, as is obvious without any farther illustra tion than what has been given in several preceding experiments.

457. A differently formed wire, and a more simple mode of suspension, is shown in fig. 13. Here a brass or copper wire, AC, rests at its bent end A, in a cup containing a little mercury, and is very moveable in azimuth round this point. The other end passes through the centre of a circular piece of pasteboard, and then forms spira turnings in the plane of this circular piece. The wire is attached by thread or silk to the pasteboard disc; and at the point B it turns and descends till its extremity reaches the quicksilver in the cup D. The communication being now made at A and D with the battery, the spiral will immediately arrange itself, as in the last

case, in a plane perpendicular to the magnetic mer.dian. This experiment is originally due to M. Ampere, but the mode of suspension described is that of rofessor Van den Boss.

458. A needle, upon a different construction, also due to M. Ampere, is shown in fig. 14.

459. The directive quality of the galvanic wire has been since exhibited in a variety of ways, much more simple than that above described, of which we shall only state the following:

460. M. de la Rive's apparatus consists of a small galvanic combination attached to a cork; the plate of zinc is nearly half an inch wide, and extends about one and a half, or two inches, below its cork, its upper end passing through the same; the slip of copper is of equal width to the zinc, but passes round it, being thus opposed to both its surfaces, as in Dr. Wollaston's construction: its upper end also appears through the cork. A piece of copper wire, covered with silk thread, is coiled five or six times, and tied together, so as to form a ring about an inch in diameter; and the ends of the wire are connected by solder, one with the zinc, and the other with the copper slip above the cork. See fig. 15.

461. When this small apparatus is placed in water, slightly acidulated with sulphuric or nitric acid, the ring becomes highly magnetic, and will arrange itself in a plane perpendicular to the magnetic meridian, or it will at least indicate a tendency to take up that position; but the escape of the bubbles, arising from the decomposition of the water, prevents it from preserving a fixed direction.

462. Its magnetic qualities, however, are more obviously shown by bringing to it a strong magnet. Mr. Barlow used a cylindrical one, about three quarters in diameter, and eighteen inches in length. This being applied at the distance of several inches, the ring was immediately attracted or repelled accordingly, as one or the other of the poles of the magnet was presented, or accordingly as one or the other side of the wire was opposed to the latter. When the result of the application is attraction, the cork will advance towards the extremity of the magnet; and if the latter be held horizontally, and in a line with the centre of the former, this will continue to advance till the pole of the magnet is within the ring, and then proceed with considerable velocity till it reaches the middle of the magnet, where it remains perfectly stationary. If now the magnet be withdrawn, and changed end for end, and re-introduced into the ring, the latter will go off from the magnet, turn itself round when quite free from it; again advance, and settle itself as before in the centre.

middle of the latter. At the same time all the above facts will be found perfectly consistent with the hypothesis that has been advanced; for it will be seen, when the wire and cork are in equilibrium, as above stated, that an observer will have the north end of the magnet to his left hand, and the south to his right, at equal dis tances; and acting therefore with equal and opposite powers; consequently the wire itself ought to be in equilibrio, and, when disturbed from, it will have a tendency to regain it, and hence be subject to all the conditions and motions that have been described.

464. As the current of electricity, produced by a Voltaic battery when passing through a metallic conductor, powerfully affects a magnet, tending to make its poles pass round the wire, and in this way moving considerable masses of matter, it was supposed that a re-action would be exerted upon the electric current capable of producing some visible effect; and the expectation being, for various reasons, that the approximation of a pole of a powerful magnet would diminish the current of electric.ty, the following experiment was made -The poles of a battery of from two to thirty four-inch plates were connected by a metallic wire, formed in one part ifto a helix, with numerous convolut ons, whilst into the circuit, at another part, was introduced a delicate galvanometer. The magnet was then put, in various positions, and to different extents, into the helix, and the needle of the galvanometer noticed; no effect, however, upon it could be observed. The circuit was made very long, short, of wires of different metals and different diameters down to extreme fineness, but the results were always the same. Magnets more and less powerful were used, some so strong as to bend the wire in its endeavours to pass round it. Hence it appears, that however powerful the action of an electr.c current may be upon a magnet, the latter has no tendency, by re-action, to diminish or increase the intensity of the former.

465. M Prechtl has a very curious experimental illustration of the effects of a spiral wire. He coils round a glass tube or wooden cylinder, steel-wire covering the surface as with a continuous sheath. To one end of this cylindric spiral, he applies the south or north pole of a magnet, and draws it along the cylinder in a straight line, parallel to the axis. In this way a magnet is formed, which possesses the following properties:

(a) Along its whole length, it has on one side the north, and on the opposite side, the south pole.

(b) These transversal magnetisms are in every point of the length of the wire-cylinder, equally strong.

dividual point of the whole length. Thus the transversal magnet is in the same condition as the conjunctive wire of the Voltaic column.

463. This very simple apparatus, which may (c) Both of its ends exhibit on the contrary be made at the expense of about a shilling, no particular polarity, and they have no other throws great light upon the nature of the electro-magnetism than that which belongs to every inmagnetic action, and proves most satisfactorily, that notwithstanding the intimate relation between the electro-magnetic, and simple magnetic fluids, they are not identical; for no possible arrangement of simple magnets can be made that would lead one of them beyond the pole of another, to find its state of equilibrium in the

(d) If we hold this transversal magnet over a magnetic needle in the declination-plane, it repels exactly like the conjunctive wire, the north pole of the needle to the right or to the left, ac

cording as the effective north pole of its transversal magnetism lies to the left or to the right hand; and with greater or less force, according to the strength of its magnetism, even to 90°. (e) If we draw the one pole of a magnet along this transversal-magnet, in a spiral direetion, the wire becomes magnetised longitudinally; the transversal magnetism disappears, the two poles are found at the two extremities, and it now resembles an ordinary magnet.sed steel wire. The longitudinal and transversal magnet.sms are not compatible with each other, in their full exhibition.

466. Besides this transversal magnetism, with single polarity, magnets may be easily made, possessing several transversal magnetisms. Take four magnetic bars, about a quarter of an inch thick; provide a small disc of wood, with an aperture of about an inch diameter in its centre, and four grooves, cut into one face of the disc, leading from the circumference, inwards: in these grooves the magnetic bars are to be fixed, with their narrow edge outwards, and their ends projecting into the aperture. Let a north pole alternate with a south pole in the circle, so that the ends of the bars right opposite to each other may be homogeneous; and adjust the whole, so that they may touch the circumference of a steel wire-coil, about half an inch diameter. We then draw this spiral wire through the opening, and between the four magnets, taking care that the cylindric coil does not revolve on its axis, but that the direction of each individual magnetic pole remains in the same plane with the axis; for otherwise the wire would acquire the longitud:nal magnetism. Instead of the wire-coil, we may take a massive cylinder of steel. This will acquire, when treated in the same way, the compound transversal magnetism, without its extremities having a stronger magnetism than each of its cross sections. These transversal magnets with manifold polarities, have all the above enumerated properties of the single transversal magnets, but their phenomena are st 1 more in unison with those of the electrical conjunctive wire; for, under this manifold polarised magnet, the deviation of the needle is the reverse of that above the magnet.

467. In the following manner we may produce a section of a manifold polarised transversal magnet in a larger rod, and with a proportionally greater number of poles. We take a ring of steel wire, from four to six inches diameter, and furnish it with as many poles as we can apply to its circumference. Lay the ring flat on a table, and apply to its outer edge the two poles of a horse-shoe magnet, that are as near each other as possible; then remove the magnet and apply it in succession all around, at distances equal to the width of its own poles. In this way it is easy to induce on a steel ring, of about five or six inches diameter, from twenty to thirty opposite poles.

468. This steel ring, which represents a section of the manifold polarised transversal magnet, exhibits, relatively to the greater extension and smaller number of the existing magnetisms, the phenomena which a conjunctive wire presents. In every part of it the magnetic needle, with its

north end above, stands to the right hand; below it, to the left; and inversely. M. Precht! thinks that these direct results leave no further doubt concerning the magnetical condition of the conjunctive wire.

469. Although success had attended the experiments made to magnetise steel needles and bars by ordinary electricity when in motion, yet every attempt to occasion the deviation of the magnetic needle by a current from the machine had failed, until M. Colladon instituted his experiments. Conceiving that the quantities of electricity operated with had been too small, he used a battery of thirty jars, containing 4000 square inches of surface for the previous accumulation of the electricity, and operated with a galvanometer constructed on Nobili's principle, having 100 revolutions of wire. The wire was covered with a double thickness of silk, and the instrument placed in a room away from that containing the battery. Two copper wires covered with silk, and suspended on silk cord, connected it with the electrical apparatus. A very fine point was soldered on to each of these wires, which were then named the extremities of the galvanometer.

470. The battery was charged: one galvanometer extremity was connected with the outer coating, and the other approached towards a ball in connexion with the inner coating. When at four or five centimetres' distance, (an inch and a half or two inches) the needle deviated; when at less than half that distance, the deviation was 23°, and, gradually diminishing, continued for five seconds. The direction of the deviation accorded with the course of the electricity. Every repetition of the experiment gave the same result, and when the points brought to the battery were changed, or the charge of the battery itself was altered, the direction of the deviation corresponded with it. The quantity of deviation varying with the distance of the points was often 40°; and the return of the magnetic needle to its original position, upon the cessation of the current, was in every instance ascertained.

471. On making experiments with a Nairn's machine, and also with a plate machine, no accumulative apparatus being used, deviations of the needle, amounting to 3° or 4°, were obtained in dry weather.

472. It was thus demonstrated that the electrical machine could, like the Voltaic pile, produce a current competent to the deviation of the needle, and that the electricity accumulated in a given time in a battery, or even in a conductor, was a finite portion of that which circulated in the same time in a closed electro-motive circuit. To establish this comparison in a more definite manner, a wire of platina was soldered to the two extremities of the galvanometer, one of the junctions heated, and the other cooled; at 125°, C. (268° F.) the needle of the galvanometer deviated 45°, the same quantity which had been produced by the battery. Hence it appears that the galvanometer may, in certain cases, be a very useful indication of the quantities of ordinary electricity.

473. The insulation of the 'different turns of

the galvanometer wire appearing to be an im

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