seven.

403. Again, fused tin was substituted for mercury in a porcelain vessel, into which wires of copper and steel were alternately ground and fixed; the elevations were produced as in the mercury, and the phenomena of rotation by the magnet: and it was found by direct experiment, that the conducting powers of the tin, at and just before its point of fusion, did not perceptibly differ, and that they were much higher than those of mercury. Lastly, the communication was made from the battery by two tubes having nearly the same diameter as the wires, filled with mercury, so that the electricity, for some inches before it entered the basin, passed through mercury; and still the appearances continued the same.

399. In the first experiment which I made, the conical elevations or fountains of mercury were about the tenth or twelfth of an inch high, and the vortices apparently as low; but, in the experiments made at the London Institution, the mercury being much higher above the wire, the elevations and depressions were much more considerable, amounting to the fifth or sixth of an inch. Of course the rotation took place of either pole of a magnet, or either wire, or both together, according to the well-known circumstances which determine these effects.

400. To ascertain whether the communication of heat diminishing the specific gravity of the mercury, had any share in these phenomena, I placed a delicate thermometer above one of the wires in the mercury, but there was no immediate elevation of temperature; the heat of the mercury gradually increased, as did that of the wires; but this increase was similar in every part of the circuit. I proved the same thing more distinctly, by making the whole apparatus a thermometer terminating in a fine tube filled with mercury. At the first instant that the mercury became electro-magnetic, there was no increase of its volume.

401. This phenomeron cannot be attributed to common electrical repulsion; for, in the electro-magnetic circuit, similar electrified conductors do not repel, but attract, each other; and it is in the case in which conductors in opposite states are brought near each other on surfaces of mercury that repulsion takes place.

402. Nor can the effect be referred to that kind of action which occurs when electricity passes from good into bad conductors, as in the phenomena of points electrified in air, as the following facts seem to prove. Steel wires were substituted for copper wires, and the appearances were the same in kind, and only less in degree; without doubt, in consequence of a smaller quantity of electricity passing through the steel wire: and by comparing the conducting powers of equal cylinders of mercury and steel in glass tubes, by ascertaining the quantity of iron filings they attracted, it was found that the conducting powers of mercury were higher that those of steel; the first metal taking up fifty-eight grains

404. From the rapidity of the undulations round the points of the cones, I thought they would put in motion any light bodies placed above the mercury; but I could not produce the slightest motion in a very light wheel hung on an axle; and when fine powders of any kind were strewed upon the surface, they merely underwent undulations, without any other change of place; and fine iron filings, strewed on the top of the cone, arranged themselves in right lines at right angles to the line joining the two wires, and remained stationary, even on the centre of the cone. The effect, therefore, is of a novel kind, and in one respect seems analogous to that of the tides. It would appear as if the passage of the electricity diminished the action of gravity on the mercury. And that there is no change of volume of the whole mass of the mercury, appears from the experiment; and this was shown likewise by enclosing the apparatus in a kind of manometer terminating in a fine tube containing air enclosed by oil; and which, by its expansion or contraction, would have shown the slightest change of volume in the mercury: none, however, took place when the contacts were alternately made and broken, unless the circuit was uninterrupted for a sufficient time to communicate sensible heat to the mercury.'

405. We may now direct the reader's attention to professor Barlow's admirable collection of experimental illustrations of this science. Mr. Barlow published a very interesting Treatise on Magnetic Attractions in 1823, but a new edition has since appeared. We shall only advert to the last section of the third part, referring our readers to the work itself for the fullest information on the subject.

406. We may commence with the most simple experiment, and show Mr. Barlow's mode of magnetising a steel bar. Take a piece of steel wire, as for example a sewing needle, and dip its ends first into steel or iron filings, in order to ascertain that it has no magnetism already in it, which will be the case if the particles of iron do not adhere to it; if they do, another needle must be tried, till one is found free from every species of magnetic action; this being done, connect the ends of the battery by the conducting wire, C Z and place the needle, NS, across it, fig. 10, plate II., drawing the latter backward and forward a few times, and it will be found to have acquired the magnetic property; for, on

immersing its extremities again in the filings, they will be found to adhere to it, in the same manner as to a needle magnetised in the usual way. 407. This very interesting experiment is strictly conformable to professor Barlow's hypothesis; for, according to this, the action of the galvanic particles in the wire, being tangential, will act upon the latent magnetic particles in the needle, in the direction of its length, and cause a displacement of them, precisely in the same manner as would be done by a magnet; and also, as in that case, the cohesive power of the steel preventing the return of the fluids to their natural state, the needle will remain magnetic. 408. This experiment was performed nearly at the same time by Sir H. Davy, and M. Ampere; but Sir H. Davy also succeeded in effecting the same with the common electrical machine, and showed that the magnetism might be excited at considerable distances, and consequently not only without rubbing the needle on the wire as we have described, but even without the contact. It requires, however, to effect this at the distances here alluded to, a very powerful apparatus.

409. If the needle be made a part of the galvanic circuit, or if it he placed lengthwise of the wire, no perceptible permanent magnetic power will be developed, which is also consistent with the hypothesis; because, in this case, the action of the wire will be transverse of the needle, which is the least favorable direction for the development of the magnetic power; the tendency of the action being to place the poles transversely, instead of lengthwise.

410. To ascertain the polarity of needles magnetised as in the last experiment, the wire and needle being placed as in the last figure, that is, the needle being above the wire, and Z denoting the zinc end of a battery of two plates only, it will be found that the extremity N will attract the south end of a compass needle, and the extremity S the north end; in short, that the north poles of the latent magnetic particles have been carried towards the left hand, and the south to wards the right hand.

411. Let the needle now be placed under the wire, instead of being placed over it, and in other respects the process described in the last example repeated, and it will be found that the polarity of the needle will be exactly the reverse of that in the last experiment, which ought to be the case according to the principle of the preceding experiment; because by this the north polarity is always carried to the left hand of the observer, who conceives himself to form the galvanic circuit, his head being towards the zinc end, and his face towards the magnet; for thus, his position being now the reverse of what it was in the preceding experiment, the polarity ought to be the reverse

also.

412. If we wish to magnetise a needle by placing it in a spiral conducting wire, let ZČ (fig. 11) represent a conducting wire bent into a spiral form, and let the needle ns be placed either naked in the spiral, or enclosed in a glass tube, or in a tube of any other matter; make the connexion with the battery, and in an instant it will be found that the needle ns has become strongly magnetic, having its poles posited, as shown in

the figure, viz. having its north end towards the zinc extremity of the battery.

413. This is of course precisely similar to the first experiment, the only difference being that, by means of the spiral form given to the wire, the action upon the needle is repeated as many times as there are spires of the wire covered by it; the power excited is therefore proportionably stronger, and the magnetism more quickly communicated. The explanation of the effect produced is exactly the same as in the last experiment. If the direction of the contact be changed, by supposing Z to communicate with the copper side of the battery, the effect will be in all respects the same, except that the polarity of the needle will be reversed. The end towards Z, in this case, becoming the south instead of the north pole. Or if a spiral, having its spires turned the contrary way, as shown in fig. 12, be used, and Z be supposed to communicate with the zinc side of the battery, the polarity will also be the reverse of that in the first case; viz. the poles will have the direction marked in the figure; and if here again the contact be changed by connecting Z with the copper side, the poles will be once more inverted, and have the same direction as at first. These facts, as we have stated above, are explained exactly in the same manner as those for the single wire.

414. In performing this experiment, Mr. Barlow employed a glass tube about five inches in length and half an inch in diameter; and it was observed, when the needle was placed in it, so that one half of it projected beyond the end, that the moment the plates reached the acid, the needle was drawn instantly to the middle of the tube, and while the contact was continued it was held suspended in the centre of the tube when the latter was held vertically, the suspending power of the spiral exceeding the power of gravity.

415. This effect is very curious, because the needle here remains suspended in the open space, directly in the axis of the tube, and not attached to either sides as in the usual cases of suspension by attraction.

416. To examine the effects of a spiral conducting wire on a floating magnetised needle, let a wire be wound about a glass tube of about half or three-quarters of an inch diameter, and hang it within a basin of water, as shown in fig. 13, so that the surface of the water rises to about the axis of the bore; then having pierced a small piece of cork with a needle previously magnetised, so as just to preserve it from sinking when immersed in the basin, make the connexion with the battery. The needle will instantly be agitated, and will soon arrange itself in front of the spiral in a direction parallel to its axis, and then suddenly dart into the interior of the tube with a force nearly sufficient to carry it to the other extremity; it then returns again towards the other end, and at length becomes stationary in the middle of the axis, arranging itself exactly parallel to it.

417. If the spirals have the direction shown in the figure, and Z communicates with the zinc side, the needle, if placed near the extremity of the tube A, will enter with its sout!

end; if placed near the other extremity it will enter with its north end; but, if the direction of the spiral be changed, the needle will enter in both cases the reverse way, as it will also if the direction of the spires remain the same, but the contact be changed. This experiment will succeed equally well if the tube be placed upright in the water; the needle will then dive like a fish,

and remain below till the contact is broken.

418. This entertaining and instructive experiment is due to Mr. Faraday; the explanation of it by the previous hypothesis is obvious, for the north pole of the particles of the needle being carried to the left of an observer conceiving himself coinciding with the direction of the wire, and with his head towards Z, all the effects ought to take place precisely as above stated. M. Ampere had assimilated a spiral wire of this kind with an actual magnet, and Mr. Faraday instituted the above experiment to prove that there was not that identity which had been assumed; for, by suspending a hollow cylindrical magnet in the same way, the needle was always attracted to the nearest extremity of its edge, and indicated no tendency to enter the tube.

419. To show the effect produced by a galvanic wire on steel or iron filings, we must strew a quantity of iron dust or filings on a table, and bring the connecting wire near to them, when the filings will immediately be affected by the action of the wire, some few flying towards it, and adhering to it as a magnet; and, if the wire be brought into actual contact with them, a very considerable quantity may be taken up by it, exactly the same as at the extremity of a bar magnet; but the moment the contact is broken the filings fall.

420. In order to produce the best effect in this experiment, the wire intended to be operated upon should be smaller than the conducting part of the circuit. This latter, in all cases, is the better for being stout, at least three-sixteenths of an inch in diameter; but in this, as in several other experiments, it is best to have the extremities of the wires terminated by a much smaller wire, wound round the former as a spiral, or by simple contact; for by this means, the transmission being made through a smaller space, the intensity of action is proportionably increased.

421. This experiment is due to M. Arago, and it seems at first sight somewhat at variance with Mr. Barlow's hypothesis; because we have here an appearance of actual attraction between the iron and the wire, whereas we have supposed that there is no attraction between them. A little consideration will, however, show, that instead of contradicting, this fact will serve to confirm the hypothesis in question.

422. Let us, for example, conceive W, fig. 14, to denote the section of our conducting wire descending vertically from the zinc end of the battery; then, the first and direct action of this wire will be to excite magnetism in any small particle of iron, ns, according to the direction indicated by the letters in the figure.

423. After which, the action of the wire will be to urge the point n in the line nn, perpendicular to n W, and the point s, in the line ss, perpendicular to s W; and, in consequence of

the combined action of these forces, the particle ns ought necessarily to approach the wire in the same way as it would do by a direct attractive force. This effect is therefore still consistent with our hypothesis, and strongly confirmatory of it.

424. We have seen that, by giving the con. ducting wire a spiral form, its power of magnetism is much increased; and in the same way. the power of the wire on the iron filings may be rendered very great. The best form for the spiral, however, here, is that in which the wire lies all in one plane, as was shown in a previous figure.

425. This, being connected by its two extremities with the poles of the battery, will take up an astonishing quantity of filings, which, by their reciprocal attraction towards each other, exhibit the most pleasing appearance.

425. To exhibit the rotation of a magnet round a galvanic wire, let ABED, fig. 15, represent a cup of glass, wood, or any other non-conductor, and NS a small magnet, having a hole drilled at S, whereby it may be fixed by a short piece of silk Sc', to the copper wire c' C, passing through the foot of the cup, and let mercury be poured into the latter till the needle floats nearly vertical. Conceive, also, Zz' to be part of the conducting wire, descending from the zinc side of the battery, and slightly immersed in the quicksilver. If now the contact be made at C with the copper side of the battery, the magi.et NS begins to rotate about the wire Z z', passing towards the left hand of the observer. This rotation will be greater or less according to the power of the battery, and will continue while there is sufficient force in the latter to overcome the resistance of the quicksilver to the motion of the magnet. If the descending wire proceed from the copper side of the battery, the motion will take place in a contrary direction, that is, from left to right.

426. Or, if the contact remain the same, and the magnet inverted, then also the motion will be reversed; but if the contact and magnet be both reversed, the rotation will be the same as in the first instance. This highly curious and important experiment, which is due to Mr. Faraday of the Royal Institution, is immediately explained by the same hypothesis; according to which, the extremity N of the magnet is always acted upon by two forces, one being the galvanic force, which is tangential to the wire, and the other the tension of the silk Sc', in the direction of the needle. Let this latter be resolved into two forces, one vertical and the other horizontal, and we shall find the extremity N under the influence of two horizontal forces, one always central and the other tangential. The result of which must be a rotation of that point about the wire; and it will be made with the position and arrangement shown in the figure from right to left.

427. To exhibit the rotation of a galvanic wire about the magnet, let ABDE, fig. 16, be a cup or vessel of wood or glass, and NS a magnet passing tight through its foot; Zz a conducting wire descending from the zinc side of the battery, and rendered free to move by the chain counexion at g. Let mercury be poured into the vessel till the extremity of the wire is slightly

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London Published by Thomas Tegg, 73 Cheapside April 1.1828.

J.Shury scalp.