Imatges de pÓgina
PDF
EPUB

the arc of flame of the voltaic battery would be affected by the magnet. From the imperfection of the apparatus be could not obtain any decisive result; but hopes to repeat the trial. He also states, that he has made many trials, with the hope of affecting electrified wires by the magnetism of the earth; as also, of effecting chemical changes by magnetism; but without any successful results.

He also mentions that he has passed the common elec trical shock, in the same way as M. Ampere did the galvanic, along a spiral wire, by which a bar of steel enclosed in it with a glass tube, was powerfully magnetized.

He concludes the note with some observations on Ampere's theory of the identity of magnetism and electricity.

"Without meaning," he says, " to offer any decided opinion on that gentleman's ingenious views, I shall beg permission to mention two circumstances, which seem to me unfavourable to the idea of the identity of electricity and magnetism; first, the great distance to which magnetism is communicated by common electricity. (I found that a steel bar was made magnetic at fourteen inches distance from a wire transmitting an electric shock from about seventy feet of charged surface): and secondly, that the effect of magnetizing at a distance by electricity, takes place with the same readiness, through air and water, glass, mica, or metals; i. e. through conductors and nonductors."

With the greatest deference to the opinion of Sir H. Davy, we cannot help observing, that as far as we understand M. Ampere's theory, he seems to consider the magnetic effects, not as produced by the common electric currents, but by other currents formed by what is termed the conflict of electricities. He has, in fact, (as we learn from an abridged account of his memoir in the Annals of Philosophy for October, 1821, p. 276,) pointed out the distinction between the two classes of phenomena, and contrasted them in a very particular manner. He shews that the contrast between the electro-magnetic attractions and repulsions, and the common electrical, is very striking; and that they differ essentially, both in the circumstances of their production, and in their effects. If then such a distinction be admitted, it follows that the two circumstances noticed by our author, need not prevent the belief of the identity of magnetism with that particular modification of electric action which M. Ampere supposes. We will not however proceed any further in relation to this theoretical part of the subject; and more especially, as it is confessedly involved in much obscurity in t present stage of the enquiry.

We shall now proceed therefore to the consideration of Sir H. Davy's second paper, containing some fresh researches, relating to the same branch of science:-and in order the more properly to introduce our readers to this latter excellent paper, we will previously mention one or two discoveries, connected with the subject, which were made known in the interval between the -appearance of Sir H. Davy's first and second memoir.

The most generally interesting, perhaps, of these discoveries, was that of M. Von Buch, who in repeating Sir H. Davy's experiments on magnetizing by common electricity, found that a strong discharge was not necessary, nor even a Leyden phial: but by merely passing the common spark through a spiral, he magnetized a needle placed in it.

MM. Gazzeri, Ridolfi, and Antinori, at Florence, tried a number of curious experiments. One singular result which they obtained was, that a needle magnetized outside of a spiral, had its poles opposite to one within.

M. Berzelius passed the current diagonally through a plate of tin, and found the magnetism strongest at the other angles; thus, he observes, proving that the electro-magnetic, like the electric, and the magnetic, polarity, has a tendency to go to the opposite extremities of bodies which imbibe it.

Many other observations of inferior importance have also been made by various philosophers, and several improvements in the mode of experimenting have been adopted: but of these it would be foreign to our present purpose to take a more minute survey. Those we have mentioned are simply such as are to be considered the most important advances in discovery, and as naturally leading us in the order of time to the consideration of Sir H. Davy's second paper. The object of the investigations described in this paper, was, according to the author, to gain some distinct knowledge on the relation of the different conductors to the magnetism produced by electricity. This enquiry was suggested from some circumstances which he incidentally noticed in the course of his former experiments; and it was extended as well to the comparison of the nature of the conductors, as to their temperature; and with respect to both points, he has established several very curious and important conclusions.

In the first place, he found that the magnetic 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

[ocr errors]

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 magnetized needle.

Amongst other observations of this kind, he found that imperfect conducting fluids do not give polarity to steel when electricity is passed through them: but electricity passed through air, produces this effect. It was this consideration. which led him to try a very beautiful experiment: On charging. very strongly the extremely powerful battery at the London Institution, connecting the poles with charcoal, and thence in the usual way forming an arc of electrical light, he found that when a powerful magnet was presented to it, the arc or column of light was attracted or repelled according to certain circumstances of position; these positions, as far as the author has explained them, (which he has not done fully,) will be found capable of being truly represented by the simple method before described, the arc of light being in the place of a moveable connecting wire. It will be recollected that in his former paper he alludes to this experiment, in which he had then not succeeded, owing to the want of greater power in the apparatus.

Thus far his researches had an immediate connection with the magnetic effects of electricity: he now proceeded with some enquiries which relate to the circumstances of the conducting power of different substances for simple electricity; these enquiries, however, are of course to be regarded as forming a part of the general examination of electro-magnetic phenomena. As, however, they may not be so generally interesting as those more immediately concerned with the magnetic effects, we shall content ourselves with a very cursory review of them. The principal part of the investigation is thus given by the author:

"Metals, it is well known, readily transmit large quantities of électricity; and the obvious limit to the quantity which they are capable of transmitting, seems to be their fusibility, or volatilization by the heat which electricity produces in its passage through bodies. Now I found in several experiments, that the intensity of this heat was connected with the nature of the medium by which the body was surrounded; thus a wire of platinum, which was readily fused by transmitting the charge from a voltaic battery in the exhausted receiver of an air pump, acquired in air a much lower degree of temperature. Reasoning on this circumstance, it occurred to me, that by placing wires in a medium much denser than air, such as ether, alcohol, oils, or water, I might enable them

to transmit a much higher charge of electricity, than they could convey without being destroyed in air; and thus not only gain some new results as to the magnetic states of such wires, but likewise, perhaps, determine the actual limits to the powers of different bodies to conduct electricity, and the relations of these powers."

These expectations were to a certain extent realized, by many curious experiments. A wire, which would have been fused in air, was, by being passed through water, enabled to develope very strong magnetic effects. Sir H. Davy however ascertained that even when thus cooled, there was a limit to the degree of electricity which a wire would transmit.

In proceeding further in his enquiries, a remarkable result was, that the conducting power of metallic bodies, varied with the temperature, and was lower in some inverse ratio as the temperature was higher, and the consequences were the same whether the heat was occasioned by the electricity passing through the wire, or by any external means.

One beautiful experiment illustrative of this doctrine, which our author has related, we must give in his own words:

"Let a fine wire of platinum of four or five inches in length be placed in a voltaic circuit, so that the electricity passing through it may heat the whole of it to redness; and let the flame of a spirit lamp be applied to any part of it, so as to heat that part to whiteness, the rest of the wire will instantly become cooled below the point of visible ignition. For the converse of the experiment, let a piece of ice, or a stream of cold air be applied to a part of the wire, the other parts will immediately become much hotter, and from a red, will rise to a white heat. The quantity of electricity that can pass through that part of the wire submitted to the changes of temperature, is so much smaller when it is hot than when it is cold, that the absolute temperature of the whole wire is diminished by heating a part of it, and vice versa, increased by cooling a part of it."

Sir H. Davy then proceeded to try the conducting powers of different metals, in which he found very considerable variations. In the same wire he found the conducting power to be nearly as the mass, and independent of the extent of surface. In comparing the powers of fluid and solid media, it appeared that the power of the best fluid conductors is some hundreds of thousand times less than those of the worst metallic conductors. He also made some experiments, with the hope of ascertaining the exact change of ratio of the

conducting powers dependent upon the change of the intensity and quantity of electricity; but did not succeed in gaining any other result, than that the higher the intensity of the electricity, the less difficulty it had in passing through bad conductors; and several remarkable phenomena depend upon this circumstance. Recommending the account of these to the attention of our readers, we will only proceed to mention one or two statements in the concluding part of this paper, which are peculiarly deserving of attention. We will give them in the author's own words; and we must observe, that we know of no scientific writer, whose language is in general more admirably adapted to the subject; it exhibits such an excellent union of brevity with perspicuity, that it is usually impossible to describe any experiment, or state any reasoning, given in his papers with more conciseness than he has done, and needless to explain it more at large.

"If," says he, "a chain be made of wire of platinum and silver, in alternate links soldered together, the silver wire being four or five times the diameter of the platinum,' and placed in a powerful voltaic circuit, the silver links are not sensibly heated, whilst all those of the platinum become intensely and equally ignited. This is an important experiment for investigating the nature of heat. If heat be supposed a substance, it cannot be imagined to be expelled from the platinum, because an unlimited quantity may be generated from the same platinum, i. e. as long as the electricity is excited, or as often as it is renewed. Or if it be supposed to be identical with, or an element of, electricity, it ought to bear some relation to its quantity, and might be expected to be the same in every part of the chain, or greatest in those parts nearest the battery."

On these remarks we will only observe, that the whole question concerning the nature of heat is involved in great obscurity, and so it must continue till we know more of the internal constitution of bodies, and what effect it may have on the different manner and degree in which they are affected by heat, as also by electricity, magnetism, or other similar agents.

He concludes with an account of the relation which the magnetic effects bear to the heat of the conductor. With the same conductor, he had before found, it increases with the heat, yet with different conductors he found it follow a very different law. Thus when a chain is made of different conducting wires, and they are placed in the same circuit,

« AnteriorContinua »