Imatges de pÓgina

ART. X. Philosophical Transactions of the Royal Society, 1821.

Part I. No. 2. On the Magnetic Phenomena produced by Electricity. By Sir Humphry Davy, Bart. P.R.S. Part II. No. 29. Further Researches on the Magnetic Phenomena produced by Electricity: with some new Experiments on the Properties of Electrified Bodies, in their Relations to conducting Powers and Temperature. By the Same.

ONE of the most interesting features presented to us in contemplating the progress of scientific discovery, is the union we often see between two branches of the stream, which rising, perhaps, from widely distant sources, after various wanderings, at length unite into one channel; and such confluences frequently take place under circumstances (to continue the metaphor) of peculiar picturesque beauty, from their abruptness, and its effects on the subsequent course of the stream,

The discoveries to which the papers under consideration relate, are such as must be contemplated in this point of view. The wonderful discoveries of Galvani and Volta were justly the boast of the commencement of this century. The production of effects, by the chemical action of a certain succession of substances on each other, which were strictly analogons to those formerly obtained by the friction of certain bodies, was a curious coincidence, and afforded a subject of generalization to philosophers, who were desirous of uniting both classes of phenomena into one. Many facts were brought forward in support of the identity of the two powers of electricity and galvanism, by which that idea was generally established. If further proof were wanting, the discoveries of the present day would amply supply it, in showing that both these powers are alike effectual in the production of a third class of phenomena, which, as derived from other sources, had been known long before either of the two former. But the mere establishment of the identity of galvanism and electricity, from their common influence in producing magnetism, is a very secondary and inferior part of the discoveries we are about to notice. Their great, and most striking, feature is the circumstance itself, that magnetic effects should be produced by electricity of either kind; and the

inferences which have been drawn with respect to the identity of magnetism and electricity. This point, indeed, we cannot, by any means, consider as actually proved, in the present stage of the enquiry; it has, nevertheless, been a subject of investigation to many of the philosophers who have followed this line of research; and has received from them all, very strong confirmation. It is this confluence, as it were, of the hitherto separate sciences of electricity and magnetism which we wish to point out to the notice of our readers, as one of those epochs in the progress of discovery which must be regarded with a peculiar interest. This union of the two branches of knowledge has been effected in the most unforeseen, as well as the most complete, manner. The new science of electro-magnetism has suddenly been brought into existence, and has at once sprung up, invested, by its first discoverers, with a very large share of all that is requisite to establish the perfection of the union which it has effected between two highly interesting branches of knowledge, and which acquire a yet higher interest from the intimate connection thus established between them.

M. Ersted, professor of natural philosophy at Copenhagen, had been for some years engaged in experiments respecting the supposed identity of chemical, electrical, and magnetic, forces. After many fruitless researches he was rewarded, in the winter of 1819, by the discovery of a curious fact, of which no one but himself had the slightest suspicion, and which, like almost all great discoveries, when once known, seemed so simple and obvious to try, that it appeared a matter of surprize that it should not have been observed before. On placing a compass-needle near the wire connecting the two poles of a galvanic battery, he found that it deviated very considerably from its previous direction. By this fact, then, a connection was established between the two powers of galvanism and magnetism; and a wide field of enquiry was laid open to philosophers respecting the laws according to which this effect was regulated under different circumstances. These researches were followed up to a considerable extent by Ersted himself, and since him, by the most eminent philosophers in every country of Europe; who, struck with the beauty and importance of these discoveries, as connected with some of the most interesting subjects of chemical and physical enquiry, have become his disciples.

The researches of (Ersted himself comprize a very large portion of the actual facts at present known. He discovered the law of attraction and repulsion of the poles of the needle,


according to its position, as moved round the wire, and as related to the side at which the positive or negative electricity is communicated. He has laid down the rules by which the different cases of the north or south pole being attracted or repelled by the wire, according to circumstances, are regulated. It certainly requires the exercise of considerable attention to apply these rules in the different cases: and in a paper published by this philosopher, some time subsequent to his first, he has given directions for the representation of all the different cases by one simple method. We confess, however, that we do not think this method to be the best adapted for an elementary study of the phenomena. It represents the course of the electro-magnetic effects, as continued in a spiral round the connecting wire; which idea coincides exactly with the directions given to the needle, when placed in different positions above, below, or on either side of, the wire. The student has only to frame such a representation for himself, and from the course of the electricity, thus indicated, he will immediately see the direction which will be given to the needle in any particular position.

As these different positions are necessary to be understood in all the subsequent parts of the researches, we cannot refrain from communicating to our readers a much more simple method of representing them generally, which has been given by a writer profoundly versed in the subject. It is simply this: let two slips of wood be fixed in the form of a cross; let that which crosses over the other be called the needle, and marked at its ends N. and S.; then the lower piece will represent the connecting wire; and if the N. end of the former be to the left-hand of the observer, then the end of the wire from him will be the positive; and this and the other end of the same piece must be named accordingly. Then, if the piece representing the needle be held so as to be above, below, or on either side of, the other, in any position, it will represent the direction which the magnetic needle has a tendency to take, when the wire is in a position relative to it, corresponding with that of the other piece of wood. We mention this easy and universal method of representing the phenomena, instead of describing any of the positions actually observed, as we consider them scarcely intelligible without a reference to diagrams: and the method we have described puts it easily in the power of any student to gain the clearest notions of the several positions. At the same time it is to be observed, that the positions here spoken of are such as the needle would assume if it were wholly unaffected by the

magnetism of the earth: and to obtain such a state of circumstances it is necessary that the apparatus be very powerful, and the needle possessed of great sensibility.

We have only given this very slight notice respecting the law by which the attractions and repulsions are regulated, because, though it is continually requisite in understanding the phenomena described by Sir H. Davy, in the papers under review, it would be impossible to explain it more fully without diagrams; yet, we trust, what we have said, will be sufficient. In a subsequent paper, M. Ersted stated, that he had found the intensity, and not the quantity of galvanic effect, to be the circumstance which is essential to the production of the greatest magnetic power. He found that even a single galvanic arc of considerable surface was suffi cient. He also found out a method of making an apparatus of this kind, so light and easily moveable, that a magnet presented to the end of the connecting wire attracted it, and caused the apparatus to revolve. Such, then, was the first discovery of these interesting facts. The papers of M. Ersted. are given at large in the Annals of Philosophy, vol. xvi.

In the course of the year 1820, these interesting discoveries excited much attention among the scientific men in dif ferent parts of Europe. That eminent French philosopher, M. Ampere, was among the first to repeat, and extend the experiments and researches begun by Ersted. He first established a curious relation between two currents of galvanism, by contriving two connecting wires easily moveable; when, if they had their poles situated both at the same end, they attracted; if at different ends, they repelled each other. Upon these facts, and some further considerations, he founded an ingenious theory of the nature of electric and magnetic action; the leading feature of which is, to consider these currents as composed of assemblages of currents in spirals, round the supposed line of the direction of the action, or the axis of the magnet. This philosopher was inclined to maintain, that the powers of electricity and magnetism are absolutely the same; differing only from the common phenomena of electricity, in being produced under different circumstances, and acting in a somewhat different manner. This point is considered by Sir H. Davy; and will be further noticed as we proceed.

M. Arago directed his attention to these subjects at about the same period. He found that the connecting wire, of whatever metal it was composed, was, for the time, itself

magnetic, and attracted iron filings, but not the filings of any other metal.

The same philosopher, in conjunction with M. Ampere, reflecting on the spiral form in which the action appeared to take place round the connecting wire; and Arago having concluded, from the last mentioned experiments, that the wire has the power of communicating magnetism to the iron filings, they inferred, that if the wire was bent into the form of a spiral, or helix, its magnetizing power would be much stronger; and on trying this experiment, they found needles inclosed in such spirals powerfully magnetized. They observed also, that the poles of these magnets always preserved an invariable relation to the direction in which the spiral twisted, and the position of the poles of the apparatus." These positions they found to be exactly such as are indicated by the method before pointed out; supposing the straight line, which before represented the connecting wire, to represent now an indefinitely small part of the spiral wire, which may be regarded as a straight line.

An equally curious result, which M. Ampere obtained, was the contrivance of a galvanic apparatus, with its connecting wire so arranged, and so delicately suspended, as to be moveable by the attraction of the earth, like a magnetic. needle; and displaying both the horizontal motions and the dip. He also illustrated his theory of the nature of magnetic currents, by spiral lines, which were found to exhibit all the phenomena of magnets.

Such, then, is a very slight sketch of the course of previous discovery on these subjects: some knowledge of which is absolutely requisite for understanding the researches of Sir Humphry Davy, contained in the two papers which are the immediate subject of this article. In the researches already noticed, we have observed, first, the attraction and repulsion of the needle by the connecting wire; the law of this action; and the attraction and repulsion of the connecting wire by a magnet, by another connecting wire, and by the earth: secondly, the magnetism of the wire itself, and its power of communicating magnetism. These may be considered the most essential parts of the discoveries hitherto noticed. These effects were all produced by the operation of a galvanic battery. We have now to examine some further researches, which present to us a third great and essential point, in respect to the completeness of this series of discoveries. The continuation of them also takes up a new branch of the subject, and investigates the relation of electro-magnetic

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