Imatges de pÓgina

powers to another agent, not hitherto noticed. These re searches present the latest improvement which the subject has received; and, in our opinion, constitute a very considerable part of the whole science. In the former experiments, the galvanic apparatus alone was used; in these, the skill and sagacity of Sir H. Davy have enabled him to obtain the same results by the application of common electricity: and he has also investigated various interesting points relative to the effects of heat in modifying the electro-magnetic effects. We must acknowledge the former parts of these discoveries to be the productions of foreign genius and perseverance; while we may claim the subsequent improvements, now about to be noticed, as the sole offspring of British talent and skill: and we must feel particular satisfaction, in having to connect them with a name, which has of late years been by far the foremost in raising the scientific reputation of England among neighbouring nations.

We will now proceed to the more particular examination of the papers before us. The first of these appears to have been drawn up long before its publication; and the experiments detailed in it, to have been of yet earlier date: the reason of the delay does not appear. Sir H. Davy commences the paper, by making some mention of the approaches which were made some years ago to the discoveries of Ersted; and shews that any former observers were very far from arriving at the same results which have since been obtained. The views of such enquirers appear to have been generally obscure, and their experiments inaccurate; hence their supposed discoveries met with no attention and it was reserved for M. Ersted and his followers to develope the true connexion which subsists between the powers of electricity and magnétism.

The line of enquiry in which they at first proceeded, has been ably and successfully followed by Sir H. Davy. His first observation was, that the connecting wire itself became strongly magnetic, forming round it a coating of steel filings ten or twelve times its own thickness. On breaking the communication, they instantly fell off; proving that the magnetic effect depended entirely on the passage of the electricity through the wire.

He next proceeded to attach needles in different positions to the wire, some parallel, others transverse, above and below in different directions; and found that they all became magnetic. Those parallel to the wire attracted filings, but lost their magnetism as soon as the wire did. Those which were

transverse, exhibited distinct poles, in the same directions, with respect to the wire, as were before explained; and retained their magnetism permanently. It was also found, that contact of the needles with the wire was not necessary. The magnetic power was even transmitted through glass.

When some iron filings were spread on a plate of glass over the connecting wire, the filings arranged themselves in right lines, always at right angles to the axis of the wire.

Our author then proceeds to make some observations relative to the comparative magnetic powers arising from different electrical intensities. He found that when two pair of plates were so arranged that the zinc and copper plates formed respectively parts of one combination, the effect was greatly increased. The principle of this is the same as that estab lished by Ersted. He also concluded that the magnetism produced increases with the degree of heat to which the wire is raised.

The phenomenon of the arrangement of iron filings, just noticed, appears to us to afford a strong presumption in favour of M. Ampere's theory of currents in planes at right angles to the axis of the wire. The fact is ocularly demonstrated in the case of electricity, but we are not aware that any similar phenomenon has been exhibited by means of a magnet, which would perhaps be necessary for the complete establishment of M. Ampere's idea. We quit, however, this point, which is entirely theoretical, to allude to the claim of M. Arago to the discovery that magnetism might be communicated by common electricity. He announced it verbally to the Royal Academy, November 6, 1820, but gave no detailed account of his experiments or conclusions. The pre-: sent paper, by Sir H. Davy, is dated November 12th, and in that part of it immediately following the last mentioned, he has announced the same discovery as M. Arago; so that both appear to have a claim to originality: but our countryman was certainly the first to give any thing like a particular account of this important discovery, and of the steps, by which his researches were conducted. His account of this

discovery we will give in his own words:

"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 I found was 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 suf. ficient 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 I found 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.

"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 of two inches long and from one-twentieth to onetenth of an inch 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."

In order to ascertain the relations of the north and south poles of steel magnetized by electricity, to the positive and negative sides of the apparatus, Sir H. Davy placed short steel needles round a circle made on pasteboard, of about two inches and a half diameter, bringing them near each other, but 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 the battery, and its lower part with the negative. After the shock all the needles were found magnetic, and each had two poles: the south pole of one needle being opposite to the north pole of the next, and the poles had the same situation relatively to the direction of the electrical discharge as was before described; the same law of polarity was found to hold good in the communication of magnetism both by galvanism and common electricity. It was also found to hold good in all other arrangements of the needles with relation to the electric poles.

Such then is the most striking part of Sir H. Davy's discovery. In the experiments here noticed he has opened a new connection between two neighbouring departments of enquiry. In those described in the remaining part of his paper he has returned to the consideration of galvanic influence alone.

Supposing powerful electricity to be passed through several wires forming part of the same circuit, parallel to each other, either in the same or different planes, it seemed probable to Sir H. Davy that each wire, and the space round it, would become magnetic in the same manner as a single wire, though in a less degree; and this he found to be actually the case. When the circuit was made by four wires, parallel to each other, they were all found to be magnetic at once, and took up separate cylinders of iron filings; and when the sides of any two of these which were opposite to each other were brought near one another, the filings on one attracted those on the other. As the filings on the opposite sides of the wires attracted each other in consequence of their being in opposite magnetic states, it was evident that if those on similar sides of the wires could be brought together, the filings would repel each other. This was easily tried by placing two batteries parallel to each other, but with their poles in opposite directions. Steel-filings adhering to the wires of each, when brought together, repelled each other. In analogy to this mode of operating, the former experiment was varied by placing the wires of two batteries in the same direction, when the filings were attracted as before. In the same manner also the wires themselves exhibited the phenomena of attraction and repulsion according to the same law.

These experiments, it will be perceived, are similar to those made by M. Ampere on the attraction and repulsion of two connecting wires, or as he considers them, two electrical currents. The same fact, somewhat differently exhibited, being shewn, and the same principle established by both.

Another subject of research was suggested to Sir. H. Davy by the consideration that as bodies magnetized by electricity put a needle in motion, it was natural to infer that a magnet would put bodies, magnetized by electricity, in motion. He accordingly tried this by fixing two knife edges of platinum in the circuit of a powerful voltaic battery, and laid upon them pieces of wire of platinum, silver, and copper, successively, so that these might also be in connection with the circuit, and be at liberty to roll freely. A magnet being presented to them they all were found 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; and vice versâ, changing the pole of the magnet. The same effect was observed, but of course more feebly, with some folds

of gold leaf placed across the apparatus. It will be easy to perceive that this law of their motions is precisely the same as that before described.

We will give the conclusion of this paper in the author's own words:

"I will not indulge myself by entering far into the theoretical part of this subject; but a number of curious speculations cannot fail to present themselves to every philosophical mind, in consequence of the facts developed; such as whether the magnetism of the earth may not be owing to its electricity, and the variation of the needle to the alterations in the electrical currents of the earth, in consequence of its motions, internal chemical changes, or its relation to solar heat; and whether the luminous effects of the auroras at the poles are not shewn, by these new facts, to depend upon electricity. This is evident, that if strong electrical currents be supposed to follow the apparent course of the sun, the magnetism of the earth ought to be such as it is found to be.

"But I will quit conjectures to point out a simple mode of making powerful magnets, namely, by fixing bars of steel across, or circular pieces of steel fitted for making horse-shoe magnets round the electrical conductors of buildings in elevated and exposed situations."

In relation to the latter point, a note is subjoined, mentioning that several instances of the production of magnetism, by the means here stated, are recorded in former parts of the Transactions. One in particular deserves notice, where a stroke of lightning passing through a box of knives, rendered most of them powerful magnets."

Another note is added, which relates to the historical account of these discoveries. We alluded before to the supposed priority of M. Arago's discovery, announced Nov. 6. In this note, however, Sir H. Davy assures us, that all his experiments were made in October. It does not appear that he was aware of M. Arago having laid any claim to the discovery of the production of magnetism by common electricity. In his note he says,

"I find that M. Arago has anticipated me in the discovery of the attractive and magnetizing powers of the wires in the voltaic circuit; but the phenomena presented by the action of common electricity, (which I believe as yet has been observed by no other person) induce me still to submit my paper to the Council of the Royal Society."

In the same note he also mentions, that he has tried an experiment, which M. Arago likewise thought of, whether

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