Scientific Bepartment.

ELECTRO-MAGNETISM.

As this is a branch of Science but recently brought to light, it is not unlikely that many of our readers who take pleasure in such pursuits, have not seen any connected statement of the facts relating to it, but merely had their curiosity excited by occasional notices. We have therefore selected the most im-portant paragraphs of the chapter on the subject, in the new · edition of Dr. Henry's Chemistry:.a work which we cordially recommend to our readers.

All the effects of galvanic arrangements, that have hitherto been described, are produced in bodies when interposed between the extremities of conductors proceeding from the positive and negative poles; in other words, so placed that the galvaric current is imperfectly continued through the body intended to be acted upon. It was not known that the electric cur› rent, passing uninterruptedly through a wire connecting the two ends of a galvanic battery, is capable of being manifested by any effect, till professor Oersted of Copenhagen, in the winter of 1819, discovered an unequivocal test of its passage-in its effect on the magnetic needle. The opposite poles of a battery of sufficient magnitude, in full action, were joined by a metallic wire, which, for shortness, he calls the uniting conductor, or uniting wire. This wire was either placed horizontally, or bent in any other direction required by the nature of the experiment. When the wire was placed horizontally over, and parallel to, a magnetic needle properly suspended, and at a distance not exceeding three quar'ers of an inch, the needle was moved, and the end of it next to the negative pole of the battery turned westwards. The circumstances of the experiment remaining in every respect the same, except that the uniting wire was placed under the nee lle instead of over it, the decli

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nation of the needle was in an opposite direction; for the pole next the negative end of the battery now turned eastwards. Again, when the connecting wire and needle were situated in the same horizontal plane, no declination took place, either to the east or west, but an inclination, or vertical dip of the needle was observed. When the uniting wire was west of the needle, the pole next to the negative end of the battery was depressed; when the wire was to the east, the same pole was elevat. ed.

When the uniting wire was situated perpendicularly to the plane of the magnetic meridian, the needle, whether above or below the wire, remained at rest, unless the pole were very near the wire. In that case, the pole was elevated, if the negative electricity entered from the west side; and depressed, if from the east. M. Von Buch has since, however, shown that this state of rest does not continue in two of the four positions of the wire provided a sufficient galvanic power be employed in the expe

-riment.

When the uniting wire was perpendicularly opposite to the north pole of the suspended needle, and the upper extremity of the wire connected with the negative end of the battery, the pole, when brought near the wire, moved towards the east. But when the wire was opposite to a point between the pole and the middle of the needle, the pole moved westwards. When the upper end of the wire was made to receive positive electricity, the phenomena were reversed.

The amount of these effects diminished with the decreasing power of the battery, and with the distance of the needle from the uniting wire. This wire, it was found, may consist of almost any metal; nor does it lose its effect though interrupted by a column of water, provided the column does not extend to several inches in length. It is remarkable, also, contrary to what is observed, in any other effect of electricity or galvanism, that the influence of the uniting wire passes to the needles through plates of glass, metal, or wood, the disc of an electro

phorus, or a stone-ware vessel of water; nor does the sudden interposition of any of these bodies destroy or sensibly dimi nish the effect. On needles of brass, glass, or gum lac, no effect whatever is produced.

The common electrometer indicates the tension or intensity of electricity; but till the discovery of M. Oersted, we had no instrument to shew the direction of its current. The effect on the needle depends, indeed, entirely on the current. So long as this current is interrupted, no effect is produced on the needle; but the moment it is restored, the north pole of the needle is turned to the left of the observer supposing him to have his face directed towards that pole. This may be more briefly expressed by saying, that the north pole is carried to the left of the current which acts upon the needle. We thus acquire a galvanometer capable of pointing out the direction of the elec tric current under all circumstances.

By an instrument nicely constructed on this principle, M. Ampere ascertained that the current in the voltaic battery it self, from the negative to the positive extremity, has the same influence on the needle, as that current, which in the uniting. wire, goes on the contrary from the positive to the negative pole. This is best shewn by two needles, the one placed upon the pile, the other above or under the conductor. In each, the north pole of the needle will be seen turned to the left of the current near which it is placed; the two needles are both carried to the same side, and are nearly parallel when one is above the pile, and the other beneath the conductor.

When two rectilinear portions of two conducting wires, joining the extremities of two voltaic piles, are so disposed that the one is fixed and the other suspended so as to be moveable, the latter will approach the former if the currents be in the same sense, and will be repelled when the currents are in opposite directions. In common electrical attractions and repulsions of electric currents, it is precisely the reverse, the repulsion taking place only when the wires are so situated that

The attractions and rev currents are in opposite directions. pulsions of these currents, unlike the mutual action of bodies electrized in the common way, take place equally in vacuo as in air.

The discovery of M. Oersted was limited to the action of the electric current on needles previously magnetized. But it was afterwards, and about the same time, ascertained both bySir H. Davy, and M. Arago, that magnetism may be developed in steel not previously possessing it, by being placed in the ́ electric current, and may even be excited in the uniting wireitself. Both philosophers ascertained, independently of each other, that the uniting wire, becoming a magnet, attracts iron filings, and collects sufficient to acquire the diameter of a common quill. The moment the connexion is broken, all the fitings drop off; and the attraction diminishes also with the decaying energy of the pile. Filings of brass or copper, or wood' shavings, are not attracted at all.

The communication of magnetic properties to a steel needle, was effected, by Sir H. Davy and M. Arago, in different ways." The former observed that steel needles, placed upon the connecting wire, became magnetic; those parallel to the wire actad like the wire itself; those placed across it each acquired two poles. Such as were placed under the wire, the positive end of the battery being east, had north poles on the south of the wire, and south poles to the north. The needles above were in the opposite directions, and this was constantly the case; whatever might be the inclination of the needle to the wire. On breaking the connexion, the steel needles, placed across the uniting wire, retained their magnetism, while these placed parallel to it lost it at the moment of disunion. Contact with the uniting wire was not found necessary, for the effect was produced though thick glass intervened..

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Similar effects were produced in Sir H. Davy's experiments by the electricity excited by a common machine. A battery of 17 square feet, discharged through a silver wire 1-20th-of

an inch diameter, rendered bars of steel, two inches long, and from 1-10th to -20th thick, so magnetic as to lift up pieces of steel wire and needles; and the effect was communicated to needles at the distance of five inches from the wire, even with the intervention of water or of thick plates of glass or metal.

On the suggestion of M. Ampere, M. Arago, in a different manner, also communicated magnetism to the needle, both by.. voltaic and common electricity.

Any wire, through which a current of electricity is passing, has a tendency to revolve round a magnetic pole, in a plane perpendicular to the current; and that without any reference to the axis of the magnet, the pole of which is used. Also a magnetic pole has a tendency to revolve round such a wire.

Suppose the wire perpendicular, its upper end positive, or attached to the positive pole of a voltaic battery, and its lower end negative; and let the centre of a watch dial represent the magnetic pole: if it be a north pole, the wire will rotate round it in the direction that the watch hands move; if it be a south pole, the motion will be in the opposite direction. From these two, the motions which would take place if the wire were inverted, or the pole changed or made to move, may be readily ascertained; since the relation now pointed out remains constant.*

The theory of Oersted, which, though it appears to have led him to his principal discoveries, is not stated in a very intelligible manner, rests on the assumption of two different and opposite electricities, positive and negative, the former of which is developed by the more oxidizable, the latter by the less oxidizable metal of galvanic arrangements. Each of these forces has a repulsive activity for itself, and an attractive activity for the opposite force. In the wire connecting the two opposite poles of a galvanic battery, and in the space around it, there

* Several ingenious pieces of apparatus, have been invented to illustrate these eets.