The discoverer remarks, in his Latin publication, that the magnetical action of the current being ne cessarily propagated and not instantaneous, the as sociation of a progressive and revolving motion must give origin to a spiral motion; still, he adds, this seems not to be required for the explanation of the electromtignetical facts hitherto discovered. His words are, " Prxteres motus per gyros cum motu progressivo, juxta longitudinem conductoris conjunctus, cochleam vel lineam spiralem formare videtur, quod tamer, nisi fallor, ad phxnomena hucusque observata explicanda nihil concert." Se veral writers upon the continent have considered it as an essential point in Oersted's_ theory, that the magnetical motions in the current should be of a spiral form; but it is evident that he has well dis tinguished this theoretical but still necessary con sequence from the fundamental law, deduced from the facts. Supposing here spirals in the place of parallel circles, their windings must be so near to parallelism, that the deviations from it must be im perceptible. Thus the question belonging to the spirals may be left for farther research, in which, perhaps, the whole doctrine of vibrations might be considered.
In an appendix published two months later, (in Schweiggcr's Journal,) Professor Oersted explain ed the apparent difference observed between the effect of the galvanical battery, and that of a simple galvanical circuit. In the battery, which is acorn pound galvanical circuit, as well as in the simple one, the electrical current goes from the more oxidable metal (zinc), through the liquid conductor, to the less oxidable (copper): and when the water is taken away in one of the elements of the battery, and a wire put in its place, the direction of the cur rent remains of course the same; but when we make use of a simple circuit, the water remains at its place, and the uniting wire connects the two pieces of metal in a place, where the direction of the current is the opposite to that of the water. Fig. 6 will make this more perspicuous; Z repre sents here the zinc, C the copper, W the water, U the uniting wire; the arrows marked with + e and — e indicate the direction of the electrical current. It is visible that when in W the current goes from zinc through the water to the copper, it must in U go from the copper to the zinc.
In this appendix it is remarked that the magnet ical efficacy of the electrical current depends not on its intensity, but on its quantity of electricity, and that the simple galvanical circuit is preferable for electromagnetical experiments. Some time after the discovery of electromagnetism, the great Swedish chemical philosopher Berzelius was of opinion that all the effects of the uniting wire could be explained in assuming four magnetical poles in its circumference. Plate DXXII. Fig. 7, where A indicates the austral, B the boreal poles, represents such a distribution. As the appearances in the first electromagnetical experiments may, until a certain degree, be represented by this scheme, it had many adherents, even since Berzelius had abandoned it. In order to decide the question upon this subject, Professor Oersted made a direct ex periment which will be understood by Fig. 8. AB is a wooden pillar more than twelve feet high; C is a magnetical needle, protected with glass against motions in the air; DE a wire of brass; K a gal vanical apparatus; HGF and OJL brass wire; M and N small cups with mercury. The whole mov able part of this arrangement was supported by a wooden frame, not here represented. It appears that the apparatus K with its conductors, whose extremities are plunged in the mercury, can turn around nearly through the whole circle, without an interruption of the continuity of the conductors; thus the same point of the perpendicular wire, though immovable itself, changes every moment its relative place in the circuit, when the movable part FG1L is turned round. The experiment shows that the deviation remains the same, whatever the position of the movable part may be, and that of consequence the polarity must be the same in all points of the circumference of the conductor. The great distance of the other parts of the circuit is the reason that DE is the only one which can have a sensible effect upon the direction of the needle.
A most useful application of electromagnetism is the electromagnetic multiplier, invented by Professor Schweigger at Halle, and improved by several other philosophers. We have already seen that when the uniting wire is bent so as to form two pa rallel branches, each of them acts in the same di rection upon one of the poles of a maguetical nee dle placed between them (Fig. 5.). On proceeding upon this principle it is clearly shown that when the uniting wire is bent several times, as ABCDE, Fig. 9, and a magnetic needle is suspended in the space, inclosed by the windings of the wire, each of its horizontal parts must produce upon the needle an equal effect; thus in the figure the effect is quadrupled. It is to be remarked, that the wind
ings should be as near each other as possible, in order to keep them all very near to the needle. At the same time the windings must be isolated from each other, which is effected by covering the wire with silk. As the windings can be repeated a great number of times, the multiplication of the effect may go very far. It should be nearly without limits, were it not that the conducting power de creases when the length of the wire increases. In order to give the instrument the solidity necessary, the wire is wound upon a frame. As it is required that the needle should be as movable as possible, it is suspended by a fibre of silk, such as is found in the cod of the silk-worm. The instrument may be made much more sensible by means of another magnet placed so as to diminish the directive power of the needle. Mr. Nobili has made a new improvement in this apparatus. In the place of one needle he introduces a compound index, con sisting of two needles, NS and S'N', Fig. 10, in opposite directions, and joined by a piece of wood or of stout wire, GIL When these two needles are of equal strength, the directive power of the index is reduced to nothing; so that the most feeble im pulse will move it. But even when one of them has some-preponderance, the force required for making the index deviate is still inconsiderable. At the same time this arrangement has the advan tage, that both needles receive an impulsion, the needle NS from the inferior side of the conductor, and S'N' from the superior. The needles being in opposite situations, one will receive the same direc tion by the superior, as the other by the inferior side of the wire. When the needles approach as much to equality as is required for some nice ex periments, the index is too easily moved in some others. In order to make the instrument proper for experiments with various degrees of force, though all of the feebler kind. Professor Oersted added a bent magnet, IKL, which can be placed so as to repel the nearest end of the index, or so as to attract it. The first of these positions is represent ed in Plate CXXII. Fig. 10. The magnet can also be approached to the index or removed from it. Fig. 11 represents the whole instrument of half its dimensions. AB is a stand of wood, having a screw on each corner for levelling it. CCC, CCC are two supporters likewise of wood, bearing the frame defg, upon which the multiplying wire is wound. This wire may be conveniently 50 to 60 feet long, and make WO or more windings. From the windings each end of the wire passes through little ring k, (the other is not to be seen in the figure,) at the ends of the wire passes also through rings, which are here covered by the other parts of the figure; KKK, are two small pil lars of ivory or wood, supporting the transverse piece //, through which passes the cylindrical piece nip, having a head at in, and being movable up wards and downwards. At the centre r of the in ferior extremity,p is a little hole, communicating with a transverse hole, which here is represented as shut with a pin, seen immediately under the ring o. Through the hole at r is introduced one end of the silk rx, which is drawn out through one of the openings of the transverse hole, and fasten ed by means of the pin abovementioned. By the silk rx is suspended the index, consisting of the superior magnetic needle ns, and the inferior one, of which the extremity 72 is here visible, the other being covered by other parts of the figure. The boreal pole of one of these needles is turned in the same way as the austral of the other, and both con nected with a piece of wire. The circle at whose divisions the index points, is made of glass, pre ferable to brass, which often is magnetic. At q is a slit to receive the needle and keep it; when the instrument shall be transported, a similar one is on the other side of the instrument. The index is cleared from the slits when the instrument is to be employed. Having been thus cleared, it is still at rest until the piece Imp is drawn upwards, the ring o stops it, so that it shall not be elevated too much. The index is sheltered from the air by means of a case of glass which covers the whole frame in cluding the index, and has in the upper part a hole through which the head nz of the piece nip passes; tt is a pile movable in the slit yy, which has a scale, showing the distance from a point in the same plane, perpendicular below the centre of the index; uv is a bent magnet, which has two points, one of which is visible at ?V, the other is placed in a hole in the pile tt. This magnet can be taken out, and the point ?V introduced in the pillar, in or der to augment or diminish the directive power of the index, as the purpose may require. When this instrument is to be used, the index must, as al ready mentioned, be taken out of the slits, and the piece rap be elevated, so that the index can move freely.