On the power of the Electrical Current in developing Magnetism in other Bodies.
In a paper read before the French Institute, the 25th September 1820, Air. Arago showed that the electrical current possesses, in a very high degree, the power of developing magnetism in iron and steel. Sir Humphry Davy stated the same facts in a letter to Dr. Wollaston on the 12th November 1820. Dr. Seebeck communicated to the Royal Academy at Berlin, the 14th December, an excel lent series of experiments upon the same subject. Thus treated in the space of three months by three so highly distinguished philosophers, the subject was nearly exhausted in the same year that the dis covery was made. The uniting wire of a powerful galvanic apparatus attracts iron-filings often with such a power as to form a coating around the wire ten or twelve times bigger than itself. Mr. Arago found that this attraction did not take its origin from any previous magnetism in the iron-filings, which could touch iron without adhering to it; nor was the attraction to be considered as a com mon electrical one, since brass and copper filings were not attracted. He found also that the iron filings began to move before they came in contact with the uniting wire. Hence it must be admitted that this attraction is operated by converting each little piece of iron into a temporary magnet. Greater pieces of soft iron were also converted into temporary magnets, and small steel-needles into permanent magnets. Sir Humphry Davy had, in his researches, obtained the same results, before he had got notice of the experiments of the French philosopher. Dr. Seebeck seems to have been in the same case, when he made his experiments; but he had received notice of Arago's experiments when he published his own. The direction of the magnetism produced is always according to the fundamental law. Let the circle in Fig. 14 repre sent a horizontal section of a perpendicular conduc tor, in which the current comes from above; let the little arrows indicate the direction of the revolving magnetism, and BA, BA, BA, BA, some steel needles; then these needles will obtain austral mag netism at A, and boreal magnetism at 13.
Dr. Seebeck found that a steel needle was strong ly magnetized when it was drawn around the con ductor. The direction of the magnetism was the same as it should be, if the needle had been laid closely around the conductor, and afterwards re moved. He laid also an armour of soft iron on both sides of the conductor, which hereby was made able to bear a considerable weight of iron.
Mr. Arago and Mr. Ampere, employed in the development of magnetism the principle of the mul tiplier, without having notice of the discovery of Schweigger. A steel needle AB covered with pa per, was surrounded by a winding of the uniting wire EE, as represented in Plate CXXII. Fig. 15. The steel-needle may also he included in a glass tube. The great galvanic apparatus of the London Institution is now found to develope magnetism in such an eminently high de,;ree, that a little steel bar, by falling through a glass tube, around which the windings of the uniting wire passed, was mag netized to saturation.
The electricity produced by friction, when em ployed in sufficient quantity, developes likewise magnetism in steel. The discharge of an electric battery, and even of a single bottle, magnetises a steel needle. All these magnetical effects are sub
mitted to the same law as those of the galvano electrical current, and hence they are also increas ed upon the principle of the multiplier. When the discharge passes through the air across the steel needle, the magnetism developed is feebler than it is when the electricity passes across it through a metallic conductor.
Mr. Savary, at Paris, has of late discovered that steel-needles placed at different, yet small distances from a wire, through which passes an electric dis charge, do not all obtain magnetism in the same direction. In one of his experiments he caused to pass the discharge of a battery having twenty-two feet surface through a platina-wire of about three feet in length and one-hundredth of an inch in dia meter. The needles in contact with the wire be came magnetised in the direction commonly ob served, which he calls the positive direction, but a needle placed at the distance of 1.1 millimetre, about inch, becomes magnetic in the opposite direction, which he calls the negative. At the dis tance of 2 millimetres a needle was not made mag netic by the discharge. At the distance of 3 to 8 millimetres the needles become magnetic in the positive direction, but most at the distance of 5.5 millimetres. From 8.6, to 21.4 millimetres, the magnetic direction was negative, with increasing intensity from 8.6, to 14.6, and with decreasing from this point until 21.4, where it was nearly at zero. From 23 millimetres distance the magnetic direction became again positive. As for different conducting wires, he found, that within certain limits the maximum of effect is the more distant from the wire, and the numbers of alternating direc tions the greater, in the same degree that the wire is shorter in comparison to its length. In a helix of narrow winding, needles placed parallel to its axis obtain all the same kind of magnetism, but by varying the electrical power, from that of one bot tle of Leyden, to that of a battery of twenty-two feet surface, he obtained, in one experiment, six alternations, viz. three positive and three negative. When the needles are included in a metal coating, for instance, wrapt in a lamina of tin, the effect is changed. If the coating is thick, the effect is nothing, but by a coat sufficiently thin the effect may be increased. When the conducting wire is straight, a plate interposed between the wire and the needle, if thin, augments the effect, if thick, di minishes it; a certain thickness may also be found by which the plate is without effect. The needle is in all these experiments in contact with the plate. When the plate is not interposed, but the wire placed upon the plate, the effect of a very feeble discharge is increased by the plate, and still more the thicker it is. At a certain degree of discharge a thin plate diminishes the effect, a thick plate aug ments it. The effect of very considerable dis charges is always reduced to nothing, or inverted by thick plates. By the galvanic arrangement the same effect is not produced, when the current is un interrupted, but analogous effects to those mention ed may be produced by an apparatus which has in tensity enough to give sparks at the moment of closing the circuit. The current must, for this purpose, only be established for a moment; a con stant current destroys the alternations.