The analogy of these effects, with those alterna tions, which may be produced in bad conductors by common electric experiments, is obvious.
Mr Ilill, at Lund in Sweden, has found that when the discharge passes along a magnetical needle, ex actly through its axis, all its magnetism is destroy ed. He even considers this as the best means to take away the magnetism of a needle. At the same time he remarks that when the electric charge does not go through the axis, a feeble magnetism is de veloped on both sides of the line of passage, which probably has led preceding philosophers into an er ror respecting the magnetical effects of electricity. (Schweigger's Journal for the year 1822, No. 3.) Professor Ermann at Berlin found that when the electrical discharge passes perpendicularly through the centre of a round plate of steel, it re veals no magnetism, but when a split is afterwards made in the plate, or a sector cut out of it, the op posite side of the gap shows the opposite magnet ism. The celebrated Gay Lussac and Mr. Welther, without knowing the experiment of the Prussian philosopher, discovered the same fact in a steel ring. This experiment is very illustrative; it shows that the steel disc or steel ring, whose circumfer ence has been in the same state as that of the unit ing conductor, preserves after the cessation of the current a latent magnetism, resembling that of a magnetic circle, composed of small magnets, con nected by their opposite poles. Such a circle is in effectual when the circumference is closed, but be comes a magnet when opened. This magnetism was, however, effectual during the time that the ring or disc was comprehended in the current, wherein its magnetism at every moment received a new impulse. Hence we may conclude that the cir cumference of' the uniting conductor is not to be compared with a magnetic circle, wherein the powers are at rest, which is the theory brought for wards by Air. Prechtel, director of the polytech nic school at Vienna; but our experiment confirms the original idea of the magnetical effect of the cur rent as produced by a revolving magnetism.
This view of the subject, that the magnetism of the electrical current is a magnetism in motion, has been overlooked by a great number of authors who have written upon electromagnetism: while it has been adopted by two highly distinguished philoso phers, Dr. Wollaston and Mr. Biot. The differ ence between magnetism in motion and at rest being until our time unexemplified, this view ap peared to many philosophers as a mere postulate, which they tried to avoid, by adopting some other theory, particularly the elaborate theory of Ampere, of which we shall afterwards speak. Now the theory of revolving magnetism has obtained a considera ble support by the discovery of Mr. Arago, who, in his researches on the effect of metals upon the os cillations of the magnetic needle, found that it was much affected by a metallic plate, for instance a cop perplate, when either the needle or the plate was put in motion. There is certainly but few philoso phers who have not repeated Arago's remarkable experiment by which a rotatory plate of copper or some other metal puts a magnetic needle conve niently suspended, into a revolving motion. We
must pass in silence the numerous and skilfully con ducted experiments of Mr. Barlow and Dr. See beck, and only quote for our purpose those of Messrs. Herschel and Babbage, by which it is prov ed that a rotating magnet causes a conveniently sus pended metallic plate to turn round. Mr. Poisson has read before the French Institute an elaborate mathematical treatise upon the theory of moved magnetism. Thus the theory of revolving magnet ism has obtained the only confirmation which could still be desired.
Effects of the Magnet upon the Uniting /Tire.
Professor Oersted, in the prosecution of his ex periments, was well aware that a movable part of the electric circuit must be attracted and repelled by a magnet after the same laws by which the unit ing wire acts upon the magnet. He published, two months after his first electromagnetical paper, an other paper, in which he gives an account of an ex periment he made; he found that a little galvanical circuit, suspended by a thin metallic wire, was put in motion by a magnet. He complains himself in this paper, that he had not succeeded hitherto in getting an apparatus sufficiently movable to be di rected by the magnetism of the earth (Schweigger's Journal.) Professor Schweigger at Halle, and pro fessor Erman at Berlin, both invented, without knowing Oersted's experiment, apparatuses fit for the same purpose. It would be tedious to give an account of all the experiments made upon this sub ject; a short description of those which arc consid ered as the best will be sufficient. Plate DXXII. Fig. 16 represents, with some slight modifications, an apparatus invented by Mr. Ampere. A B CD EFGH is a bent wire, of which the two ascending parts at B and G arc isolated from each other by some non-conductor and tied together. At A, and also at is soldered a steel point, which reposes on the bottom of a small iron cup filled with mer cury, at K and M. J K and L M are brass wires, N 0 a piece or wood, in which they are inserted, and by means of which they can be fixed at a con venient place. It appears that when the current enters at the end of one of these wires, for instance at J, it is obliged to pass through the whole mov able conductor A B C D E F G H, and go out at the other end L. This conductor is put in motion with much promptitude by means of the magnet. In comparing this arrangement with Fig. 5, it is ob vious that the part D E F G of the movable con ductor, in which the current enters at D, is quite analogous with B D C A, Fig. 5, and that there fore the austral magnetism on the interior side of both is turned toward a spectator placed over against the place represented by the figure. It is also evident that the magnetical direction is the same in the part B C D, which turns the same side to the space included by the movable conductor. Thus a magnet whose austral pole is directed against this space will repel the conductor, but placed near to a point of the exterior side it will attract it. On the opposite side of the plane B C D E F G, all the effects are opposite to those here mentioned.