Among the electromagnetical experiments which preceded the discovery of electromagnetism, ought to be mentioned an experiment of Professor Mojon at Genoa, who found that a steel needle having been 22 days in communication with a galvanical apparatus of 100 elements, had become magnetical, —an experiment which would have been of no his torical interest, if its author had not founded upon it, 18 years later, a pretension to the discovery of electromagnetism. He seems not to have been aware that his pretended discovery, were it true, should be considered as new even now; for the mag netical effect, hitherto proved by experiments, is not in the direction of the electrical current, but perpendicular to it. The experiment of Mojan is described in fildini's Essai Thi.orique et Experi mental sm. le Galvanisme. Paris, 1804, tom. i. pag. 339 and 340. .1ldini mentions, at the same place, that a certain Mr. Romancsi at Trent had confirm ed the experiment of Mojon, and at the same time observed that galvanism makes the magnetical needle deviate. Professor whose work upon galvanism comprehends two volumes, does not say a word more upon this subject.
It is, therefore, not surprising, that neither the French institute, nor the other learned societies, nor the numerous natural philosophers, to which the work was presented in the year 1804, took any notice of this observation, which would have aced crated the discovery of electromagnetism by sixteen years. Romanesi seems likewise to have forgot his observation, until electromagnetism was discovered.
Two or three years before the discovery of elec tromagnetism, Professor Maschmann at Christi ania, in Norway, observed that the silver tree, formed in a solution of nitrate of silver, when put in contact with mercury, (the arbor Diana?,) takes a direction towards the north; and the celebrated Professor Hanstcen found that this direction can likewise be determined by a great magnet. As the metallic precipitation is also of galvanical nature, this observation may be considered as one of the precursors of electromagnetism.
Electromagnetism itself was discovered in the year 1820, by l'rofessor Hans Christian Oersted, of the university of Copenhagen. Throughout his literary career, he adhered to the opinion, that the magnetical effects are produced by the same pow ers as the electrical. He was not so much led to this, by the reasons commonly alleged for this opin ion, as by the philosophical principle, that all phe nomena are produced by the same original power. In a treatise upon the chemical law of nature, pub lished in Germany in 1812, under the title Ansich ten der chemischen Naturgesetze, and translated into French, under the title of Reeherehes stir l'identite des forces Neetriques et ehy2niques, 1813, he endea voured to establish a general chemical theory, in harmony with this principle. In this work, he proved that not only chemical affinities, but also heat and light are produced by the same two pow ers, which probably might be only two different forms of one primordial power. He stated also, that the magnetical effects were produced by the same powers; but he was well aware, that noth ing in the whole work was less satisfactory, than the reasons he alleged for this. His researches upon this subject were still fruitless, until the year 1820. In the winter of 1819-20, he delivered a course of lectures upon electricity, galvanism, and magnetism, before an audience that had been pre viously acquainted with the principles of natural philosophy. In composing the lecture, in which
be was to treat of the analogy between magnetism and electricity, he conjectured, that if it were pos sible to produce any magnetical effect by electri city, this could not be in the direction of the cur rent, since this had been so often tried in vain, but that it must be produced by a lateral action. This was strictly connected with his other ideas; for he did not consider the transmission of electricity through a conductor as an uniform stream, but as a succession of interruptions and re-establishments of equilibrium, in such a manner that the electri cal powers in the current were not in quiet equi librium, but in a state of continual conflict. As the luminous and heating effect of the electrical current goes out in all directions from a conduc tor, which transmits a great quantity of electricity; so he thought it possible that the mag-nctical effect could likewise eradiate. The observations above recorded, of magnetical effects produced by light ning, in steel-needles not immediately struck, con firmed him in his opinion. He was nevertheless far from expecting a great magnetical effect of the galvanical pile; and still he supposed that a power, sufficient to make the conducting wire glowing, might be required. The plan of the first experi ment was, to make the current of a little galvanic trough apparatus, commonly used in his lectures, pass through a very thin plating wire, which was placed over a compass covered with glass. The preparations for the experiments were made, but some accident having hindered him from trying it before the lecture, he intended to defer it to another opportunity; yet during-the lecture, the probability of its success appeared stronger, so that he made the first experiment in the presence or the audi ence. The magnetical needle, though included in a box, was disturbed; but as the effect was very feeble, and must, before its law was discovered, seem very irregular, the experiment made no strong impression on the audience. It may appear strange, that the discoverer made no further experiments upon the subject during three months; he himself finds it difficult enough to conceive it; but the ex treme feebleness and seeming confusion of the phe nomena in the first experiment, the remembrance of the numerous errors committed upon this sub ject by earlier philosophers, and particularly by his friend Ritter, the claim such a matter has to be treated with earnest attention, may have determin ed him to delay his researches to a more conveni ent time. In the month of July 1829, he again re sumed the experiment, making use of a much more considerable galvanical apparatus. The success was now evident, yet the effects were still feeble in the first repetitions of the experiment, because he employed only very thin wires, supposing that the magnetical effect would not take place, when heat and light were not produced by the galvanical cur rent; but he soon found that conductors of a greater diameter give much more effect; and he then dis covered, by continued experiments during a few days, the fundamental law of electromagnetism, viz. that the magnetical effect of the electrical current has a circular motion round it.