|:n its natural state. The intensity of this ac-
- ion diminishes rapidly with the distance, par-
ticularly in the case where the cylindrical con- luctor is short and the excited body is small ; jut in the case of atmospherical electricity, in ,vhich the charge is on the surface of a large
- loud, the inductive action takes place through
several miles of intervening space. An at- tempt was made by ^Epinus, Poisson, and others, to apply the same hypothesis to the phenomena of magnetism. Between these and those of electricity a striking analogy was ob- served. For example, bodies which are dissimi- larly electrified attract each other ; those which are similarly electrified repel each other. In like manner, two similar poles of a magnet re- pel, and two dissimilar poles attract each other. Again, if the north pole of a magnet be brought near an unmagnetized bar of soft iron, the near end exhibits southern polarity and the further end northern polarity, apparently simi- lar to the result of the action in the example we have just given of electrical induction. There is however this remarkable difference, that if we magnetize a piece of hardened steel in the same way by the induction of a power- ful magnet, and afterward break the bar into two pieces, each half will exhibit a north and south pole of equal intensity ; and if we con- tinue to break each piece into two others, however far the division may be continued, the same result will be produced, namely, a pole at each end of each piece and a neutral point in the middle. From this experiment we infer that the polarity of magnetism results from the development of the magnetic power in each atom of the mass ; while if the same ex- periment be made with an electrical conductor, that is, if it be separated into two parts while under the influence of the excited body, each half will exhibit a charge of only one kind of electricity. By considering therefore that elec- trical induction is produced by a bodily trans- fer of fluid from one end of the conductor to the other, and limiting the disturbance in agnetism to the particles of gross matter, a athematical expression of most of the phe- )mena known previous to the discovery of Oersted was obtained. Still electricity and agnetism were so dissimilar in some particu- lars that they continued to be studied as dis- nct branches of science. The fact had long n noticed that discharges of lightning fre- quently gave polarity to bars of steel, and in some cases reversed the mariner's compass. A series of experiments to imitate these effects were made by Franklin and others by passing shocks through darning needles. The results were unsatisfactory, since the needle was some- times magnetized in one direction and some- times in the other, and frequently not at all, without any apparent change in the conditions. Indeed, ordinary electricity was not favorable to the study of the connection of electricity and magnetism, since the phenomena which belong to both are exhibited during the continuance of ELECTRO-MAGNETISM 513 an electrical current ; and in the case of the dis- charge of a Leyden jar the transfer is so instan- taneous that we are only able to study effects which have taken place, without being able to make any observations as to the manner in which they have been produced. This was the condition of the science up to the winter of 1819-'20, when Prof. Oersted of Copenhagen put a new interrogation to nature by asking what would take place in regard to a magnetic needle when a wire transmitting a current of galvanism was brought near it. He found that when the wire was brought parallel to and near the needle, the latter tended to turn at right angles to the former. This was a new result, unlike any phenomenon before discovered. Previous to this, the connection between elec- tricity and magnetism had been sought in the analogy of the polarity of the two ends of a magnetic bar and the two extremities of a gal- vanic battery, both of which exhibited polarity. An account of this remarkable discovery was published in all parts of the civilized world, and everywhere excited the interest of men of sci- ence. It was repeated in England, France, and Germany. The additional fact was discovered by Arago in France and Davy in England, that the wire joining the two poles of a galvanic bat- tery while the latter was in action was capable of imparting magnetism to iron filings ; but the person who seized on the phenomenon with the greatest avidity, and who in the course of a few months developed the whole subject to such an extent as to elevate it to the rank of a new science, was Ampere, of the French academy. He discovered an additional fact which gave a key to all that had previously been found by his contemporaries, namely, that two parallel wires transmitting currents of electricity in the same direction attract each other, while similar wires transmitting currents moving in opposite direc- tions repel each other. On this fact, combined with the hypothesis that all magnetic action consists in the attraction or repulsion of elec- trical currents, he founded his celebrated the- ory of electro-magnetism, which gives in a single sentence a generalization from which all the known phenomena of electro-magnetism as well as ordinary magnetism can be deduced. This theory is based upon one fact and one hypothesis. The fact is this, that currents mov- ing in the same direction attract, and moving in opposite directions repel, each other; the hypothesis is, that the magnetism of a bar of steel consists in currents of electricity revolving at right angles to the length of the bar around each particle of the metal, the resultant of which would be a current around the circum- ference of the bar. Let us suppose a num- ber of shillings or cents piled one on the other, and cemented together so as to form a cylindri- cal column or rod 8 or 10 in. in height; and let us further suppose that on account of some molecular action a current of electricity is per- petually circulating in the circumference of each piece of coin, and that the direction of
