120 



ELECTRO-MAGNETISM. 



which obtained little attention, being unaccompanied by any new facts, and the 

 community of science being then engrossed by the various and interesting ex- 

 perimental applications of the pile, and the magnificent series of discoveries 

 which Davy was beginning to unfold. In 1 820, however, Oersted supplied 

 all that was wanting in 1807 to fix the attention of scientific inquirers — a cap- 

 ital experiment. In that year he announced the fact, that a magnetized needle 

 placed near a metallic wire connecting the poles of a pile was compelled to 

 change its direction ; that the new direction which it assumed was determined 

 by its position in relation to the wire, and to the direction of the current trans- 

 mitted along the wire ; that when the current was sufficiently strong, and the 

 needle sufficiently sensitive, the latter always assumed a position at right an- 

 gles to the wire ; and that whenever the direction of the current along the wire 

 is reversed, the needle, making half a revolution, reverses the direction of its 

 poles, keeping still perpendicular to the wire. This discovery being made 

 known, caused unqualified astonishment throughout Europe ; the more espe- 

 cially, as all the attempts made before to trace the relation between the elec- 

 tric current and the magnet had been unavailing. The enthusiasm which had 

 been lighted up by the great discovery of Volta twenty years before, and which 

 time had moderated, was relumined, and the experimental resources of every 

 cabinet and laboratory were brought to bear on the pursuit of the consequences 

 of this new relation between sciences so long suspected of closer ties. The 

 inquiry was taken up by Ampere, Arago, Biot, Savart, and Savary, in France ; 

 by Davy, Cumming, and Faraday, in England ; and by De la Rive, Berzelius, 

 Seebeck, Schweiger, Nobili, and others, in various parts of Europe. 



Among these, in the inquiry now before us, Ampere has assumed the first 

 and highest place. No sooner was the fact discovered by Oersted made known, 

 than that philosopher commenced the beautiful series of researches which has 

 since surrounded his name with so much lustre, and brought electro-dynamics 

 within the pale of mathematical physics. On the 18th of September, 1820, 

 within less than three months of the publication of Oersted's experiments in 

 France, Ampere communicated his first memoir on electro-magnetism to the 

 Academy of Sciences. 



In this paper was explained the law which determined the position of the 

 magnetic needle in relation to the electric current. In order to illustrate this, 

 he proposes that a man should imagine the current to be transmitted through 

 his body, the positive wire being applied to his feet and the negative wire to 

 his head, so that the current of positive fluid shall pass upward from the feet 

 to the head, and that of the negative fluid downward from the head to the feet. 

 This being premised, a magnetic needle freely supported on its centre of grav- 

 ity, so as to be capable of assuming any direction, and placed before him, will 

 throw itself at right angles to him : the north pole of the needle pointing toward 

 his left, and the south pole toward his right. 



If the person through whose body the current thus passes turn round, so as 

 to present his face in different directions, a magnetic needle, still placed before 

 him, will have its direction determined by the same condition : the north pole 

 pointing always to the left, and the south to the right. 



In the same memoir were described several instruments intended to be con- 

 structed ; especially spiral, or helical wires, through which it was proposed to 

 transmit the electric currents, and which, it was expected, would thereby ac- 

 quire the properties of magnets, and retain these properties so long as the cur- 

 rent might be transmitted through them. The author also explained his theory 

 of magnets, ascribing their attractive and directive powers to currents of elec- 

 tricity circulating constantly round their molecules, in planes at right angles 

 to the line joining their poles ; the position of the poles on the one side or the 



