122 



ELECTRO-MAGNETISM. 



M. Ampere, with the view of more completely developing the action of 

 electric currents and magnets separately and on each other, contrived various 

 methods by which wires, formed into parallelograms, circles, and other geo- 

 metrical figures, could have a current transmitted round them, and be at the 

 same time so supported or suspended as to be capable of assuming any posi- 

 tion or direction to which their mutual attraction, or the attraction between 

 them and magnets placed near them, or the influence of the magnetism of the 

 earth upon them, might dispose them. These contrivances afterward became 

 instruments by which many important experiments were made ; the first of 

 which was communicated to the Academy on the 30th of October, 1820. 

 This was the fact, that a wire forming a plane geometrical figure through 

 which the electric current is transmitted will, if free to move, dispose itself so 

 that its plane shall be at right angles to the dipping needle. 



On the same day, MM. Biot and Savart communicated to the Academy the 

 results of experiments made with the view to determine the law of the mutual 

 attraction and repulsion of electric currents. The results of these experiments 

 were reduced to analytical investigation by Laplace, who showed that the 

 law resulting from them was, that the attraction or repulsion of each elementary 

 part of the current diminishes in the same ratio as the square of the distance 

 of the object on which it acts increases : a law identical with that of all 

 other modes of electrical attraction and repulsion. The effect of the obli- 

 quity of the current to the direction in which the force acted was also deter- 

 mined. 



On the 4th of December following, M. Ampere read to the Academy the 

 memoir which contains the reduction of the phenomena of electro-magnetism 

 to mathematical analysis. He showed that all the various phenomena attend- 

 ing the action of magnets on each other, of electric currents on magnets, and 

 of magnets on electric currents, and, in fine, of electric currents on each other, 

 could be derived, by mathematical calculation, from formulae expressing the 

 action of two infinitely small elements of electric currents, acting on each other 

 in the direction of the line joining their middle points. The discussion of this 

 subject was concluded in another memoir, read to the Academy on the 8th and 

 15th of January, 1821. 



This year, 1821, was signalized by the commencement of the labors of Far- 

 aday in electro-magnetism. This philosopher, who has since attained such 

 well-merited celebrity, realized a suggestion which originated with Dr. Wol- 

 laston. A magnet being placed in a vertical position, a wire was so suspended 

 that, while the electric current was passing through it, it was capable of mo- 

 ving round the axis of the magnet so as to describe a conical or cylindrical 

 surface. While the current was maintained, the wire took spontaneously this 

 motion ; and when the direction of the current along it was reversed, it re- 

 versed its motion, and turned round the magnet the contrary way. Reversing 

 these conditions, and instead of fixing the magnet and leaving the wire free, he 

 fixed the wire, and so adjusted the magnet that it was at liberty to revolve 

 round the wire as an axis. When the current was transmitted through the 

 wire, the magnet spontaneously revolved round it ; and when the direction of 

 the current through the wire was changed, the motion of the magnet was re- 

 versed. 



Faraday attempted, without success, to cause a' magnet to revolve on its own 

 axis ; but, the memoir containing the account of his experiments being pub- 

 lished in France, Ampere succeeded in producing rapid rotatory motion of 

 magnets on their own axes, and showed that this and the two former results of 

 Faraday's experiments followed as necessary consequences of his own mathe- 

 matical principles. He also showed that the same effects could be produced 



