280 Historical Sketch of Electro-magnetism. [Oct, 



and the wires being connected with the poles of a batteiy, the 

 parallelogram immediately moved towards a position in the plane 

 perpendicular to the dipping needle ; when the communication 

 was broken, it returned towards its first position; and when 

 renewed, it resumed the second, evidently indicating the mag- 

 netic influence of the earth over it. In consequence of the diffi- 

 culty of placing the centre of gravity in the centre of suspension, 

 and keeping it there, this conductor did not take its position 

 exactly in a plane perpendicular to the dipping needle, but 

 approached towards it till in equilibrium between the magnetic 

 and the gravitating power of the earth. 



On Oct. 30, MM. Biot and Savart read a memoir to the Aca- 

 demy of Sciences, the object of which was to determine the law 

 by which a connecting wire acted on magnetised bodies. Small 

 rectangular plates, or cylindrical wires, of tempered steel were 

 made magnetical by the double touch, and being then suspended 

 by silk worm threads were placed in different positions with, and 

 at different distances from, the wire connecting the poles of the 

 battery. The terrestrial magnetism was sometimes combined 

 with that of the wire, sometimes opposed to it, and sometimes 

 neutralised by the vicinity of another magnet. The different 

 positions of equilibrium, and the number of oscillations of the 

 needles, were then observed, and data gained, by which MM. 

 Biot and Savart were conducted to the following result, which 

 expresses the action exerted by a molecule of austral or boreal 

 magnetism, placed at any distance from a very fine and indefinite 

 cylindrical wire, rendered magnetic by the voltaic current. Let 

 a line pass from this molecule perpendicularly to the axis of the 

 wire, the force which draws the molecule is perpendicular to 

 this line and to the axis of the wire; its intensity is reciprocal 

 to the distance. The nature of the action is the same as that of 

 a magnetised needle placed on the surface of the wire in a direc- 

 tion determinate and constant in its relation to the direction of 

 the voltaic current; so that a molecule of boreal magnetism and 

 A molecule of austral magnetism would be drawn in different 

 directions, though constantly according to the preceding expres- 

 sion. 



Having succeeded in magnetizing iron and steel by the wire 

 discharging the voltaic apparatus, M. Arago was led to expect 

 the same etifects from common electricity ; and on trial found the 

 results to be the same. He announced this fact verbally to the 

 Royal Academy on Nov. 6, stating that he had produced all the 

 phenomena in this way that he had observed in using voltaic 

 electricity. No account of these experiments has, I believe, 

 been published, but it is easy to conceive the general way in 

 which they would be formed. ■ They are very important, as iden- 

 tifying voltaic and common electricity, though few, I believe, 

 still retain doubts on this point, and, also, as proving the 

 magnetic phenomena not to depend upon this or that mode of 



