S8S On the Magnetism impressed on Metals 



found that the force exercised by the wire was transverse to' its 

 length, and revolutive, as M. Oersted had observed ; but we 

 discovered besides that it decreased in a ratio exactly proportional 

 to the distance. However, the force which we thus observed was 

 in reality a compound result; for on dividing in imagination the 

 whole length of the conjunctive wire, into an infinity of segments 

 of a very small altitude, we perceive that each segment ought to 

 act on the needle, with a different energy according to its dis- 

 tance and direction. Now these elementary forces are precisely 

 the simple result which it is important to know, for the total 

 force exercised by the whole wire, is merely the sum of their 

 actions. Calculation enables us to remount from this resultant 

 to the simple action. This has been done by M Laplace. He 

 has deduced from our observations, tliat the individual law of 

 the elementary forces, exercised by each section of the conjunc- 

 tive wire, was the inverse ratio of the square of the distance ; 

 that is, precisely the same as we know to exist in ordinary mag- 

 netic actions. This analysis shewed that in order to complete 

 the knowledge of the force, it remained to determine if the 

 action of each section of the wire was the same, at an equal dis- 

 tance, in all directions; or if it was more energetic in a certain 

 direction than in others. I have assured myself by delicate 

 experiments that the last is the case. 



What we now know of the law of the forces, is sufficient for 

 explaining and connecting together a multitude of results, of 

 which I now proceed to indicate briefly some of the most 

 curious. For example, let us conceive as we have done above, 

 an indefinite conjunctive wire, stretched horizontally from south 

 to north. Let us present laterally to it a magnetic needle, of a 

 cylindrical shape, and suspended so that it can take no move- 

 ment but in the horizontal direction. For greater simplicity, 

 let us withdraw it from the influence of the terrestrial magnet- 

 ism, by neutralizing this influence with the action of a magnet 

 suitably placed. This being done, when the needle rests at the 

 same height as the wire, so as to point exactly to its axis, it is 

 neither attracted nor repelled ; but if we raise it above the 

 wire, it presents one of its poles to it, and makes an effort of 



