18*22.] Historical Sketch of Electro-magnetism, 115 



ments would account for the phenomena. M. Ampere is, I 

 believe, inchned to adopt the latter.* 



Conceiving a magnet then to be formed in this way of electric 

 currents, and reverting to the experimental results obtained by 

 the action of a wire and a magnet on each other, if one end of the 

 magnet be presented to one side of the wire, it will attract it; if 

 to the other side, it will repel it. The reason according to the 

 theory is evident : the currents pass in different directions on 

 the two sides of the magnet up on one side down on the other. 

 When that side is towards the wire in which the currents move 

 in the same direction as in the wire, attraction takes place; when 

 the opposite side is towards the wire, repulsion takes place, 

 because the currents are in opposite directions. If the magnet 

 be turned round, and the other pole be brought near the wire, 

 the direction of the currents in the magnet will be turned also, 

 and motions opposite to those which before took place will now 

 occur, because the place of the similar and dissimilar currents 

 has been changed. 



In consequence of the idea which had been formed of a mag- 

 net as an assemblage of electric currents in planes perpendicular 

 to the magnetic axes, M. Ampere endeavoured to obtain an 

 imitation by forming a spiral or helix of wire, and passing a cur- 

 rent of electricity through it. As the electricity traversed the 

 spirals, it would nearly resemble the different currents in the 

 magnet ; and the effect of the obliquity of the spirals was coun- 

 teracted by returning t'he wire from the extremity down the axes 

 of the helix. This instrument has been described before,t and 

 the similarity of the effects produced by it to those of the magnet 

 stated. 



It would lead me far beyond my original intention were I to 

 extend further on this part of M. Ampere's theory, nor is there 

 any occasion ; for I am sure all those who are anxious to under- 

 stand or pursue the subject, will think it necessary to read M. 

 Ampere's papers ; and for those who may think a sketch suffi- 

 cient, I have already said enough. Let us, therefore, notice 

 very briefly that philosopher's opinions on terrestrial magnetism. 



Naturally led by his elaborate views to substitute terrestrial 

 magnetism for the magnet he had previously used in experiments 

 on the wire, M. Ampere was induced to suspend a circle very 

 dehcately, in hopes the earth's magnetism would make it tra- 

 verse ; for as according to his theory, wire and magnets moved 

 each other, not by any supposed pole or point of attraction and 

 repulsion, but by the attraction and repulsion of the currents 

 passing through them, he hoped to be able to make a current 

 inove also by those he assumed to exist in the earth. The suc- 

 cess of this experiment has been related,^ and was certainly 

 sufficient to make the author trust very confidently to a theory 



* Journal de Physique, xcii. 1 63. 



t Annals of Philosophy, ii. 281, New Series. X Ibid, ii. 279. 



I 2 



