1821.] Prof . Oersted on Electro-magnetism. 329 



It is to the well-conducted researches of M. Ampere that we 

 owe the law, that the conductors or parts of parallel conductors 

 attract each other when they both receive the electric current in the 

 same direction, and repel each other when they receive it in con- 

 trary directions. He does not endeavour to derive this law from 

 the nature of electric forces, but considers it as a law which is 

 independent of any laws already known. 



I shall show that this law is necessarily derived from that 

 which I have discovered. 



Let us regard the thing at first as if the effect of electricity 

 upon the magnetic needle had not been discovered. The attrac- 

 tions or repulsions of conductors of which we see no trace, unless 

 they are pervaded by electrical powers, can be attributed only 

 to those powers, and they must have such a direction in the 

 conductors as to enable them to produce the effects discovered. 



Let us consider the various modes of action which may be 

 conceived, in order to discover that which agrees best with the 

 circumstances demonstrated by experience. Fig. 6 represents 

 the transverse sections of two conductors, which receive the 

 current in the same direction. Neither of the electrical forces 

 can be in any sensible excess, for such an excess would cause 

 the conductors to repel each other mutually. The effects of the 

 two forces cannot either, possess the same direction ; for in this 

 case, they would destroy each other. Still less can any inequa- 

 lity be suspected in the states of the two conductors, because 

 they are supposed to be equally, and in the same manner, per- 

 vaded by the two forces. Thus the forces must leave each point 

 of the surface in opposite directions ; consequently their direction 

 cannot be in the lengthened radius, but each of the forces must 

 follow the direction of one of the tangents opposite to the point 

 from which they set out, pe, to the point C on the conductor A, 



— E will go towards t, while + E will go towards s. Let us call 

 also in this place -+- E and — E, which act transversely, + e and 



— i, in order to distinguish them from the forces in the longitu- 

 dinal direction, as besides they agree absolutely with what we 

 have before marked with these letters. 



If any one should adopt the improbable idea, that the forces 

 leave each point in two opposite directions, which will be found 

 on contrary sides between the tangent and the lengthened radius, 

 as a b and a c, fig. 7, each would nevertheless resolve itself into 

 two directions, one of which would be a d, and would produce 

 no effect in consequence of the union of the two forces, and the 

 other would be for one force a e, and the other force a f\ conse- 

 quently the effect would depend upon direction in a tangent. 



I have observed that this supposition is improbable, but it is 

 very likely that the forces may act at the same time according 

 to the tangent, and in every direction between the tangent and 

 the lengthened radius, so that they may form pencils from 

 the pointy?, fig. 8, + e in the directions p q, p r, p s, See. ; — e 



