the Lines of Magnetic Force. 411 



tery presents such a case, and resembles a magnet in the dispo- 

 sition of the external lines of force. But the sustaining action 

 (as regards the induction) being dependent upon the necessary 

 relation of the opposite dual conditions of the force, is external 

 to the conductor, or the battery ; and in such a case, if the con- 

 ductor or battery be separated in the middle, no charge appears 

 there, nor any origin of new lines of inductive force. This is, 

 no doubt, a consequence of the fact, that the lines of static in- 

 ductive force are not continued internally ; and, at the same time, 

 a cause why the two divided portions remain in opposite states 

 or absolutely charged. In the magnet such a division does de- 

 velope new external lines of force ; which being equal in amount 

 to those dependent on the original poles, shows that the lines of 

 force are continuous through the body of the magnet, and with 

 that continuity gives the necessary reason why no absolute 

 charge of northness or southness is found in the two halves. 



3265. The well-known relation of the electric and magnetic 

 forces may be thus stated. Let two rings, in planes at right 

 angles to each other, represent them, as in Plate X. fig. 1. If 

 a current of electricity be sent round the ring E in the direction 

 marked, then lines of magnetic force will be produced, corre- 

 spondent to the polarity indicated by a supposed magnetic 

 needle placed at NS, or in any other part of the ring M to which 

 such a needle may be supposed to be shifted. As these rings 

 represent the lines of electro-dynamic force and of magnetic 

 force respectively, they will serve for a standard of comparison. 

 I have elsewhere called the electric current, or the line of electro- 

 dynamic force, " an axis of power having contrary forces exactly 

 equal in amount in contrary directions" (517.). The line of 

 magnetic force may be described in precisely the same terms ; and 

 these two axes of power, considered as right lines, are perpendi- 

 cular to each other ; with this additional condition, which deter- 

 mines their mutual direction, that they are separated by a right 

 line perpendicular to both. The meaning of the words above, 

 when applied to the electric current, is precise, and does not 

 imply that the forces are contrary because they are in reverse 

 directions, but are contrary in nature; the turning one round, end 

 for end, would not at all make it resemble the other ; a consi- 

 deration which may have influence with those who admit electric 

 fluids, and endeavour to decide whether there are one or two 

 cleft lifit ic-s. 



3200. \Vh< -n these two axes of power are compared, they have 

 some remarkable correspondences, especially in relation to their 

 position at right angles to each other. As a physical fact, Am- 

 pere* and Davyf have shown, that an electric current tends to 



* Ann. de Chim., 1822, vol. xxi. p. 47- t Phil. Trans. 1823, p. 153. 



