466 ON PHYSICAL LINES OF FORCE. 



Beyond the conductor, in the space round it, 



< = 2Ctan-' % (24), 



a = ^=-2C7 y . = ^ = 2(7^* ,, 7 = ^ = (25). 



If p^Jaf + y* is the perpendicular distance of any point from the axis of 



2(7 

 the conductor, a unit north pole will experience a force = , tending to move 



it round the conductor in the direction of the hands of a watch, if the observer 

 view it in the direction of the current. 



Let us now consider a current running parallel to the axis of z in the 

 plane of xz at a distance p. Let the quantity of the current be c', and let 



the length of the part considered be I, and its section s, so that is its 



S 



strength per unit of section. Putting this quantity for p in equations (12, 13, 

 14), we find 



per unit of volume ; and multiplying by Is, the volume of the conductor con- 



sidered, we find 



X= -c'l 



(26), 



shewing that the second conductor will be attracted towards the first with a 

 force inversely as the distance. 



We find in this case also that the amount of attraction depends on the 

 value of p, but that it varies directly instead of inversely as /i; so that the 

 attraction between two conducting wires will be greater in oxygen than in air, 

 and greater in air than in water. 



We shall next consider the nature of electric currents and electromotive 

 forces in connexion with the theory of molecular vortices. 



