MAGNETISM AND ELECTROMAGNETICS 45 



Thus a unit north pole is associated with each 4 TT lines leaving 

 a surface and a unit south pole with each 4w lines entering a 

 surface. 



34. Magnetic Potential. The magnetic potential of an iso- 

 lated magnetic pole is the work done in carrying a unit north 

 pole from an infinite distance to the point against the forces in 

 the magnetic field. 



Fig. 38 shows a north magnetic pole of strength m. Its field 



FIG. 38. Magnetic potential. 



extends out radially in all directions and the magnetic force at a 

 distance of r cm. from m is 



nr m J 



SIC = -5 dynes. 



The magnetic potential of the point P at a distance of ri cm. 

 from m is the work done in carrying a unit north pole from an 

 infinite distance to the point against the force of repulsion of m. 

 The work done is 



TT^ 



W 



^j 



&Cdr 



r dr m 



I m 9 = 

 J ri T ri 



ergs. 



Therefore the magnetic potential of a point at a distance of r\ cm. 

 from an isolated magnetic pole of strength m is 



nf 



M = . 



(71) 



The difference of magnetic potential between the points P and 



Q in Fig. 38 is 



m m 



