THE MAGNETISM OF IRON. 



General relation between pole strength and flux. A magnet 

 pole may be defined as a place where magnetic lines of force pass 

 from iron into air (north pole) or from air into iron (south pole). 

 A piece of iron may be magnetized 

 so that the magnetic flux does not 

 pass out of the iron. In such a 

 case, there are no magnetic poles. 

 Thus, the iron ring shown in Fig. 

 35 has no magnetic poles when it 

 is magnetized by a current flowing 

 through the winding of wire. 



The relation between pole strength 

 and magnetic flux which is given in 

 equation (19) is entirely general ; 

 4777/2 lines of force emanate from any north pole of which the 

 strength is m, whatever the shape and size of the pole may be ; 

 and 47rm lines of force converge upon any south pole of which 

 the strength is m. 



40. Magnetic field in the neighborhood of a long slim pole. 



Consider a long slim magnet having one of its poles spread uni- 

 formly over / centimeters of its end, as indicated by the shaded 

 area in Fig. 36. 



The lines of force emanate from this uniformly distributed pole 

 in planes at right angles to the axis of the rod as indicated by 



I centimeters 



Fig. 35. 



steel ! 



win; 



E&ma%s&&%$%^^ 



aide view 



end view 



Fig. 36. 



the fine lines in Fig. 36, and the intensity H of this field at a 

 point distant r centimeters from the axis of the rod is 



