MAGNETISM AND ELECTROMAGNETICS 



71 



the gradient becomes small again and a large increase in mag- 

 netizing force is required to produce any considerable increase 

 in density, the permeability decreases to a low value and the 

 material is said to be saturated. Such a curve is called a " per- 

 meability curve," "magnetization curve" or " saturation curve" 

 of the material. 



2800 3200 3COO 4000 



I Cast Iron 



II Cast Steel 



III Sheet Steel(Armature Punchings) 



300 400 500 



Ampere Turns per Inch 



FIG. 59. Magnetization curves. 



Curve 2, Fig. 58, shows the relation between the permeability 



B 

 fj, = and the induction density B; when the density is low 



eJi 



the permeability is low; as the density is increased the per- 

 meability increases until it reaches a maximum at the point m 

 where the tangent from the origin touches curve 1; above this 

 point the permeability decreases again. 



In Fig. 59 are shown permeability curves for materials used in 

 electrical machine design. For cast iron the maximum value of 



