ELEMENTARY LAWS OF CONTINUOUS CURRENTS 19 



Permeability. The permeability, ^ is generally given 

 as the ratio of the flux density to the m.m.f. per cm. length. 

 It is therefore evident that (i may be obtained from the 

 magnetization curve for any required density. A straight 

 line is drawn from the point on the magnetization curve 

 where the density has the required value, to the origin. 

 The tangent of the angle between this line and the X axis 

 (i.e., the ratio of the vertical value for the point to the 

 horizontal value) is evidently eqaul to \L because by definition 



li = B/H, (10) 



where B = the flux density in lines per sq.cm., 

 H = the m.m.f. per cm. length. 



For example, the permeability of cast steel when the 

 density is 10,000 lines per sq.cm. as shown by the curve, 

 Fig. 1, is 10,000/4.0 = 2500. The maximum permeability 

 will occur at that density which makes the line drawn 

 through the origin tangent to the curve. 



Hysteresis. If the iron is carried through a complete 

 magnetic cycle, i.e., from zero to a positive maximum 

 magnetization, then to zero, reversed, and to negative 

 maximum, etc., the complete magnetization curve will form 

 a loop as given in Fig. 2. This is called the hysteresis loop 

 of the iron; its area is a measure of the amount of work 

 used (in the iron) in carrying the iron through the cycle. 

 The exact nature of the action involved in the dissipation 

 of this energy is not known but it is supposed that the iron 

 is made up of molecular magnets and that as these molecular 

 magnets are reversed heat is generated by some sort of 

 molecular friction. 



Occurrence of Hysteresis Loss in a Generator. In the 

 armature core of an electric generator or motor the magnetic 

 flux is continually reversing its direction as the armature 



