90 ADVANCED ELECTRICITY AND MAGNETISM. 



The intensity of magnetization 3 approaches a definite lim- 

 iting value for a given kind of iron when *K is increased in- 

 definitely. This limiting or saturation value of 3 is about 

 l>73O units pole per square centimeter for annealed wrought iron, 

 and about 1,250 units pole per square centimeter for ordinary 

 cast iron. 



The value of 9 for a good permanent magnet made of a 

 straight bar of hardened tungsten steel, the length of the bar 

 being about 15 times its diameter, is about 700 or 800 units pole 

 per square centimeter. 



20000 



6000 



t-2000 



COO 



Fig. 65. 

 $ and cK curves for wrought iron and cast iron. 



The normal cB and &f curve. The curve of which the 

 abscissas represent values of magnetizing force ctf and of which 

 the ordinates represent the corresponding normal values of <$ 

 for a given kind of iron or steel is called the normal & and &f 

 curve of the iron or steel. Figure 65 shows the normal $ and 



