ELECTROMAGNETS. MAGNETISM OF IRON. 363 



of the previous magnetic state of the rod. Under these condi- 

 tions, therefore, definite values of cB and 3 are produced by a 

 given value of 3-C. These values are called the normal values * 

 of cB and 3 corresponding to the given value of e/ For exam- 

 ple, a dynamo having a given exciting current in its field coils, 

 tends to settle to a state in which the magnetization of its field 

 magnet is the normal value corresponding to the given value of 

 its field current, on account of the mechanical vibrations of the 

 machine and on account of slight variations of magnetizing 

 action due to slight changes of field current and of armature 

 current. 



The normal 3 and 3f curve. f The curve of which the ab- 

 scissas represent values of magnetizing force dt and the ordinates 

 represent the corresponding normal values of 3 for a given kind 

 of iron or steel is called the normal 3 and &C curve of the iron or 

 steel. Fig. 9 shows the normal d and df curves for ordinary an- 

 nealed wrought iron and for ordinary cast iron. The intensity 

 of magnetization, 3 y approaches a definite limiting value in a 

 given kind of iron when d{ is increased indefinitely. This limit- 

 ing, or saturation, value of 3 is about 1,730 units pole per square 

 centimeter of sectional area for annealed wrought iron, and about 

 1,250 units pole per square centimeter sectional area for ordinary 

 cast iron. 



The normal cB and &C curve. The curve of which the abscissas 

 represent values of magnetizing force 3f and the ordinates repre- 

 sent the corresponding normal values of cB for a given kind of 

 iron or steel, is called the normal cB and df curve of the iron or 

 steel. Fig. 10 shows the normal cB and &C curves for annealed 

 wrought iron and for ordinary cast iron. The value of cB in- 

 creases indefinitely in a given sample of iron when &C is increased 

 indefinitely. 



* A normal curve of magnetization of iron. W. S. Franklin and S. S. Clark, 

 Physical Review, Vol. VIII. , pp. 304-309. 



f Methods for determining these curves experimentally are explained in Nichols 

 and Franklin's Elements of Physics, Vol. II., pages 134-7. 



