ELECTROMAGNETS. MAGNETISM OF IRON. 



89 



by a given value of cV. These values are called the normal 

 values of &C and 3 corresponding to the given value of &C. For 

 example, 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, because of the mechanical vibrations 

 of the machine and because of slight variations of magnetizing 

 action due to slight changes of field current and of armature 

 current. 



The normal 3 and 3f curve.* The curve of which the 

 abscissas represent values of magnetizing force df and of which 

 the ordinates represent the corresponding normal values of of 

 for a given sample of iron or steel is called the normal 3 and 

 c^ curve of the sample of iron or steel. Figure 64 shows the 

 normal 3 and &C curves for annealed wrought iron and for 

 ordinary cast iron. 



1600 



1200 



200 



600 



300 400 



Fig. 64. 

 ef and &C curves for wrought iron and cast iron. 



* Methods for determining these curves experimentally are explained in labora- 

 tory manuals. See especially Franklin, Crawford and MacNutt's Practical 

 Physics, Vol. II, pages 113-124, The Macmillan Co., New York, 1908. 



