ELECTROMAGNETS. MAGNETISM OF IRON. 



&C climes for annealed wrought iron and for ordinary cast iron. 

 The value of cB increases indefinitely in a given sample of iron 

 when $ is increased indefinitely. 



The sharp bend in the cB and M curve is called the "knee" 

 of the curve. The iron is approximately saturated when &C has 

 been increased beyond the knee of the curve, and any further 

 increase of &C produces only slight increase of cB. Thus a 

 magnetizing force of 10 units produces a flux density of 12,400 

 lines per square centimeter in a certain sample of annealed 

 wrought iron, and a magnetizing force of double this intensity 

 produces only 14,330 lines per square centimeter; that is, doub- 

 ling the value of M (from 10 to 20 units) produces only about 15 

 per cent increase of <$. Wrought iron has been subjected to a 

 magnetizing field of about 20,000 units by Ewing who found the 

 corresponding value of cB to be about 40,000 lines per square 

 centimeter, and the corresponding value of 3 to be 1,730 units 

 pole per square centimeter. 



The accompanying table gives the corresponding values of cB 

 and eV for wrought iron, for cast iron and for soft cast steel. 



TABLE. 



Magnetic Properties of Iron and Steel. 



57. Magnetic permeability. A long coil of wire produces a 

 magnetic field of intensity &C in its (air-filled) interior, and this 

 field intensity is of course equal to the magnetic flux per unit 



* Thompson, Knight, and Bacon, Transactions American Institute of Electrical 

 Engineers, Vol. IX, 1892. 



