ELECTROMAGNETS. MAGNETISM OF IRON. 



365 



The sharp bend in the cB and 3f curve is called the "knee" 

 of the curve. The iron is approximately saturated when 3{ has 

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

 crease of 3{ produces only slight increase of $$. Thus a magnet- 

 izing 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 cm., or about ten per cent, increase, in 

 the value of cB. Wrought iron has been subjected to a magnet- 

 izing field of about 20,000 units by Ewing who found the cor- 

 responding value of cB to be about 40,000 lines per square centi- 

 meter, 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 3{ for wrought iron, for cast iron and for soft cast steel. 



TABLE. 



Magnetic Properties of Iron and Steel. 



13. 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 

 sectional area, that is, to the flux density, in the air-filled interior 

 of the coil. If the region inside of the coil is filled with iron, the 

 flux density is many times as great as df, as is evident from an 

 inspection of the $> and 3f curves of Fig. 10. From this fact 



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

 Engineers, Vol. IX., 1892. 



