CHAPTER II 

 THE MAGNETIC CIRCUIT WITH IRON 



10. The Difference between Iron and Non-Magnetic Materials. 

 Steel and iron differ in their magnetic properties from most other 

 known materials in the following respects : 



(1) The permeability of steel and iron is several hundred and 

 even thousand times greater than that of non-magnetic materials. 



(2) The permeability of steel and iron is not constant, but 

 decreases as the flux density increases. 



(3) Changes in the magnetization of steel and iron are 

 accompanied by some sort of molecular friction (hysteresis) with 

 the result that the same magnetomotive force produces a different 

 flux when the exciting current is increasing than when it is de- 

 creasing (Fig. 7). 



Besides iron, the four adjacent elements in the periodic system, 

 viz., cobalt, nickel, manganese, and chromium, are slightly mag- 

 netic. Some alloys and oxides of these metals show considerable 

 magnetic properties. Heusler succeeded in producing alloys of 

 manganese, aluminum, and copper which are strongly magnetic. 

 These alloys have not been used in practice so far. 1 



11. Magnetization Curves. The magnetic properties of the steel 

 and iron used in the construction of electrical machinery are shown 

 in Figs. 2 and 3. These curves are called magnetization curves, or 

 B H curves-, sometimes also the saturation curves of iron. 

 The flux density, in kilolines per square centimeter of cross-sec- 

 tion, is plotted, in these curves, against the ampere-turns per 

 centimeter length of the magnetic circuit as abscissae. 



The student may conveniently think of these curves as represent- 



1 For the preparation and properties of Heusler's alloys see Guthe and 

 Austin, Bulletins of Bureau of Standards, Vol. 2 (1906), No. 2, p. 297; Dr. C. 

 P. Steinmetz, Electrical World, Vol. 55 (1910), p. 1209; Knowlton, Physical 

 Review, Vol. 32 (1911), p. 54. 



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