366 



ELEMENTS OF ELECTRICAL ENGINEERING. 



there arises the idea of the multiplying effect of an iron core, 

 insofar as the production of magnetic flux is concerned. In con- 

 formity with this idea we may write : 



# = A*<W (9) 



in which /i is the ratio of a given value of flux density <$ to the 

 corresponding value of magnetizing force &(. This quantity, /*, 

 is called the permeability of the iron. It is of course not a con- 

 stant for a given kind of iron (if it were constant the cB and &C 

 curve would be a straight line), but its value varies with the flux 

 density cB, as is clearly shown in the tabulated values of <9f, <$, 

 and p ( = c&IM) given in Art. 12. 



1200 

 1000 



8oo 



600 

 400 



2OO 



\ 



\ 



4OOO 



8000 



16000 



Fig. 11. 

 <$ and fi curves for wrought iron and cast iron. 



The ordinates of the curves in Fig. 1 1 show the values of the 

 permeability /x of wrought iron and of cast iron for various values 

 of the flux density cB. 



The permeability of air and of all non-magnetic materials such 

 as copper, cotton, wood, etc., is unity. 



14. Magnetic circuit calculations. When an iron rod of uni- 

 form size and quality, which forms a closed magnetic circuit, is 



