ADVANCED ELECTRICITY AND MAGNETISM. 



sectional area (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 3f, as is evident from an inspec- 

 tion of the & and <#' curves of Fig. 65. From this fact there 

 arises the idea of the multiplying effect of an iron core, in so far 

 as the production of magnetic flux is concerned. In conformity 

 with this idea we may write: 



This quantity, ju, is called the permeability of the iron. It is 

 of course not a constant for a given kind of iron (if it were con- 

 stant the $> and 3f curve would be a straight line), but its value 

 varies with the flux density c&, as is clearly shown in the tabu- 

 lated values of c^, <$, and /z in Art. 56. 



1200 



tooo 

 8oo 

 600 

 400 

 coo 









4000 



8000 



10000 



and 



Fig. 66. 

 curves for wrought iron and cast iron. 



The ordinates of the curves in Fig. 66 show the values of the 

 permeability M of wrought iron and of cast iron for various 

 values of the flux densicy <$. 



The permeability of air and of all non-magnetic materials 

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



