ELEMENTARY LAWS OF CONTINUOUS CURRENTS 17 



wood, paper, etc. For iron y. may have values between 

 50 and 12,000 depending upon the quality of the iron, 

 the intensity to which it is magnetized, and its temperature. 

 The permeability of iron as this material is ordinarily used 

 in electrical machinery is about 2000. 



If the magnetic circuit is made up of several parts in 

 series, as is generally the case, the expression for reluctance 

 will comprise several terms also. 



Then we shall have 



'' 



where the subscripts refer to different parts of the mag- 

 netic circuit. 



9. Magnetization Curves Hysteresis. The magnetic 

 quality of a piece of iron is well shown by its magnetization 

 curve (sometimes called saturation curve). This curve 

 shows the relation between the magnetomotive force per 

 cm. length (or ampere-turns per cm. of length) acting 

 on a substance, and the density (i.e., lines per sq.cm.) of 

 the magnetic flux which is produced in the substance. 

 Flux density in substances which may be magnetized is 

 usually represented by the symbol B. 



Use of Magnetization Curves. Magnetization curves are 

 extremely useful in calculating the necessary winding for 

 the field coils of a machine. Knowing the flux density, 

 B, required in a certain part of the magnetic circuit we 

 can obtain at once from the magnetization curve the nec- 

 essary ampere-turns per cm. of length; and this value 

 multiplied by the length of that part of the magnetic circuit 

 under consideration gives the ampere-turns required by 

 that part of the circuit. The sum of the ampere-turns 

 thus obtained for the separate parts of the circuit gives 

 the total ampere-turns required for the magnetic path. 



