18 



PRINCIPLES OF ELECTRICAL DESIGN 



calculations; but so long as engineers persist in expressing linear 

 measurements in feet and inches, the curves of Fig. 3 will gen- 

 erally be preferred by the designer. Fig. 4 may be used for high 

 values of the induction in armature stampings of average 

 quality. 



The value of /* is, of course, the ratio between B of Fig. 2 and 

 the corresponding value of H, and curves or tables giving the 

 relation between /* and H could be used ; but it is generally more 

 convenient to read directly off the curves of Figs. 2, 3, or 4, the 



130 



200 400 600 800 1000120014001600180020002200240026002800300032003400360038004000 

 Ampere -Turns per Inch 



FIG. 4. Magnetization curve for armature stampings (high values of 



induction). 



flux density in the iron coresponding to any known value of the 

 magnetizing force. As a matter of fact, it will be found that 

 the curves are more frequently used for the purpose of determin- 

 ing the necessary ampere-turns to produce a desired value of the 

 flux density. 



4. Magnetic Circuits in Parallel. As an illustration of the 

 fundamental relations existing between magnetic flux and excit- 

 ing ampere-turns, it will be convenient to work out a numerical 

 example. A simple case will be chosen of magnetic paths in 

 series and in parallel, with small air gaps in # circuit consist- 

 ing mainly of iron, and the effect of any leakage flux through 

 air paths other than the gaps deliberately introduced will be 

 neglected. 



