72 



ELECTRICAL ENGINEERING 



JJL occurs at a density of about 4000 lines per square centimeter or 

 26,000 lines per square inch, and for steel at about 6500 lines per 

 square centimeter or 42,000 lines per square inch. Densities be- 

 low these points are not of great importance. These curves are 

 plotted with induction density B expressed in lines per square 

 inch on a base of ampere turns per inch instead of magnetizing 

 force &C. This results in a change of scales only since B lines per 

 square inch = (2.54) 2 US, and T ampere turns per inch corre- 



0.4 irT 

 sponds to a magnetizing force 3-C = 



The curves showing the relation between permeability n and 

 induction density B in lines per square inch for the same materials 

 are shown in Fig. 60. 



1000 



900 



800 



700 



^600 



g500 



^400 



300 



200 



000 



\ 



111 



Cast 

 Cast 5 



Sheet 



tccl 



Steel 



20x10 3 40x10 3 COxlO 3 8Cxl0 3 lOOxlO 3 180x103 HQxlO 3 160x1 



Magnetic Induction B Line per Sq.In. 



FIG. 60. Permeability curves. 



50. Hysteresis. If the exciting current in the solenoid in 

 Art. 47 after being increased to its maximum value 7 is gradually 

 decreased again, the flux density decreases very slowly and when 

 the current i and therefore the magnetizing force 3-C is zero it has 

 a value or, Fig. 61, which is called the residual magnetic induc- 

 tion. If now the current is reversed and increased the residual 

 induction is decreased and reaches zero when the magnetizing 



