CHAP. Ill] 



HYSTERESIS AND EDDY CURRENTS 



33 



Let now the exciting current be again decreased, reversed and 

 increased to its former maximum value corresponding to H = OP. 

 It will be found that the relation between B and H follows a differ- 

 ent though symmetrical curve, A'C'F'A, which connects with the 

 upper curve at the point A. The complete closed curve is called 

 the hysteresis loop; a sample of iron which has been subjected to a 

 van- ing magnetomotive force as described before, is said to have 

 undergone a complete cycle of magnetization. If the same cycle 



FIG. 7. A hysteresis loop. 



be repeated any number of times, the curve J^etween B and H 

 remains the same, as long as the physical properties of the sample 

 remain unchanged. 



The lower half of the hysteresis loop is identical with the 

 inverted upper half, so that the residual flux density OC' = OC, 

 and the coercive force OF' = OF. The shape of the loop for a 

 L'ivin sample is completely <lt< i mined 1>\ the maximum onlinate 

 .I/', or th<- HIM \imum excitation <>/'. If the excitation be camVd 

 further, for instance, to the point D on the virgin curve, the 

 hysteresis loop would be larger, beginning at the point D, and 

 would be similar in its .neiu-ml >ha|>e to the loop shown in Fig. 7. 



