MAGNETIC METHODS 



171 



With such apparatus the magnetizing current may be increased by 

 steps to any desired amount, plus or minus, and at these various stages 

 the value of the magnetizing field H, intensity of magnetization I, and flux 

 density B can be determined.^ 



The data thus obtained may be presented in two types of hysteresis 

 curves. In one type, the IH curve, / is used as ordinate and H values for 

 abscissa. In the other type or BH curve, B is the ordinate with H again 

 as the abscissa. 



With a ferromagnetic substance placed in the above-described appar- 

 atus, the magnetizing field is made to vary in small increments : proceeding 

 from the zero value of H in Figure 72(a) to such positive value P as 

 desired, and then back again to zero at point D. The current is then reversed 

 and the necessary negative values are obtained, to P' and then again back 

 to zero, point /. The intensity of magnetization, or the flux density B, of 

 this substance as it varies with the field H is shown in a curve like that in 

 Figure 72(a). 



♦ B or 4 nl 



(b) 



Fig. 72. 1 — Typical hysteresis curve for a magnetizable material, showing varia- 

 tion of intensity of magnetization, or flux density B, vs. magnetizing field H. 



There are three stages in the magnetization cycle. The first is the prelim- 

 inary or initial magnetization stage. Here the value of H is small and a 

 change therein produces only a very small change in B. Near the origin 

 the curve is substantially a straight line and /x = B/H and is constant. 

 This portion of the curve is represented at a in Figure 72(b). 



The second or magnetization stage shows that the curve rises rapidly 

 and a small change in H produces a large change in B. The permeability 

 /x = B/H has its maximum value at the inflection point of the B-H curve 

 in this stage. Note the portion h in Figure 72(b). 



The third or saturation stage is reached when the curve becomes nearly 

 horizontal, or where a large increase in H causes a relatively small increase 

 in B. Here /x, acquires quite low values, as may be seen by the portion c of 

 the curve. 



t Other procedures are often used for measuring these quantities; however this is one of the 

 simplest. 



