172 EXPLORATION GEOPHYSICS 



Upon reducing the magnetizing field, after the third stage has been 

 reached, the plotted curve follows along a different path than it traced as 

 H was being increased. It is of note that during this reduction when H 

 reaches zero, there is a certain amount of flux left in the specimen. This 

 flux is designated as the remanent magnetism. See OA, Figure 72(a). 



The phenomenon of the lagging behind of the flux density or magnet- 

 ization as regards the intensity of the field is called magnetic hysteresis.'^ 



Remanent Magnetism and Retentivity. — When there is remanent 

 magnetism, it can be assumed that it is due to internal forces which cause 

 the remaining flux (after the field has reached zero) to stay in the material. 

 This characteristic, known as coercive force, is measured by OD of Figure 

 72(a). It is the force or coercive field necessary to reduce the flux in the 

 specimen to zero. The usual symbol for coercive force is He. The reversed 

 current portion of the hysteresis curve is shown in the third and fourth 

 quadrants of Figure 72(a). 



Retentivity defines the ability of a magnet to hold magnetism. Strictly 

 speaking, it is the remanent magnetism of a substance, or remanence, in 

 relation to the maximum magnetizing field. Brem — remanent flux. Reten- 

 tivity would be expressed by an equation thus : 



Retentivity = ^^^^^ (65) 



-'-' max 



A high retentivity magnet steel is desirable for magnetic instrument systems. 



The complete figure of variations of flux density or intensity of mag- 

 netization with field intensity is called the liystercsis loop. (Figure 72a.) 

 The area formed by the loop is the energy in ergs per cubic centimeter per 

 cycle involved in the operation. 



In considering hysteresis curves, it is of note that the descending branch 

 of the curve obtained with a reversed H is always above the ascending 

 branch, hence the zero value of / or 5 is reached later in the cycle than 

 the zero value of H. 



If the field is removed after point A (Figure 72a) is reached, there 

 will be a remaining magnetization equal to OA, and certainly the material 

 is not demagnetized. 



The only satisfactory way to demagnetize a specimen in hysteresis 

 experiments is to take it repeatedly through cycles of continually decreas- 

 ing range, ending with extremely small cycles (by alternating current). 



TYPES OF HYSTERESIS 



Figure 7Z illustrates three typical hysteresis curves, (a.) The steep curve 

 indicates a rapid increase of induction with magnetizing force, (b.) The 

 area of the curve is small, indicative of the small amount of work necessary 

 to reverse the magnetization. This iron will have a low hysteresis loss 



t R. W. Hutchinson, "Advanced Text Book of Magnetism and Electricity," 1927, 2nd Edition, 

 in two volumes. University Tutorial Press, Ltd., High St., New Oxford St., W. C, London. 



