394 



ELECTROSTATICS AND MAGNETISM. [PT. II. CH. IX. 



phenomenon, discovered by Warburg*, and thoroughly investigated 

 by Ewingf, was named by the latter Hysteresis, to denote the after- 

 effects of the fields to which the substance has been submitted. 

 Warburg and Ewing found that if the field was increased to a 

 certain value, then decreased, and then varied successively between 

 the same limiting values, the path of the representative point on 

 the F-% diagram was a closed curve, which was re-traversed after 

 the first periodic cycle. This is called the hysteresis-loop, and its 

 area has an important physical significance. Such a loop is shown 

 in Fig. 82. If instead of continuing to repeat the same cycle 

 we vary F between different limits the point may take any 

 position between the two limiting curves of the loop, as 

 shown in Fig. 83, both these figures being copied from Ewing. 



FIG. 82. 



FIG. 83. 



If the cycle be so chosen that at some point, F, while decreasing, 

 passes through the value zero, the value of / calculated as 

 the corresponding value of 8/4?r is the residual magnetization. 

 If the force Fis still further decreased, its value when 7=0, $ = F, 

 is called, after Hopkinson, the coercive force, since it measures the 

 negative force necessary to destroy the residual magnetization. 



Besides these phenomena of hysteresis, there is another more 

 complicated effect, which causes the magnetization to arrive at 

 its final value only gradually, taking a certain time to reach 

 its permanent value. This is denoted by the name of viscous 

 hysteresis, magnetic lag, or after-effect (Nachwirkung), to dis- 

 tinguish it from the proper or static hysteresis just described. 



* Warburg, Wied. Ann. 13, p. 141, 1881. 

 + Ewing, Phil. Trans. CLXXVI., p. 523, 1885. 



