CONTEMPORARY ADVANCES IN PHYSICS 323 



axis of / or B, and increase the intercept upon the axis of //.* The 

 former of these intercepts, representing as it does the magnetization 

 which the metal retains when the external field has been reduced to 

 zero, is known under the names of residual magnetization and remanence 

 and retentiveness. The last two of these words, and also residual 

 magnetism, are used in a general sense, to denote the property of not 

 losing magnetization altogether when the magnetizing field is with- 

 drawn. The intercept on the axis of //, representing as it does the 

 force which must be applied oppositely to the direction of the prior 

 magnetization in order to annul it altogether, is known under the 

 names of coercive force and coercivity. 



Residual magnetism was the first of magnetic phenomena to come 

 under human notice. If the pieces of magnetite (lodestone) in the 

 fields of Asia Minor had not been able to retain the magnetization 

 which they had acquired in past ages, the Greeks would never have 

 observed nor produced a magnetized metal; if steel needles rubbed 

 against pieces of magnetite or held parallel to the earth's field and 

 "smartly tapped," as the English textbooks say, could not retain the 

 magnetic moment they so acquire, there would have been no compass- 

 needles; the discovery of magnetism would probably have waited 

 upon that of electric currents. Residual magnetism is the property 

 to which the intercept of the hysteresis-loops gives a definite and 

 definable meaning. 



The greatest remanence, usually called retentivity, is attained after 

 the material is magnetized to saturation. It may be as much as three- 

 quarters of /max., or more.f Occasionally one finds samples of 

 materials for which the ratio of the greatest remanence to /max. lies 

 close to some simple fraction — in nickel, for instance, to 1/2. The 

 greatest ratio of remanence to previously-attained magnetization, 

 however, is obtained by choosing //o somewhere in the second segment 

 of the pristine curve. In fact, there may be a long range of the second 

 segment over which the difference (/i — I2) between the ordinates of 

 the initial curve for any values //i and II2 of the magnetizing field 

 is practically equal to the difference between the values of the rema- 

 nence in the hysteresis-loops for which //a = //i and //o = H2 respec- 

 tively. In other words: along the second segment of the curve, 

 whatever added magnetization is given to the metal by increasing 



* This is not a universal rule; samples of electrolytic iron studies by E. Gumlich 

 and W. Steinhaus (£. T. Z., 36, pp. 675-677, 691-694; 1915) which had been annealed 

 at constant temperatures and cooled at various rates displayed intercepts on the 7- 

 axis which were much lower when the cooling had been rapid than when it had been 

 slow; but the intercepts on the //-axis remained nearly unchanged. 



t Ewing records an instance of remanence 0.96 as great as prior magnetization 

 (hardened nickel under strong compression). 



