Certain Chrome and Tungsten Steels. 739 



magnetism is held. The hysteresis curve o£ a specimen then, 

 in indicating the values both of the residual magnetism and 

 the coercive force, gives us a considerable amount of in- 

 formation regarding its permanent magnetism. 



Professor Silvanus P. Thompson, in a paper * read before 

 the Institution of Electrical Engineers, has collected and 

 described the results obtained by various workers on the 

 subject. He shows first of all that, while in the preparation 

 of a satisfactory magnet the material of which it is to be 

 composed is naturally of primary importance, the shape and 

 size must also be considered, and in addition the heat treat- 

 ment to which it is subjected before magnetization, a steel 

 hardened by rapid quenching from a high temperature having 

 in general a much greater coercive force than a specimen of 

 the same steel cooled more slowly. 



Even when a suitable steel of suitable dimensions is chosen, 

 however, and carefully hardened before magnetization, it is 

 found that it does not retain all of its residual magnetism 

 permanently, but that mechanical shock, change of tempe- 

 rature, contact with other magnets, exposure to demagnetizing 

 forces, and even mere lapse of time, all tend to diminish its 

 value. It is found, indeed, that while a part of the residual 

 magnetism seems to be really permanent, a part is only 

 apparently so, and this part gradually disappears. Fig. 1 



illustrates the law of decay 

 Fig. 1. of magnetism, the heights of 



the ordinates representing 

 the residual magnetism di- 

 minishing as time goes on. 

 The height OP represents the 

 really permanent part of the 

 magnetism, the height PT 

 representing that part which 

 is removable. Now this de- 

 cay of magnetism can be 

 hastened in several ways : by the repeated application of 

 mechanical shock for instance, or by alternate gentle heating 

 and cooling, or by prolonged exposure to a gentle heat, and 

 thus the magnet can be brought rapidly into a permanent 

 state. This process, by which the removal of the non- 

 permanent part of the residual magnetism is hastened, is 

 termed " maturing " the magnet. 



In the investigation of this subject, a number of carbon 

 and alloy steels have been examined by various experimenters. 



* Journ. lust. Elec. Engineers, Feb. 1913, p. 80. 



