740 Dr. Margaret Moir on Permanent Magnetism of 



Many steels with high values of the residual magnetism have 

 been found to have a very small coercive force, and therefore 

 lose their magnetism readily, while others again have a large 

 coercive force, but only a comparatively small amount of 

 residual magnetism. Neither of these then makes the best 

 kind of permanent magnet, and the ideal at present aimed at 

 is the production of an alloy steel which shall have, after 

 suitable treatment, a coercive force of 80 c.G.s. units and a 

 residual magnetism of 800 c.G.s. units. 



Though many steels have been examined then, there is 

 still need for further investigation of the permanency of the 

 magnetism of various series of alloy steels. I therefore 

 decided to undertake the examination of some of the series, 

 and commenced my work with a graded series of chrome 

 steels supplied by Sir W. G. Armstrong, Whitworth & Co., 

 Ltd., and containing respectively 1, 4*05, 8, 12, 16, and 20 

 per cent, chromium. 



Hysteresis curves obtained in the annealed and quenched 

 conditions showed the coercive force in the former case to be 

 so small as to make it not worth while investigating the 

 permanent magnetism, and accordingly attention was con- 

 centrated on the quenched specimens, which, though they do 

 not fulfil the desired ideal, yet retain sufficient permanent 

 magnetism to make useful magnets. 



The method of procedure adopted in magnetizing the 

 specimens before testing the permanence of the magnetism 

 was as follows : — Each specimen after being quenched from 

 900° 0. in water at room temperature was placed inside 

 the magnetizing helix of a Gray-Ross magnetometer *, and 

 after having been demagnetized by reversals to remove the 

 sensitiveness to magnetism induced by quenching, was sub- 

 mitted to a magnetic test from which its hysteresis curve could 

 be obtained. The maximum field, which was of sufficient 

 strength to saturate the specimen, was then applied once 

 more, and the field gradually reduced to zero (a gradual 

 diminution having been found to give more satisfactory 

 results than a sudden removal oii the magnetizing force), and 

 the residual magnetism noted. The value of the residual 

 magnetism obtained in this way is much less than that ob- 

 tained from the hysteresis curve, owing to the demagnetizing 

 influence of the ends of the magnet — an influence for which 

 corrections were made in obtaining the absolute values of I 

 and H belonging to the hysteresis curve. This influence 

 depends for its value largely on the dimension ratio of the 



* Proc. Roy. Soc. Edin. xxix. p. 182 (1909). 



