744 Dr. Margaret Moir on Permanent Magnetism of 



thermal treatment, it is eventually only a little better 

 than IV. 



It would appear then, that in making a chrome-steel 

 magnet of the dimension-ratio of the specimens examined, 

 the choice would be between that containing 8 and that 

 containing 12 per cent, chromium. If retentivity were the 

 first consideration, then the 12 per cent, specimen would be 

 superior, while if a considerably stronger magnet were 

 desired, and retentivity might be to some extent sacrificed, 

 the specimen containing 8 per cent, would be the more 

 satisfactory. 



It is perhaps of some interest to compare these results 

 with those obtained by W. Brown,* who has also examined 

 the effect of percussion on the permanent magnetism of 

 chrome steels. The series examined by him also consisted 

 of six specimens, none of which, however, contained as much 

 as 10 per cent, chromium. Table II. below shows some of 



Table II. 



Percentage of 



Magnetic moment 



Percentage loss 



Chromium. 



per gm. 



by percussion. 



1-75 



38-2 



1-9 



1-96 



50-4 



0-5 



2-11 



52-f> 



2-2 



3-50 



41-7 



7-9 



5-79 



38-7 



70 



9-22 



42-2 



1-3 



his results, and from it, it will be seen that the most satis- 

 factory percentage of chromium for a permanent magnet 

 seems to be about 2 per cent. The dimension-ratio of the 

 specimens used in this investigation was however 33, as com- 

 pared with about 21 in the case of my work ; and it is well- 

 known that the steel which makes the most satisfactory 

 magnet of one dimension-ratio, is generally not the best, if 

 a magnet of a different dimension-ratio be desired. 



Madame Curie f has also examined some chrome steels in 

 the form of square bars, 20 cm. long and 1 cm. broad, the 

 dimension-ratios of which are nearly equal to those of the 



* Proc. Roy. Dub. Soc. vol. xii. p. 349 (1910). 

 t Bull, de la Societe d' Encouragement, pp. 36-76. 



