2 Dr. J. Gr. Gray on the Magnetic Properties of a 



at first, but more and more rapidly as the critical temperature 

 is approached. On the other hand, the freedom with which 

 the molecules have their axes directed increases with rise of 

 temperature. For low values of the magnetizing force the 

 increased freedom of the molecules brought about by increase 

 in the temperature more than compensates for the diminution 

 in the magnetic moment of the molecular magnets, with the 

 result that the susceptibility of the test-piece is increased. 

 For high values of the magnetizing force the reverse is the 

 case. 



From this point of view we should expect to find that the 

 effect of immersing a test- specimen of iron or steel in liquid 

 air, and thus bringing it to a temperature of —190° C, 

 would be to diminish its susceptibility for low fields and 

 increase it for high fields. That this is the case for annealed 

 iron, nickel, and cobalt, has been proved by Honda and 

 Shimizu "*, who examined specimens of these metals (1) at 

 room-temperature, and (2) when immersed in liquid air. 

 The improvement in magnetic quality brought about by the 

 low temperature for high fields is very slight, and the value 

 of the magnetizing force for which the magnetization curves 

 cross is very great, in the case of these materials. 



In 1890 Hopkinson f described the very remarkable 

 magnetic properties of the material resulting from alloying 

 iron with nickel. Although both these metals belong to the 

 magnetic group, an alloy of the two, containing 25 per cent, 

 nickel, is quite non-magnetic at ordinary room-temperature 

 when in the forged or annealed conditions. If cooled to a 

 temperature considerably below 0° C, however, it becomes 

 magnetic, and remains magnetic on warming to room- 

 temperature. The critical temperature of a specimen after 

 being cooled was found by Hopkinson to be about 600° 0. ; 

 that is, the specimen retained a considerable amount of 

 magnetic susceptibility for temperatures lying between room- 

 temperature and 600° C. On being heated to this latter 

 temperature the test-piece became non-magnetic, ami re- 

 mained so on cooling to that of the room. On cooling the 

 specimen once more the magnetic condition was restored. 



Specimens of nickel-steel containing varying amounts of 

 nickel and iron were examined by Hopkinson, ami yielded 

 results of great interest. In general the specimens were 

 found to possess a considerable amount vi' magnetic quality 

 at room-temperature in the annealed condition. The alloy 

 containing 25 per cent, nickel was tic only member of the 



* Phil. Mil- x. p. 548(1906). 



t Pro* Key. Soc, Dec. 12, 1889; May 1, 1890. 



