2 Dr. J. G. Gray and Mr. A. D. Ross 



magnetism and in a field of zero intensity, but with vastly 

 different magnetic properties for the two ways in which it 

 may be magnetized in the direction of its length. If we 

 denote the ends of the specimen by A and B. then the per- 

 meability of the material in the direction AB differs from 

 that in the direction BA for magnetizing forces which are 

 less than the maximum force employed in the operations. In 

 other words, the chains of molecular magnets, in the closedness 

 of which the external neutrality of the bar consists, are left 

 by the process referred to in an arrangement which is 

 unequally affected by a magnetic field according as it is 

 applied from A to B or from B to A. This difference of 

 response to magnetic force in the two directions is due to the 

 magnetic treatment which the specimen has sustained, and is 

 not removed by annealing unless the temperature is raised 

 above the so-called critical temperature of the material. For 

 this reason it is usual to divest a specimen of its previous 

 magnetic history before submitting it to test. This is accom- 

 plished by exposing the specimen to the action of an alternating 

 magnetic field which diminishes gradually from a high value 

 to zjro. 



But it is not generally known that purely thermal treat- 

 ment, such as that involved in the process of annealing a 

 specimen, no matter what temperature is reached in the 

 process, develops in the specimen a peculiar state which 

 renders additional precautions necessary. This fact has been 

 disclosed in experiments carried out by the authors, which 

 are described in papers * read before the Royal Society of 

 Edinburgh. It is there shown that an alteration of the tem- 

 perature produces what we have called a " Sensitive State " 

 of the material. A magnetization curve yielded by the 

 specimen following upon any change, or cycle of changes, in 

 the temperature, is not what we may here call the true 

 magnetization curve of the material. This will be clear from 

 fig. 1, which shows the results obtained on testing, at room 

 temperature by the magnetometric method, a specimen of 

 hard steel which had been heated to 900° C. and allowed to 

 cool slowly. Previous to being heated the specimen had 

 been rendered neutral, and during the process of heating and 

 cooling had been exposed to no magnetizing forces. On 

 submitting the specimen, in the condition brought about by 

 the thermal treatment described, to the action of a mag- 

 netizing field which increased gradually from to + 9 c G.s. 



* J. G. Gray and A. D. Ross, " On a " Sensitive State induced in 

 Magnetic Materials by Thermal Treatment," Proc.Rov. Soc. Edin. xxviii. 

 pp. 239. & 615(1908). 



