2 Mr. R. L. Wills on the Effects of Temperature 



The magnetizing process, according to Weber's theory, 

 consists in turning the molecular magnets so that the mag- 

 netic axis of each is in the same direction as the applied 

 magnetizing force. When the force is weak the molecules 

 will be but slightly deflected ; as the force is increased the 

 molecules are brought more nearly into alignment till, with 

 a very strong force, they are all parallel with the field and 

 the specimen is saturated. 



Prof. Ewing* divides the magnetizing process of magnetic 

 metals into three stages. During the first stage the perme- 

 ability is small and there is practically no retentiveness. In 

 the second stage the curve of magnetization rises rapidly and 

 the permeability is high. In the third stage the permeability 

 decreases and the specimen approaches saturation. 



The effect of heat is regarded as making the transition 

 from one stage of the magnetizing process to another occur 

 at lower values of the magnetic force. Heating expands the 

 structure and weakens the mutual forces between the mole- 

 cular magnets, thus enabling the applied magnetizing force 

 ^o have a greater effect in turning the molecules. Oscilla- 

 tions are supposed to be set up as the temperature rises which 

 qppose the alignment of the magnets, and by supposing the 

 oscillations to become so violent that the magnets eventually 

 rotate, the disappearance of magnetism at the critical tempe- 

 rature would be accounted for. 



The temperature effect appears to have been studied only 

 for magnetizing forces so large that the heat applied has 

 brought on the second stage of the magnetizing process, and 

 before the magnetic quality disappears the third stage has 

 even been entered upon. From this it would appear that by 

 using successively smaller magnetizing forces, so that less of 

 the third stage is brought on before the critical condition is 

 reached, the permeability will rise more rapidly with the 

 temperature, and with a suitable relation of force to tempe- 

 rature the permeability will be the maximum obtainable. 

 With lower magnetizing forces than this critical value the 

 temperature required to bring on the second stage will be 

 higher, and only a part of this stage is brought on before 

 the critical temperature is reached and magnetic quality lost. 

 As the magnetizing force is further reduced the maximum 

 permeability obtained in each case decreases, and in ex- 

 tremely weak fields it appears quite possible that the critical 

 temperature is reached before even the second stage is 

 brought on. 



The experiments described in this paper were undertaken 



* " Contributions to the Molecular Theory of Induced Magnetism. " 

 Proc. Roy. Soc. vol. xlviii. pp. 347-350 (1890). 



