720 Dr. J. R. Ashworth on the Application of 



necessary if R' is to be treated as a constant for both states 

 and equal to the reciprocal of Curie's constant. 



It is generally conceded that an intrinsic or molecular 

 field exists within a magnet, but it is the magnitude of 

 this field required by the theory which presents a 

 difficulty. Prof. P. Weiss has developed a kinetic theory 

 of magnetism which demands molecular fields equally large, 

 but he has not been able to account satisfactorily for their 

 origin*. 



The experiments of Prof. Weiss t which seem to confirm 

 with least objection the large magnitude of the molecular 

 field are those relating to the change of specific heat with 

 rise of temperature which the ferromagnetic metals exhibit. 

 The specific heats of these metals augment as the tempera- 

 ture increases up to the critical temperature, and at this 

 point there is an almost sudden reduction to the normal 

 values. Calculation shows that the energy required to 

 destroy the molecular magnetic field, if it is of the order of 

 10 7 gausses, accounts for the increase of the specific heat. 

 The agreement between calculation and experiment is very 

 close. There is another experiment which may be made to 

 bear on this problem to which I have briefly drawn atten- 

 tion J. When two pieces of the same ferromagnetic metal 

 are dipped in a dilute acid, such as acetic or oxalic acid, and 

 a strong magnetic field is applied to one of them, an electro- 

 motive force is set up between them in a direction such that 

 the ferromagnetic ions tend to travel through the electrolyte 

 from the unmagnetized to the magnetized electrode. The 

 magnitude of this E.M.F. has been determined by Hurmu- 

 zescu § and by Paillot ||, and from their experiments a cal- 

 culation can be made of the electrical energy required to 

 transfer an element of volume of the magnetized material 

 from one electrode to the other, and hence of the magnitude 

 of the intrinsic field from which it was removed. The result 

 of this calculation is to show that the intrinsic field is of the 

 order of 10 7 gausses. Another argument in favour of a 

 large intrinsic field is the fact that the curve of I =/ (T) is 

 not altered in a high degree by the application of a powerful 

 exterior field. This shows that even strong exterior fields 

 are nearly negligible compared to the intrinsic field of a 

 ferromagnetic substance. 



* Weiss, Ann. de Phys. t. i. pp. 134-162. 

 t Weiss and Beck, J. de Phys. 4 ser. t. vii. p. 249 (1908). 

 X Ashworth, Mem. & Proc. Manchester Lit. & Phil. Soc. vol. lviii. 

 part ii. (1914). 



§ J. de Phys. 3 ser. t. iv. p. 118. 

 || C. JR. cxxxi. pp. 1194-5. . 



