Physics. — "The Magneto- Thermic E^^ect according to Thermo- 

 dynamics'. (Supplement N". 47 to the Commiinicalioiis from 

 the Physical Laboratory at Leiden). By Prof. J. P. Kuknen f . 



(Communicated at the meeting of December 30, 1922). 



In experiments with ferro-magnetic substances Wkiss and Piccard ') 

 found that the heat-effect which accompanies a magnetic change, 

 assumes a relatively large value in the neighbouihood of the Curie- 

 point. According to them this phenomenon, just as the discontinuity 

 in the specific heat at the Curie-point'), is a consequence of the 

 "molecular field", which plays a prominent role in Weiss' theory 

 of ferro- magnetism. 



It is natural to apply equations to this phenomenon which ensue 

 from the second law of the theory of heat. The question suggests itself 

 whether this is allowed, as non-reversible changes occur in feii'O- 

 magnetism. Every condition — leaving disturl)ances out of account 



— is indeed a condition of stable equilibrium, but in general the 

 substance cannot pass through a definite series in both directions. 

 This difficulty may be obviated by considering only those condi- 

 tions that arise under the influence of strong mechanic or electric 

 vibrations: these neutralize hysteresis, and with it also remanent 

 magnetism, and the conditions then become reversible. The results 

 obtained by the aid of thermodynamics, will in main lines most 

 likely also hold for the phenomena occurring under normal circum- 

 stances: above the Curie-point they are, of course, strictly valid. 



The external work of a magnetized system being represented by 



— H d Ö, where H and o denote resp. the magnetic force and the 

 magnetisation, the chief equation of thermodynamics is: 



ds = T dri -\- H do (1) 



As it is most convenient if H is an independent variable, we 

 write: 



d{^ — Hö)—Tdi] — ödH (2) 



from which follows: 



1) P. Weiss et A. Piccard, J. de Phys. (5) 7, p. 103, 1917. 



2) P. Weiss, A. Piccard et A. Cabrera, Arch, de Geneve 1917 ; J. de Phys. 

 (5) 7, p. 87, 1917. 



