462 



According to Wëiss's ^) theory for the ferromagnetic state : 



H=H, + H^, (10) 



where //,. is the external magnetic tield and 



//,, = A„ /„ (11) 



the molecuhar field, /m = the magnetization per nnit of volume 

 :=QMm, if p = the density. Weiss supposes the coefficient of the 

 molecular tield Nm to be constant. 



The spontaneous magnetization ö, which belongs to He = 0, is 

 obtained by Weiss's construction in a <7, ^^-diagram by determining 

 the point of intersection of the curve 



I Ch a 1 ) 

 <i=n,, (12) 



I 8h a a ] 



with the straight line 



'y=T^<' (13) 



which corresponds to the chosen value of 7\ 

 If 



'*.o> ^ ' (14) 



none of the straight lines (13) has another point of intersection 

 with the curve (12) than the point a = 0. Hence in the assumption 

 of a zero-point energy for the molecular rotatory motion /(^r/w/ia^/^ie/ic 

 substances, characterised according to Weiss by their possessing a 

 molecular Held, loitkout the specific ferromagnetic properties: spon- 

 taneous magnetization, hysteresis, remanent magnetism, are possible. 

 We come back to this in § 6 (Suppl. N". 32^). 

 If on the contrary 



to each temperature T <:^ Tc, the temperature of the CuRiE-point, 

 which is determined by 



«re ^ (lO) 



(m^-c = the rotation energy at the Curie point), a spontaneous 

 magnetization belongs, which we may consider to be determined 

 by (12) and 



1 Ur 



a = nii.- — (17) 



6 Urc 



1) p. Weiss. J. de phys. (4) 6 (1906), p. 661. Physik. ZS. 9 (1908), p. 358. 

 Les iilées modemes sur la Constitution de la matière (Conférences Paris 1912), p. 332. 



