Physics. — "Further experiments with liquid helium. T. Magnetic 

 resenrche--<. yi^\\\. On the magnetisation of gadolinium stilphate 

 at temperatures obtainable nnth liquid helium." Bj' H. R. 

 WoLTJKR and H. Kambrungh Onnes. (Communiealion N'. 167c 

 from the Physical Laboratory' at Leiden). 



(Communicated at Itie meeting of September 29, 192.3). 



\ 1. Introduction. Previous') preliminary researches and a detailed 

 discussion') of the results (hen obtained have shown tlie importance 

 of a ('loser investigation of the magnetisation of gadolinium sulphate 

 at very low temperatures; tiiis substance is one of the compara- 

 tively few, tiiat follow Curik's law down to the region of tempera- 

 tures obtainal)le with liquid hydrogen. Now in the light of Langevin's 

 theory the Curik law holds only appi'oximately, viz. as long as the 

 susceptibility may be considered to be independent of the field strength: 

 Langevin gives for the ratio of the specific magnetisalion, o, to the 

 specific saturation magnetisation, o^, 



1 



a : O^^^ cotgh a (la) 



a 



(\b) 



R T 



(öra«, being the saturation magnetisation of one gram molecule, R 

 the gas constant per grm. mol., H the magnetic field applied and 

 T the absolute temperature). 

 For small values of a 



o:ö =-a or ■, = — = -^"^.- .... (2) 

 " 3 ^ H SR r ^ ' 



If 7' is small and thus a large, x '8 no longer independent of ^, 

 but the curve a -. a^ =f(a) deviates from the straight line a -. a^ 



=: - a, becomes concave towards the a-axis and approaches asymp- 

 o 



totically to a : o^ == \ (cf. fig.) The detailed discussion of the preli- 

 minary experiments has already made very problable the existence 



') H. Kameelingh Onnes, these Proceedings 17, p. 283; Leiden Comm. N". 140d. 

 ') H. Kamerlingh Onnes, Rapport Solvay 1921, p. 131; Leiden Gomm. Suppl. 

 N". 44a. 1. 



