468 



Physics. — "On the magnetization of ferromagnetic substances 

 considered in connection itntk the tisswnption of zei^o-point 

 energy. II. On the susceptibility in the excited ferromagnetic 

 state". By Dr. W. H. Kersom. Supplement N". 32è to the Com- 

 munications from the Physical Laboratoiy at Leiden. Com- 

 municated by Prof. H. Kamerlingh Onnes. 



§ 5 ^). The susceptibility of ferromaynetic substances above their 

 CuRiE-/)om/. According to Weiss's theory ferromagnetic substances 

 above their CuRiE-point, before eventually adopting the paramagnetic 

 state, pass into a state (eventually a succession of different states) in 

 which just as below the CuRiE-point the magnetization produces a 

 molecular field. In that case, however, spontaneous magnetization 

 does not occur any more ; hence the molecular field only comes 

 into action when it is excited by the external field. We assume in 

 this section that the condition (15) for the existence of a Curie- 

 point is satisfied. For states, in which saturation is not yet 

 appreciable ^), follows from equation f7), which then simplifies to 



Mm = -^ma, with (8), (10), (11) and (16) for the specific suscep- 



o 



tibility (for unit of mass) : 



'A-=7^r^ — i (2') 



6 (Ui—Urc) 



hence 



y-i ^ {u,—u,,) (28) 



In the first approximation, viz. for equi partition, as well as in the 

 second approximation, viz. when in the development according to 

 positive powers of x of the second member of the first of the 

 equations (5) only the first two terms need to be considered ^'), (28) 

 passes into Weiss's law for the susceptibility above the CuRiE-point: 



y-i _ (7- To) . . ..... (29) 



In fig. (2) is shown how the curve which gives /-^ as a function 

 of temperature, is derived from ih as a function of 7\ According to 



1) The §§, equations and figures of this paper are numbered as continuations 

 of those in Suppl. No. 32a. 



~) Kor these states no account has to be taken of the change of Uv under the 

 influence of the field, which lias been treated in § 3/?. 



S) With an accuracy of 1"/,, in ih- this is the case for T> 1.2 0^;, with one of 



