Magnetic Properties of Iron and Nichel. 369 



•equation assumes a new interest. When I is very small 



the term j- may be neglected and the equation then 

 becomes ° 



? = R'T or KT=^ 



K being the susceptibility, and this is at once recognized 

 as Curie's para-magnetic equation in which ^ is the 

 equivalent of his constant A. Comparing ^p, the constant 

 in the ferro-magnetic state, with A, Ave have : — 



R'. 1/R'. A. 



Iron 4-69X10 -6 -213X10 6 -281* 



Nickel 27-9 XlO -6 -036xl0 6 -048 t 



JRatio of iron to nickel 5*9 5"9 



* Mean value of A from experiments by Curie, GSuvres, p. 327, and 



Weiss & Foex, Arch, des Sc. 4^ ser. t. xxxi. pp. 4, 89 (1911). 



t Weiss & Bloeh, Arch, des Sc. t. xxxiii. p. 293 (1912). (Intensity 



_, , , , magnetic moment , 



calculated as ^ .) 



volume J 



The absolute values of the constants are connected by a 

 factor about 1*33 x 10~ 6 ; their ratios, however, are almost 

 exactly the same. This perfect agreement is partly accidental, 

 as it cannot be assumed that the iron and nickel were pre- 

 cisely the same in these experiments as in those with which 

 they are compared, and that experimental errors play no 

 part ; nevertheless there is no doubt that the metals behave 

 •correspondingly %. 



(11) If, however, the curves of I=/(T) for constant H 

 are consulted, it is evident that the equation as it stands does 

 not adequately represent them. 



As written above it implies that I varies inversely with 

 the absolute temperature, subject to a limiting value, and 

 the curves should then be convex and not concave, as they 

 are below the critical point, to the temperature axis. This 

 concavity of the curves for large intensities at lower tem- 

 peratures suggests that there is some other field of force in 

 action, in addition to the external applied field, and it is 

 highly probable that this is set up by the magnetization 

 itself of the material and is some function of the intensity. 



\ See Ashworth, Phil. Mag. vol. xxiii. p. 36 (Jan. 1912). 

 Phil Mag. S. 6. Vol. 27. No. 158. Feb. 1911. 2 B 



