664 CONSTANTS OF MAGNETISATION. 



Experiments do not seem to indicate a maximum for bismuth. 

 The following are the principal results which have been obtained: 



Weber* ............... 16-4. io~ 6 



Topler and Ettingshausenf ... ... ... 15*1. io~ 6 



Christie^ ... ... ... ... ... 14*6. io~ 6 



Ettingshausen ............ 14*5. io~ 6 



The first three numbers were determined by comparison with iron, 

 the last by absolute direct measures by the aid of a method of 

 torsion. 



1226. ANISOTROPIC BODIES. The magnetic properties of 

 substances of fibrous or crystalline structure were discovered by 

 Pliicker. || The general case of a non-homogeneous body situated 

 in a field which is not uniform gives rise to very complicated 

 phenomena, because each element of volume is attracted towards 

 maxima or minima regions of force, according as it is magnetic or 

 diamagnetic, at the same time that the axis of greatest magnetisa- 

 tion tends to set in a direction parallel or perpendicular to the 

 field (397). The action reduces to a couple when the field is 

 uniform. 



Let us consider a homogeneous body of volume u placed in 

 a uniform field <, the direction of which makes angles with the 

 principal axes of magnetisation whose cosines are A, A', and A'' 

 (392). If the principal coefficients k, k', and k" are very small, 

 the resultant magnetisation is independent of the shape of the 

 body. The magnetic moments m t ;;/', and m", parallel to the axis, 

 are respectively 



m = u , f = u<>, m = u<>, 



and the couples relative to the three axes, 



D = m'<j>\" - m"4>\' = u<P (k' - k") A' A", 

 D' = m"^\ - mW = u<^(k" - k) A"A, 

 D" = m<j>\' - m'<f>X = u<p (k - k') A A'. 



* W. WEBER. Electrodynam. Maasbestim. Diamagnetismus, p. 523. 

 t TOPLER and ETTINGSHAUSEN, Pogg. Ann., Vol. CLX., p. i. 1877. 

 CHRISTIE. Pogg. Ann., Vol. cxxxni., p. 589. 1858. 

 ETTINGSHAUSEN. Wied. Ann., Vol. xvn., p. 272. 1882. 

 || PLUCKER. Pogg. Ann., Vol. LXXIL, p. 315. 1847. 



