670 CONSTANTS OF MAGNETISATION. 



The factor </>, which has been used previously (1140) to repre- 

 sent the influence of the magnetisation induced by the earth in the 

 oscillations, is then proportional to the difference f-f, and not 

 merely to the magnetisation parallel to the length of the magnet. 



If the bar turns about a line parallel to the edges c, the com- 

 ponents of the induced magnetic moment are uUf\ and wH/'A'. 

 It may be decomposed into two others, one m parallel to the 

 greatest length a, the other m parallel to the field; m and m' satisfy 

 the conditions 



m + #j'A 



j'A/ = 

 which give 



m = 



It will be seen that the magnetic moment m' parallel to the field 

 is a constant, and that the moment m is sensibly constant when 

 the deflections are very small ; the first does not give any couple, 

 and the second may be considered as adding to the rigid mag- 

 netisation M (1140). 



Generally, whatever be the form of an isotropic body placed 

 in a uniform field, there are always three rectangular directions for 

 which the magnetisation is parallel to the field, and may be defined 

 by the mean coefficients /, /', and /". These determine the same 

 ellipsoid of induction 



as if it were the case of an anisotropic body defined by the co- 

 efficients of magnetisation f, f, and /". 



1231. MARINER'S COMPASS. The term compass is usually ap- 

 plied to the magnetic apparatus used in navigation. It consists 

 of a needle or system of needles rotating on a point; the needle 

 supports a card, on which is a line of sight corresponding to the 

 magnetic axis and a series of circular divisions. The line of sight 

 does not usually coincide with the magnetic meridian ; the iron and 

 steel of which the ship is constructed, or which forms part of its 

 equipment, produce a deviation which it is necessary to correct or 

 to compensate. The correction of the compass is based on a theory 

 which was propounded by Poisson.* 



* POISSON. Mem. de rinstitiit., Vol. v., p. 521. 1824. 



