MAGNETIC MOMENTS. 617 



CHAPTER II. 

 CONSTANT OF MAGNETISATION. 



1189. MAGNETIC MOMENTS. OSCILLATIONS AND DEFLECTIONS. 

 The oscillations of a bar magnet (1140) have already been utilised 

 for estimating the directive couple MH in a uniform magnetic field 

 of intensity H ; the moment M will be determined in absolute value 

 if we know the intensity of the field. 



If we merely wish to find the ratio of the two magnetic moments 

 M and M', two successive experiments in the same field give 



M 2 K 



when n and ri are the numbers of oscillations in the same time, 

 K and K' the corresponding moments of inertia. 



If the bars have a geometrical shape, so that their radii of 

 gyration may be calculated, and if the weight of the apparatus of 

 suspension may be neglected, then, if P and P' are the weights 

 of the bars, p and p their radii of gyration, 



M n 2 P 2 



The moment of inertia of the system may be eliminated by 

 taking care that it is the same in both cases. The two magnets 

 are suspended, for instance, to the same oscillating system, so that 

 they are parallel, first in one direction and then in the other. The 

 ratio of the squares of the number of oscillations gives the ratio of 

 the sum of their moments M + M' to their difference M - M'; from 

 this is deduced the ratio of the two moments M and M'. 



We only obtain in this way, in strictness, the ratio of the apparent 

 magnetic moments. In the latter arrangement, in particular, the 

 magnets should be placed on the mounting at such a distance that 

 their reciprocal action may be neglected. 



