568 MAGNETIC FIELD. 



PART III. 

 MAGNETIC MEASUREMENTS. 



CHAPTER I. 

 MAGNETIC FIELD. 



1139. The methods used to determine the field of a current are 

 also applicable for any given magnetic field. But the measurement 

 of magnetic fields, and particularly that of the terrestrial field, which 

 comes so often into electrical measurements, is of such importance 

 that it is necessary to resume the question in some detail more 

 particularly from this point of view. 



1140. OSCILLATIONS OF A MAGNETIC NEEDLE. The most 

 ancient method is that of oscillations. If K is the moment of 

 inertia of a magnetic needle movable about an axis, M that com- 

 ponent of its magnetic moment which is normal to its axis, N the 

 number of oscillations, reduced to infinitely small angles, which it 

 makes in unit time under the action of a magnetic field which is 

 sensibly uniform in the space occupied by the needle, and the 

 component perpendicular to the axis of which is H, we have 



(i) 7r 2 KN 2 = MH. 



This equation furnishes the product HM of the field by the 

 magnetic moment of the needle, or the directing couple, if we 

 knew the moment of inertia of the needle. 



If the observation is made successively in two different fields 

 H and H', their ratio is equal to the square of the ratio of the 

 numbers of oscillations which the same needle would make under 

 their influence in the same time. 



Nevertheless the magnetic state of the needle is modified by the 

 field itself, but the induced magnetisation is not strictly parallel to 



