628 CONSTANTS OF MAGNETISATION. 



1201. The field of a coil being proportional, by a known factor 

 G, which depends on the form of the circuit, to the intensity of the 

 current, the problem resolves itself into determining the intensity of 

 magnetisation as a function of that of the current. We measure, in 

 that case, either the intensity of magnetisation of the body by de- 

 tachment or by induction, or its magnetic moment by a method of 

 deflection. 



In this latter case, if the body is placed in the field of a coil, the 

 magnetic moment of the current itself should be determined, and 

 deducted from the result obtained for the system of the coil and the 

 magnetised body. It is then better to eliminate the action of the 

 magnetising coil on the declinometer, compensating it by that of 

 another coil placed on the opposite side, at a suitable distance, and 

 which is traversed by the same current. 



In like manner, if we wish to compare bodies of the same 

 dimensions as to their magnetic properties for instance, steel bars 

 of different kinds, submitted to different operations of tempering or 

 of annealing it is advantageous to work by a method of reduction 

 to zero. A typical bar being arranged in a fixed post on one side of a 

 declinometer, the bar to be examined is placed on the opposite side, 

 and its distance modified until the needle is restored to its original 

 position. The ratio of the magnetic moments of two bars is equal, 

 within a correction, to the cube of the inverse ratio of the distances 

 for which they counterbalance the typical bar. 



1202. With ring-shaped coils the magnetising force is not con- 

 stant throughout the entire extent of the section S of the coil, and 

 this is also the case with the coefficient k ; but the variations of this 

 factor may generally be neglected, and we may consider the internal 

 field to have a constant intensity F^I (754), the mean value F m 

 referring to the mean section S' of the ring. In these conditions 

 the intensity of magnetisation is equal to ^F m l, and the correspond- 

 ing flow of induction to 47r/F^S'. 



The total flow of induction across a closed circuit surrounding 

 the ring p times is then 



and, if the wire is close to the ring, 



the determination of Q by an induced discharge will give the value 

 of A. 



