256 



MAGNETISM 



system represented by 27ric 2 H 2 2 cos 2 2 is negligible, since it is 

 proportional to /c 2 . Any field into which the body is introduced 

 is sensibly the same after the introduction, so that in the calculation 



of the volume force the value of i maybe taken as being the same 



as before its introduction. We shall see how this is used in making 

 measurements of AC in Chapter XXII. 



Illustrations. As an illustration of the foregoing let us 

 suppose that a long bar of material with very small susceptibility 

 K is hanging with its lower end in the horizontal field H between 

 the poles of a magnet, Fig. 199. Let the upper end O be so far 



H J H 

 FIG. 199. 



away that the intensity of the field there may be neglected in 

 comparison with H. 



Let us measure x downwards from O. Let a be the CK>^ 

 section of the bar,./ its length. The force pulling the bar 

 down is 



J 











The surface force Sira* per area 1 may be neglected since K is small. 

 As an illustration of a case in which this surface force is not 



FIG. 200. 



negligible, let us suppose that two iron bars of constant permeability 

 are magnetised by coils near their neighbouring ends and placed as 

 in Fig. 200, with unlike poles near each other. Let the bars be so 



