MAGNETIC INDUCTION. 



CHAPTER IV. 

 MAGNETIC INDUCTION. 



374. GENERAL CHARACTERISTICS OF MAGNETIC INDUCTION. 

 There is probably no substance which, when placed in a magnetic 

 field, does not experience the effect of induction that is to say, does 

 not itself become a magnet, at any rate temporarily. 



When the body is isotropic, the axis of induced magnetisation 

 coincides everywhere with the direction of the magnetic force. In 

 certain bodies the induced magnetisation is in the same direction 

 as the force ; these are the bodies which we have called paramagnetic 

 or simply magnetic. In others the direction of the magnetisation is 

 opposite that of the force; these bodies are diamagnetic. In the 

 presence of a pole of a magnet, the nearest part of bodies of the 

 first class acquires polarity of the opposite kind ; bodies of the 

 second class acquire a pole of the same kind. 



We shall assume that at every point of an isotropic body sub- 

 mitted to magnetic induction, the magnetisation is proportional to 

 the resultant of all the magnetic forces which are exerted at this 

 point. These forces depend not only on the original field, but also 

 on the magnetism developed by induction on the body itself. If F 

 is the resultant force, to which the name magnetising force is some- 

 times given, I the intensity of magnetisation, we may write 



(i) I = F. 



The factor k, which expresses the ratio of the magnetisation to 

 the magnetising force, is called the coefficient of induced magnetisation ; 

 this coefficient is positive or negative, according as the body is mag- 

 netic (in the ordinary sense of the word) or is diamagnetic. 



The hypothesis of the proportionality of the magnetisation to the 

 magnetising force is verified with close approximation whenever k 



