CHAPTER XIV 



GENERAL, ACCOUNT OF MAGNETIC 

 ACTIONS continued 



Relation between magnetisation and the magnetising force producing 

 it The hysteresis loop Susceptibility and permeability Magnetisa- 

 tion and temperature Permeability and temperature Change of length 

 on magnetisation Magnetisation and strain. 



IN this chapter we shall give some account of certain less obvious 

 phenomena of magnetism, which for their complete discussion 

 require a knowledge of the methods of magnetic measurement. 

 But without quantitative details a general idea of the phenomena 

 may be obtained, and this general idea will be valuable when we 

 come to the more exact investigations of magnetic quantities in 

 later chapters. 



Relation between magnetisation and the mag- 

 netising force producing it. Without entering into detail 

 we shall now give a short account of the relation between the 



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^ /Dpooooooooooooooooas.o< 

 FIG. 128. 



magnetisation in iron and the magnetic force by which the 

 magnetisation is produced, and we shall suppose that we use the 

 electro-magnetic coil method of producing it. We have already 

 seen that a coil carrying a current is itself a magnet. If Fig. 128 

 represents the section of such a coil by a plane through its axis, 

 the course of the lines of force may be investigated by inserting a 

 card with iron filings on it, and they are somewhat as represented 

 in the figure. The lines of force are practically parallel within 

 the coil except near the ends. They diverge as they pass out from 

 one end and converge as they come in at the other. If we turn a 

 screw round so as to travel along the lines of force inside the coil 



181 



