MAGNETIC INDUCTION. 



and especially if the magnetic properties of the medium were com- 

 parable with those of soft iron. 



383. MAGNETIC SUSCEPTIBILITY AND PERMEABILITY. The 

 phenomena of magnetic induction may, as we have seen, be 

 expressed with the aid of two coefficients. 



The coefficient of induced magnetisation k expresses the ratio of the 

 intensity of magnetisation to the magnetic force ; in other words, 

 the intensity of magnetisation in a field equal to unity. This 

 coefficient is sometimes known as that of Neumann, who first used 

 it. Sir W. Thomson calls it the coefficient of magnetic susceptibility. 



The second coefficient called /*, is the coefficient of magnetic 

 induction ; it is the analogue of the specific inductive capacity of a 

 dielectric in electricity. It is equal to the ratio of the perpendicular 

 components of the force on the outside and inside of the medium in 

 question, and is connected with the preceding by the ratio 



/A I + 47T& 



Sir W. Thomson calls this coefficient /A, the coefficient of magnetic 

 permeability. The following are his reasons for using this expression 



" The analogue corresponding to conducting power of a solid for 

 heat or, as it is shortly called, 'thermal conductivity' is, in electro- 

 static induction, the 'specific inductive capacity' of the dielectric; in 

 magnetism it is not what has hitherto been called magnetic inductive 

 capacity a quality which is negative in diamagnetics but it is 

 Faraday's ' conducting power for lines of force,' and in hydrokinetics 

 it is flux per unit area, per unit intensity of energy. The common 

 word permeability seems well adapted to express the specific quality 

 in each of the four analogous subjects. Adopting it we have thermal 

 permeability, a synonym for thermal conductivity; permeability for 

 lines of electric force a synonym for the electrostatic inductive 

 capacity of an insulator; magnetic permeability a synonym for 

 conducting power for lines of magnetic force; and hydrokinetic 

 permeability a name for the specific quality of a porous solid 

 according to which, when placed in a moving frictionless liquid, 

 it modifies the flow." (Reprint of Papers, 628.) 



384. ANISOTROPIC BODIES. We have hitherto only considered 

 the induction produced in isotropic bodies. The experiments of 

 Pliicker on bodies with a fibrous texture, and on crystals, have shown 

 that magnetic action is exerted unequally in different directions. 

 Poisson had predicted the existence of such bodies, and in order to 

 explain them on his theory we must substitute for the conducting 



