180, 181] DIELECTRICS AND MAGNETIZABLE BODIES. 357 



the vector has, by (8), the property of being solenoidal in all parts 

 of space where there are no charges. That is 



The force, or field-strength F, no longer enjoys this property in 

 general, but does so in a homogeneous medium, for which //- comes 

 out as a constant factor. The vector is called the induction, and 

 is connected with the force by the equations 



The induction accordingly satisfies everywhere the law of 

 Fourier-Ohm. 



The surface integral over any surface of the induction resolved 

 normally to the surface is called the total induction, or induction- 

 flux, through the surface. The quantity //, is called the inductivity 

 of the medium. A more usual name for it is the specific inductive 

 capacity or dielectric constant, in the electric case, magnetic per- 

 meability in the magnetic case. The latter name is due to Lord 

 Kelvin, to whom the recognition of the analogy to the case of 

 flow in electricity and heat is due.* The name permeability comes 

 from the hydrokinetical analogy of water flowing through a porous 

 medium. 



The lines of induction suffer refraction in the manner described 

 in 163 when passing from one medium to another. In the 



FIG. 71 a. 



* Magnetic Permeability, and Analogues in Electro-static Induction, Conduction 

 of Heat, and Fluid Motion. Papers on Electrostatics and Magnetism, p. 487. 



