THE DIELECTRIC 103 



according to which the dielectric constant should be equal to the 

 square of the refractive index, led to a renewal of interest in the 

 matter, and this was no doubt increased on the practical side by 

 the necessity of knowing something about the electric capacity of 

 the insulating material of telegraph cables. Since then a great 

 amount of work has been done, and the specific inductive capacities 

 of a great number of solid, liquid, and gaseous dielectrics have 

 been determined in a great variety of ways. The phenomena 

 termed residual charge and discharge, which so much complicate 

 the measurements, have also been investigated. 



Before proceeding to an account of the work we shall examine 

 the effect of the existence of specific inductive capacity on the 

 various electrical relations, assuming that for a given material' it 

 has a constant value. 



The effect of specific inductive capacity on the 

 relations between electrical quantities. Let us suppose 

 that we have two condensers (Fig. 71), A with air as the dielectric 



Air 



FIG. 71. 



and K = 1, H with a substance as dielectric which has constant K, 

 and let the two be charged to the same surface density &. 



Electric strain. Since the electric strain is measured by the 

 quantity induced per unit area on a conductor bounding the 

 im-dimn. and this quantity is & in both A and B, the strain D is 

 iint- in cadi. 



Difference of potential. To produce the same difference of 

 potential we should require Ko- in B. Then <r in B will only 



produce -^ of the potential difference that the same charge 



produces in A. 



Electric intensity. If we imagine it possible to move unit 



charge through the medium B wu shall require an intensity^ of 



that in A to produce the observed difference of potential. Or, 



-inc this i> usually an imaginary and impracticable way of regard- 

 ing intensity in B, \\v must now define it as the slope of potential, 



and again, of course, we get it as r of the value in air. If then 

 in A we put E = 4-7nr= 4-TrD, in B we must put 



, _ 



-K : 



Energy of charge. If Q is the charge in each, and if V is the 



V 

 potential in A, and V = r? is that in B, the energy of charge will 



