402 ELECTROKINETICS. [FT. II. CH. X. 



media, whereas we may determine only the ratio of the inductivity 

 of a medium to that of a standard medium. 



If we make two experiments with the above combination of 

 condensers, one with A alone, which gives T, and a second with A 

 and B which gives 



if we know K' and R we may from these two results determine K, 

 and if the condenser is made in any shape suitable for calculating 

 KQ from geometrical data, we can then determine p. In this 

 manner Cohn found for water JJL 73*6, the largest value of the 

 electric inductivity yet found for any substance. In the case of 

 metals, all that we know is that T is extremely small. This is 

 of course due to the large value of X, so that whether ^ is large or 

 small we have as yet no means of knowing. 



208. Condenser with two Dielectrics. Absorption. In 



the preceding section we have seen that a charge residing in any 

 part of a conducting dielectric will gradually disappear, and that 

 no electricity will accumulate at any part of such a dielectric. 

 We have considered only the discharge or leakage of a condenser, 

 starting from a state of steady flow. We shall now consider the 

 state which precedes the attainment of the steady state when 

 an electromotive force is suddenly applied to produce a difference 

 of potential between the -plates of the condenser. We shall also 

 suppose that the condenser contains two dielectrics of different 

 properties, and for simplicity we shall consider only a plane con- 

 denser. Let the potentials of the two plates be Fi and F 2 , and 

 let that of the plane separating the two dielectrics be F 3 . 



Let the thickness, inductivity and conductivity of the upper 

 dielectric be d 1} fa, \ lt and of the lower d z , /z 2 , V The force in 

 the upper dielectric will be the same at all points, F lt which 

 however depends on the time. In the lower dielectric let the 

 force be F^, also a function of the time. 



Let the currents in the two dielectrics be q l and q z respec- 

 tively, and let F lt F 2 , q 1} <? 2 , be considered positive when measured 

 from V 1 to F 2 . Let the condenser plates, of area S, be connected 

 by a wire of no resistance, into which we can suddenly introduce 

 an electromotive force E, which can be suddenly removed. The 



