126 STATIC ELECTRICITY 



meter the condenser with that kind has the greater capacity. 

 C must be adjusted till the electrometer shows no charge and t In- 

 equality is obtained. Now let a layer of dielectric be interposed in (', 

 of thickness t. If the distance of the plates apart is </, the capacity 



rr I 



is increased in the ratio d t =-? : d. 



Jv 



To obtain equality with S the distance d must be increased by 

 t ^ = <J. This distance S can be measured exactly and so K can 



be found. 



In the apparatus actually used C was a "guard-ring" con- 

 denser of which the inner plate, sharing its charge with S, was 

 15 cm. in diameter. It was surrounded with a guard-ring con- 

 nected to the battery when charging, so as to^be at the saint- 

 potential as the disc. The- lines of force thus went straight across 

 from plate to plate, and the edge effect which would depend on 

 distance and dielectric was eliminated. Before the disc was con- 

 nected to S the guard-ring was earthed, and remained earthed 

 during the connection to S. This method was used for plates of 

 flint glass, for which the following values were found : 



for D line 



1 :> 1 1 



am 



1-710 



It is evident that /x 2 is very much less than K. 



Hopkinson also showed that the result was the same whether 

 C and S were connected to each other, and to the electrometer, for 

 a time comparable with a second or for a minute fraction of a 

 second. Residual phenomena, then, did not come appreciably into 

 play. With plate glass, however, it was necessary to have only 

 an instantaneous connection, for with long connection the method 

 failed owing to residual charge. 



To show that K is really constant, i.e. independent of the 

 potential difference, for a given specimen of glass, a flask of extra 

 dense flint glass containing sulphuric acid was placed in 

 water, the contacts of the liquids with the glass forming the 

 coatings. The capacity of this flask was found to be the same. 

 comparing it with the sliding condenser S, when charged \\ith 

 20 elements as when charged with 1800 elements of a chloride of 

 silver battery. 



To find the dielectric constant of liquids a fluid condenser 



