298 



On the Electrostatic Capacity of Glass. [June 1 % i 



III. " Electrostatic Capacity of Glass." By J. Hopkinson, D.Sc, 

 AI.A. Communicated by Prof. Sir William Thomson, F.R.S. 

 Received May 17, 1877. 



^Abstract.) 



The instruments used in the experiments were — a guard-ring condenser 

 the distance between the plates of which could "be varied and measured : 

 a sliding condenser which could be adjusted so that its capacity was 

 equal to the former ; the quadrant electrometer used as an electroscope ; 

 a battery of 72 Darnell's elements. 



The method of experiment was to connect the middle of the battery 

 with the lower plate of the guarding condenser and with the case of the 

 electrometer, to charge one condenser from one pole of the battery, the 

 other from the opposite pole, to insulate both, add their charges, and then 

 connect them to the quadrant of the electrometer. The condensers are 

 equal when there is no deflection of the image. Two experiments are 

 needed in each case — 1st, the glass plate is introduced into the guard-ring 

 condenser, and the sliding condenser is made equal to it ; 2nd, the glass 

 plate is removed, and the guard-ring condenser is made equal to the shding 

 condenser. The two readings of the screw which measures the distance 

 of the plates of the former give the specific inductive capacity of the 

 glass. 



Four samples of flint glass were successfully examined with the 

 followiDg results : — 





Density. 



Electrostatic 

 capacity. 



Eatio. 



Index of 

 Ee fraction 

 for line D. 



Light flint 



3-2 



6-85 



2-14 



1-574 



Double extra dense flint 



4*5 



10-1 



2-25 



1-710 



Dense flint 



3-66 



74 



2-02 



1-622 



Terv light flint 



, 2-57 



6-57 



2*29 



1-541 



Two samples of the first of these glasses were examined, taken from 

 different meltings, and made at different times ; they gave the same 

 result. It will be remarked that these determinations do not bear out 

 Prof. Maxwell's conclusion that the square of the refractive index for 

 long waves is equal to the product of the electrostatic capacity and the 

 magnetic permeability. The quotient electrostatic capacity -f- density for 

 these glasses and also for solid paraffin lies between 2-0 and 2-3. 



