DR. J. HOPKINSON ON ELECTROSTATIC CAPACITY OF GLASS. 
21 
Light flint, density about 3 ’2. 
Two plates were examined of different thickness, the plates were also from different 
meltings of glass made at different times, and may be regarded as two quite 
independent samples of glasses intended to be of the same composition. 
A, Thickness, 15 ‘01 turns of the screw; diameter, 220 millims. 
First Experiment.- - Plates of guard ring condenser in contact with glass plate. 
48 elements in the battery. 
S 2 =0 Si=50 when sliding condenser positive. 
= 20 „ ,, negative. 
Mean=35 
It is found that S 2 =0, Sj = 35 is equal to the guard ring condenser with the glass 
plate out, when the distance between the plates is 2 ‘18 turns of screw, 
Hence K=6'89. 
Second Experiment.' — Battery of 72 elements. 
S 2 drawn out beyond the graduation, S x =160 when slide is positive. 
= 220 „ negative. 
Mean=190 
Glass plate removed. 
S 1= =190 R (+)=3-50 R ( — )=3;43. 
Mean reading for contact of plates IT 4, when corrected l - 24. 
So plate of glass 15'0] is equal to plate of air 2’225, 
Hence K=676. 
B. Thickness, 1075 turns of screw ; diameter, 220 millim s. 
Plates of guard ring both in contact with glass, battery of 72 elements. 
S 2 =25 S x = 450 when positive. 
= 400 when negative. 
Glass plate removed. 
S x =425 equivalent to R (+) = 2’85 
R (— ) = 2*80 
or plate of glass 1075 equal to air 1-585. 
K=6-90. 
Mean of three determinations — 
6-85. 
“ Double extra dense flint glass,” or “ Triple dense flint,” density about 4-5. 
Thickness of plate, 24'27 turns ; diameter, 235 millims. 
