20 
DR. J. HOPKINSON ON ELECTROSTATIC CAPACITY OF GLASS. 
The guard ring screw reading is denoted by It. It (+) when the guard ring is 
positive, It ( — ) when it is negative. This condenser must be regarded as a circular 
plate of 151 millims. diameter with a uniform distribution of electricity on its under 
surface ; its capacity is therefore - centimetres where x = It — the reading when 
k and e e are in contact. 
3. In order to ascertain the distance between the plates from the screw reading It, 
it is necessary to know the reading when the plates khh and e e are in contact. Slips 
of thin tissue paper are introduced at the top of each of the ebonite legs, the lower 
plate is raised, and a reading is made when each slip becomes loose ; the mean of the 
three readings may be taken as the zero when the instrument is used to measure 
the thickness of plates, or when h k h is carried by an interposed plate, but it will 
require a correction when in the subsequent measurements the upper plate is carried 
by the ebonite supports only, for the upper plate must have been lifted by a greater 
or less amount depending on the compression of the paper slips and on the imperfect 
rigidity of the brass before the slips can be released. The amount of this correction 
was estimated in two different ways. 
1st. Everything on the upper plate was connected with one pole of the battery 
and also with the electrometer. The . plates were brought to contact ; it was found 
the slips became loose at 1T5, 1'13, 1 '0.9, mean 1 ‘12, the lower plate was very slowly 
lowered until the upper plate became insulated, as declared by the movement of the 
image on the electrometer scale. This occurred at 1 '22, indicating a correction 
of 0-10. 
2nd. A plate of light flint glass was introduced between the condenser plates ; 
the slips were just loose at readings — 
16-15 16-16 16-11. Mean 16-14. 
The two condensers were now connected through the switch and rendered equal, 
the screw being turned to vary the distance of the plates, and the slide being 
adjusted to make the sliding condenser equal to the guard ring. The following 
corresponding pairs of readings were obtained : — 
R 
16-10 
16-20 
16-30 
16-25 
16-40 
Si 
180 
180 
150 
170 
100 
R 
16-30 
16-27 
16-24 
16-20 
16-17 
Sj 
150 
170 
180 
180 
185 
It thus appears that the capacity of the guard ring condenser does not begin 
to diminish till It is between 16’24 and 16"27. This indicates a correction between 
0-10 and 0"13. Throughout the experiments a correction of OTO is used whenever 
the upper plate is carried by the ebonite legs alone. 
4. The glasses examined were Chance’s optical light flint, double extra dense flint , 
dense flint, a special light flint, and a piece of common plate glass. 
