18 
DR. J. HOPKINSON ON ELECTROSTATIC CAPACITY OP GLASS. 
fig. (2) in plan through h h. It consists essentially of an insulated brass disc k 
surrounded by a flat ring h In, and covered by a brass shield connected with h h. 
It is opposed by a larger disc e e parallel with k and h h, which is always connected 
to the case of the electrometer. The disc k and ring h In are connected, simultaneously 
charged, next separated, and then at one moment h is put to earth, and k discharged 
in such manner as the experiment may require. 
a b and c d are triangular pieces of iron forming with three wrought-iron stays a 
stiff frame. To the tops of these stays are screwed three legs of ebonite g g, which 
serve to support and insulate the guard ring h h. The disc e e is of brass truly 
turned, it is carried on a stem which is screwed for a portion of its length with 
exactly 25 threads to the inch, a motion parallel to itself is secured by bearings in 
each frame plate ; these are not ordinary round bearings which may work loose, 
but are of the form represented full size in fig. 3. e e is prevented from rotating 
by a pin working through a hole in the upper triangular plate and pressed against 
one side of the hole by a steel spring. The plate e e is raised or lowered by a 
milled nut f divided on the circumference into 100 parts, and bearing upon a piece 
of brass tubing secured to the lower plate of the frame, k is carried by two rods 
of ebonite l l, which insulate it from h h; both were faced in the lathe together 
so as to be truly in one plane. The diameter of the disc k is 150 millims., it is 
separated from the ring by a space of 1 millim. When the capacity of a glass plate 
is to be measured a dish of pumice and sulphuric acid is placed upon the disc k between 
the rods l l, and a second dish upon the triangular plate c d, the whole instrument 
being loosely surrounded by a glass cylinder. This instrument also serves to measure 
with sufficient accuracy the thickness of the glass plates. To ascertain when the 
plates are in contact, or when the glass plate to be measured is in contact with h k h, 
slips of tissue paper are interposed between the ebonite legs g g and the plate h h, 
and the contact is judged by these slips becoming loose, a reading being taken for 
each slip. 
The sliding condenser was the identical instrument used by Gibson and Barclay, 
kindly lent to the author by Sir W. Thomson ; it was used simply as a variable 
condenser. Although a more finely graduated instrument than the guard ring 
condenser, it was not used as a measuring instrument, because its zero readings 
had to be valued by the guard ring condenser ; it seemed better to use it like the 
counterpoise in the system of double weighing, adjusting it to the guard ring 
condenser with the glass in, then removing the glass and adjusting the guard ring 
condenser to equality with the sliding condenser. It suffices to say that the sliding 
condenser has two adjustments, a fine one denoted here by S l5 and a coarse one 
denoted by S 2 . 
The electroscope was Sir W. Thomson’s quadrant electrometer adjusted for maxi- 
mum sensibility and charged as highly as it would stand. A single Daniell’s 
element gave from 120 to 160 divisions of the scale. 
