DR. J. HOPKINSON ON ELECTROSTATIC CAPACITY OF GLASS. 
19 
The battery consisted of 48 or of 7 2 Daniell’s elements of a very simple 
construction ; a piece of copper wire covered with gutta percha is stripped for a 
short distance at each end, it is set in a test tube 6 or 7 inches long, a piece of zinc 
being soldered to its upper extremity. Some sulphate of copper in powder is put in 
the tube around the exposed wire, this is covered by a thick plug of plaster of Paris, 
and the element completed by the addition of dilute zinc sulphate solution, into 
which the zinc which is soldered to the wire of the next element dips. The element 
has a very high resistance, but that is of no consequence for electrostatic experiments. 
The middle of the series is put to earth. The battery thus gives the means of 
charging two condensers to equal bat opposite potentials. The poles of the battery 
are connected with the switch through the electrometer reversing key. In each 
case two experiments are made, one in which the guard ring is positive, in the 
other negative. 
The switch is represented in plan in fig. 4, and its place is indicated in elevation 
in fig. 1. Calling the poles of the battery A and B, its purpose is to make rapidly 
the following changes of connection : — 
(1.) A, sliding condenser ; B, guard ring, disc Jc; earth, quadrant of electrometer. 
(2.) A, B, guard ring, earth ; disc h, sliding condenser. 
(3.) To connect the disc k and the sliding condenser to the quadrant of the 
electrometer. 
The combination (1) may exist for any time long or short, but (3) follows (2) within 
a fraction of a second, and the observation of the electroscope consists in deciding 
whether or not the image moves at the instant of combination (3), and, if it moves, in 
which direction. In (2) the poles of the battery are put to earth, in order that one 
may be sure that the parts of the switch with which they are connected do not 
disturb the result by inductive action on the parts connected with the condensers. 
q q is a plate of ebonite screwed to the shielding cover of the condenser, r is a steel 
spring connected to earth, s a similar steel spring connected to one pole of the battery. 
t v are segments of brass of which the securing screws pass through to the brass 
cover. 
w u, similar segments insulated from the brass cover and guard ring, connected 
respectively to the sliding condenser and the electrometer. 
p is an ebonite handle and brass pin which turns in an insulated brass socket 
connected by a spring m with the disc k; p carries a piece of ebonite x x which 
moves the springs r s from contact with t v to contact with u w, and also a spring y y 
which may connect l v with the disc k, or, when turned into the position indicated, 
w with the disc k, and instantly after both with the electrometer. One pole of the 
battery is always connected to the guard h h. The switch is protected against 
inductive action from the hand of the observer, or from electrification of the top of the 
ebonite handle when touched with the finger by a copper shield n n connected with 
the guard ring through the cover. 
d 2 
