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XVI. Residual Charge of the Leyden Jar. — Dielectric Properties of different Glasses. 
By J. Hopkinson, M.A., D.Sc. Communicated by Professor Sir William Thomson, 
F.B.S. 
Received November 30, 1876, — Read January 18, 1877. 
I. Before proceeding to comparative experiments on different glasses, it appeared 
desirable to verify experimentally the two following propositions : — 
(a) If two jars be made of the same glass but of different thicknesses, if they be 
charged to the same potential for equal times, discharged for equal times and then 
insulated, the residual charge will after equal times have the same potential in each. 
In experiments in which potentials and not quantities of electricity are measured the 
thickness of the jar may be chosen arbitrarily, nor need any inconvenience be feared 
from irregularities of thickness. 
(b) Besidual charge is proportional to exciting charge. 
These propositions may be included in one law — that superposition of simultaneous 
forces is applicable to the phenomena of residual charge. 
To verify ( a ) two flasks were prepared of the glass afterwards referred to as No. 1. 
One was estimated to be about 1 millim., the other 6 or 7 millims. thick. These were 
cleansed and insulated in the usual way by filling with strong sulphuric acid without 
soiling the neck of the flask. They were placed in the same basin of water, which was 
electrically connected with the outside of the quadrant electrometer. The interiors of 
the flasks were respectively connected with the two quadrants ; they were also connected 
together by a wire which could at any instant be removed. One Daniell’s element gave 
a deflection of 69 scale-divisions. The two flasks were charged together with 48 elements 
for some minutes, and it was observed that the equal charge of the two quadrants did 
not deflect the needle. The flasks were discharged for 15 to 20 seconds and insulated, 
still connected. The connecting wire was then removed, and the subsequent movement 
of the image observed. If left undisturbed a maximum of about 20 divisions of the 
scale was attained. But usually the deflection in from 20 to 30 seconds reaching 10 
divisions, the thick flask was discharged, and the image was driven from the scale, 
showing that at that time the potential of either flask was represented by more than 
500 scale-divisions, and hence that the difference between them was less than 2 per cent, 
of either of them. When the charge was negative the error was in favour of the thin 
flask. This is in complete accord with anomalous results subsequently obtained with 
the same glass. Correcting for this peculiarity of the glass we may conclude that the 
law is verified within the limits of these experiments. 
The second proposition was confirmed with two different glasses ; but the results in 
MDCCCLXXVII. 4 Q 
