356 
DR. J. HOPKINSON ON THE ELECTROSTATIC 
Thomson had informed me that experiments. had proved that the capacity of a good 
insulating glass is sensibly the same, whether the period of discharge be the ten- or 
twenty-thousandth of a second, or say one-quarter of a second. This statement has 
been verified, (ii.) It appeared plausible to supj>ose that specific inductive capacity 
of glass was not a constant, but was a function of the electromotive force—in other 
rlmro’p of* ronrlpnsPT 
words, that the ratio — r~ - 5 —-—-——— was less when the electromotive force was 
clinereiice oi potcnticU. 
great than when it was small. This surmise gains some force from Dr. Kerr’s electro- 
optical results, which show that electrostatic and optical disturbance of a dielectric are 
not superposable. It has, however, been submitted to a direct test, with the result 
that, within the limits tried, specific inductive capacity is a constant, and that it is not 
possible that the discrepancy of experimental results can be thus explained. Finally, 
I have made a rough model of Mr. Gordon’s five-plate balance, and used it to make 
determinations of specific inductive capacity. 
Firstly, a brass plate was tried, and its capacity was found less than unity instead 
of infinite. 
Secondly, by varying the distances of the plates of the balance from each other, 
different values of the specific inductive capacity of the same glass w^ere obtained. In 
fact, it has been shown that the five-plate induction balance cannot be freely relied 
upon to give correct values of specific inductive capacity. 
I conclude that the values I published in 1877 are substantially accurate, whether 
the period of discharge be 2U o O U - or ^ second, whether the electromotive force be one 
volt per millimetre or 500 volts per millimetre, and that Mr. Gordon’s different result 
is to be explained by a defect in the method lie used. 
(I.) To prove that a condenser of well-insulated glass mag he almost completely 
discharged in tucToo second. 
For this experiment it is essential that the effect of conduction over the surface of 
the glass should be insensible. A jar, such as that used in Sir W. Thomson’s electro¬ 
meter, is unsuitable. The proper form for the condenser is a flask with a thin body 
and a thick neck, filled with strong sulphuric acid to the neck. Such a flask of light 
flint glass was prepared, and was instantaneously discharged in the following manner :— 
The interior of the flask was connected to a metal block, A. Upon this block rests a 
little L-shaped metal piece, B, which can turn on a knife edge, C. A and C are 
carried on a block of ebonite, and are therefore insulated. D is a piece of metal 
connected to earth, and rigidly attached to the extremity of a pendulum. The pen¬ 
dulum is drawn aside and let go ; the piece D strikes B and puts the jar to earth, and 
instantly afterwards breaks the contact with A, and drives away the piece B. In all 
cases the pendulum was drawn aside 45°, and in all the experiments but one mentioned 
below it made 93 half-oscillations per minute. The duration of the discharge was 
