112 
MESSRS. J. HOPKINSON AND E. WILSON ON THE 
is far from constant, and hence the apparent resistance of that extraordinarily high 
insulator, a flint-glass flask, must be, for very short times, but still for times 
enormously large compared with the period of light waves, much less than 80 ohms. 
[Added 11th March, 1897.—Somewhat similar considerations are applicable to 
conduction by metals. Maxwell pointed out that the transparency of gold was 
much greater than would be inferred from its conductivity measured in the ordinary 
way. To put the same thing another way—the conductivity of gold as inferred 
from its transparency is much less than as measured electrically with ordinary 
tiines. Or the conductivity of gold increases after the application of electromotive 
force. Suppose then we have a current in gold caused by an electromotive force 
which is increasing, the current will be less than it would be if the electromotive 
force were constant, by an amount approximately proportional to the rate of increase. 
If u be the current, ^ the electromotive force, u — where n is the 
conductivity as ordinarily measured. This gives us the equation of light trans- 
(Jit 
mission a^‘ — assuming that we have no capacity in the gold. 
Professor J. J. Thomson gives as a result of some experiments by Drude that the 
capacity of all metals is negative. This conclusion is just what we should expect, if 
we assume, as Maxwell has shown, that the conductivity of metals increases with 
the time during which the electromotive force is applied.] 
The experiments herein described are addressed to ascertaining the eftect of 
temperature, first on residual charge as ordinarily known, second on capacity as 
ordinarily known, third to examining more closely how determinations of capacity 
are affected by re.sidual charge, fourth to tracing the way in which the properties of 
insulators can continuously change to those of an electrolyte as ordinarily known. 
The bodies principally examined are soda-lime glass, as this substance exhibits inte¬ 
resting properties at a low temperature, and ice, as it is known that the capacity of 
ice for such times as one-tenth of a second is about 80, and for times of one-millionth 
of a second of the order of 3 or less. 
Residual Charge as affected by Temperature. 
Exjieriments on this subject have been made by one of us which showed that 
residual charge in glass incretises with temperature up to a certain temperature, but 
that the results became then uncertain owing to the conductivity of the glass 
inci'easing. These experiments were made with an electrometer, the charge set free 
in the flask being measured by the rate of rise of potential on insulation. We now 
replace the electrometer by a delicate galvanometer and measure the current directly 
without sensible rise of potential. 
Fig. 1 gives a diagram of connections. The glass to be experimented upon is 
blown into a thin flask F, with thick glass in the neck to diminish the effect of 
