120 
MESSRS. J. HOPKIRSON AND E. WILSON ON THE 
sheet of ebonite with tin-foil on either side; three such condensers are available, and 
by variation of the area of tin-foil, if necessary, a suitable value for was speedily 
obtained. Platinum-foil was used for the electrodes in the acid inside and outside the 
jar in the glass experiments instead of wire, as shown in fig. 1, in order to secure that 
the connections should not add materially to the self-induction of the circuit. 
The frequency is calculated from the formula 
Frequency = 
where 
K is the capacity in secondary in microfarads, 
L is the self-induction in centimetres. 
where 
n is the number of turns on the secondary, 
2a is the diameter of the ring = 4 feet, 
2/‘ is the diameter of ware on secondary. 
When n = 1, L = 4230 centimetres. If K be taken '00096 microfarad the fre¬ 
quency is 2 ’5 X 10®. 
The lowest frequency we have tried with this apparatus is when = 160 
L = 136 X 10®. If K be taken '0028 microfarad, the frequency is 8,400. 
That the capacity of some kinds of glass does not vary much with a moderate 
variation of temperature is known (‘Phil. Trans.,’ 1881, p. 365). Experiments wmre 
tried on the same blue flask as before, using the method in fig. 3. The results 
obtained and many times repeated for a frequency of 70 or 80 are given in Table IV. 
As the specific inductive capacity of this flask, measured in the ordinary manner, is 
about 8, it appears that at 170° it is about 21. Knowing from the results in 
Tables I. and II. how great was the residual charge for high temperatures and short 
times, it appeared probable that the result would depend upon the frequency. This 
was found to be the case, as shown by the results of November 26, 1894, Table I^ ., 
the apparent capacity being somewhat more than one-half at frequency 100 of 
what it is at frequency 7‘3. Experiments on the window-glass flask show the same 
result. 
The next step was to determine whether or not this large increase of a23parent 
capacity was due to residual charge. To do this the resonance experiments fig. 4 
were resorted to and the capacity of the flask was determined with a frequency of 
about 2 X 10® ; it was found to be sensibly the same whether the flask were hot 
or cold. The results show that the capacity varies from 185 to 198 in arbitrary 
units with a variation of temperature from 25^° to 127°. With frequency 8,400 the 
capacity varies from 240 to 285 in arbitrary units for a variation of temperature 
L = 4:7Tn~a ( log^ 
r 
