THE SPECIFIC mEUCTIVE CAPACITY OF A DIELECTRIC. 
9 
GIojSs. 
The glass condenser was made with seventeen sheets of thin microscope-slide 
cover-glass, about three inches diameter. They consisted of a soda glass of high 
conductivity; so that, although the discs were carefully cleaned and bordered with 
shellac as in the case of mica, the insulation of the condenser, as tested by the electro¬ 
meter, was never perfect. Since it is only the charging of the condenser that affects 
the galvanometer, the discharge passing sensibly ah through D R (fig. 1), the defective 
insulation introduces simply a steady current during each time of cliarging. Each 
time the condenser is charged, the quantity of electricity passing D and B consists of 
(l)the charge of the condenser, and (2) the current through the condenser, which lasts 
during the time the vibrator P is in contact with S. For our present purpose, the 
current may be considered as immediately established at full strength when the circuit 
is closed. So that, neglecting absorption, the apparent capacity exceeds the true 
capacity by the conductivity of the condenser multiplied by the duration of the contact 
between P and S. And from observations on the apparent capacity with two forks 
of known speed, the true capacity can easily be found. 
The highest temperature in the preliminary heating was less than 110° C. 
The results are given in the following table and diagram (fig. 5):— 
Temperature. 
Variable resist¬ 
ance. 
Cliange of 
capacity. 
Rate of change per 
degTee. 
1 
Fork making 99 comp 
lete vibrations 
ler second. 
17° C. 
4795 
38 
4630 
■034 
■0016 
49 
4540 
•053 
•0017 
57 
4425 
•077 
•0019 
02 
4320 
•099 
•0022 
Fork making 64 complete vibrations per second. 
17 
6070 
38 
5830 
•039 
•0018 
49 
5670 
•066 
•0021 
57 
5470 
•098 
•0024 
62 
5360 
•117 
•0026 
MDCCCXC.—A. 
c 
