MR. W. CASSIE OX THE EFFECT OF TEMPERATURE OX 
needle the residual charge comes out anti overpowers the first effect before the needle 
has time to show it. With a small ebonite condenser it was just possible to make an 
Elliott's electrometer sufficient by making the connection as short as possible; but 
with a large condenser it was useless. Consequently, this experiment would require 
an electrometer with a needle of very short time of swing. One with small 
cylindrical needle and quadrants seems most suitable. 
Mica. 
The mica condenser was made of twenty-two sheets of brown Muscovite with 
equilateral triangular markings. The sheets were about 3^ inches diameter, and the 
thicknesses varied from about a quarter to about a tenth of a millimetre. Some 
difficulty was found in getting rid of surface conduction on the mica. No cleaning 
would make it insulate, but in the end perfect insulation was secured by putting 
a border of shellac varnisli round the edges of the sheets. The shellac did not come 
between the plates of the condenser, and so did not affect the capacity. The highest 
temperatures in the preliminary heating were less than 140° C. 
The insulation was constantly tested by an electrometer, and was found perfect 
throughout for temperatures below 110° C. Above this it began to give \vay. 
The results are given in the following table and diagram (fig. 3). 
Temperature. 
Variable re.sist- 
ance. 
Change of 
capacity. 
Rate of increase 
per degree. 
Fork n 
taking 99 comp 
lete vibrations 
ter second. 
11° C. 
4890 
37 
48.50 
•0082 
•00032 
46 
4830 
•0122 
•00035 
.56 
4810 
■0163 
•00035 
70 
4790 
•0204 
•00035 
79 
4770 
•0245 
•00036 
89 
4750 
•0286 
•00035 
107 
4700 
•0.387 
•00040 
112 
4600 
•0591 
•00059 
Fork making 64 complete vibrations 
aer second. 
14 
5.530 
48 
5460 
•0127 
•OC037 
60 
5440 
•0163 
•00035 : 
70 
5425 
•0190 
•00034 
78 
5390 
•0253 
•00039 
89 
5340 
•0343 
•00045 
110 
5270 
•0470 
•00049 
