110 nePorT—1890. 
Taste III.—Giving the Cupacity of two Mica Condensers for various 
Frequencies of Charge. 
Frequency | June 12 | sune 14 | June 16 | Mean 
CONDENSER A. 
21 “04885 "04886 — ‘04886 
32 04883 04884 == 04884 
64 04868 “04868 04864 ‘04867 
80 _ ‘04859 — ‘04859 
CONDENSER B. 
21 an 09642 = 09642 
32 — “09642 — “09642 
64 — 09634 09642 ‘09638 
Taking the air condensers first, the tables show that, at any rate for 
frequencies between 32 and 80 per second, the time of charging has no 
effect on the capacity, while the individual observations in each series are 
within 1 in 2,000 of each other. 
For condenser I. the observations at frequency 64 are in all the series 
the least, but this is not the case with condenser II. 
The capacity of condenser I. shows no change between December 1889 
and June 1890. The observations in August 1890 are all rather greater 
than those in the earlier series, but the increase, about 1 in 2,000, is almost 
within the error of the experiments. With regard to condenser II. there is 
an indication of a rise in its capacity all through. It will be remembered 
that we have already shown that the insulation resistance of II. is con- 
siderably less than that of I., but it is easy to see that this leak was not 
sufficient to account for the change, for if k be the resistance of the leak 
then our approximate formula becomes 
noto=", instead of n=. 
Now, the current through the condenser when leaking most was about 
0002 = c, where & is the E.M.F. to which it is charged and ¢ the capacity 
of the condenser. 
Thus the resistance of the leak is ork ee or ‘25 x10?! C.G.S. units, 
“0002 xc 
since the value of c is ‘02 x10-!5. This resistance is 250,000 megohms. 
Hence the correction to the capacity =1/nr='0002 x c/n, and this is 
far too small to affect the result. 
There is no doubt, then, that the capacity of II. altered during the 
experiments by about 1 per cent., and it will be necessary to take it to 
pieces and set it up again. 
It will be remembered that in the early part of August the leak in II. 
was very great, and it seems probable that the steps taken to discover 
the cause of the leak have produced a change in capacity. The experi- 
ments on II., then, serve merely to show that the capacity can be found 
by the rotating commutator method to a high degree of accuracy, while 
those on I. prove that an air condenser, of ‘02 microfarad capacity, has 
been constructed which has retained its capacity unaltered for the eight 
months between December 1889 and August 1890. 
