138 
PROFESSOR R. THRELFALL AND MR. J. H. D. BREARLEY 
Specific Inductive Capacity .—Five pieces of film, measured as usual, gave the 
following values for the thickness in inches :— 
(1) -009022 
(2) -009917 
(3) -008G89 
(4) -008058 
Centre (5) "008978 
Mean "0089316 inch = *0226863 centim. 
These measurements were made with very great care and represent the greatest 
accuracy the method is capable of; individual measurements of the same bit of film 
not differing by more than "00008 inch. The differences are to be partly attributed 
to partial crushing or crumbling of the' film during measurement, though there is 
clearly also a want of uniformity in the thickness of the film itself. 
In obtaining the capacity by the de Sauty method, a balance was first obtained 
when the film was in the box, the lid was then taken off and a new balance obtained ; 
and finally the plates were taken out and supported on their ebonite legs on the 
laboratory table at some distance from anything else. Removing the lid of the box 
changed the resistance required for balance from 2517"5 to 251CP5, and placing on 
the table reduced this to 2510. The connections and keys by themselves require 
47 ohms in the adjustable arm. The other was always 100,000 ohms. We see from 
the small effect produced by lifting the plates out of the box which was to earth, that 
the free capacity may be neglected compared to the part of the capacity which is 
due to the dielectric action of the sulphur. We have used the factor 2463/10 5 in 
calculating the capacity, allowing for the capacity of the keys, &c. The standard 
required no correction at all, it was "1 microfarad. The resulting capacity is 
therefore 2463 X 10~ 6 microfarad as stared in the summary. The sulphur con¬ 
denser did not leak sufficiently to affect the galvanometer by more than one 
division with 300 volts, and hence with 40 Clarks may be considered to 
insulate perfectly. The mica standard, however, leaked quite appreciably, so that 
its resistance was only about 2"3 X 10 11 ohms. The capacity was measured again at 
71°, and the balance was found to be unchanged. In this case the sulphur condenser 
also leaked, i.e., its resistance was about 10 12 ohms, as against 7 X 10 13 , say when it 
was cold. In order to find whether these leaks (which are of course exceedingly 
small) produced any measurable result, we made use of the formula given by 
Glazebrook, ‘Phil. Mag.’ [5], vol. I, p. 376 -377, 1881. This is 
where R and R' are resistances out in the arms, and p ancbp' the corresponding insula¬ 
tion resistances of the condensers. We found that the correcting factor w-as about 
