CONTINUOUS ELECTRIC CALORIMETRY. 
65 
resistances of the two halves of the dial was frequently checked with consistent 
results, as a precaution to give warning of any accidental flaw. This precaution was 
by no means superfluous, for on one occasion in November, 1897, a fault, amounting 
very nearly to complete rupture of the wire, was discovered by Mr. King in this 
manner. It was however easily located and rectified without altering the relative 
values of the coils. 
(4.) Method of Testing. 
The method of testing the ratio of the two halves of the slide-box was as follows :— 
The slide-box was connected by the terminals AB in parallel with a 100,000-ohm box 
consisting of ten coils of 10,000 ohms each. The battery was connected as usual at 
A and B, one terminal of the galvanometer to G, and the other to the middle of 
the 100,000-ohm box. The slide-box contacts were set at 50,000 ohms. To take 
one particular experiment as an example, the deflection of the galvanometer observed 
on reversing the battery was 77 scale-divisions. When the contact was set at 
50,010 ohms, the deflection was 215 scale-divisions in the same direction, showing a 
sensitiveness of 138 scale-divisions on reversal for a change of 1 in 5000 in the 
reading. The contact was then set back to 50,000, and the two halves of the box 
were interchanged with respect to the rest of the circuit by interchanging the 
connections at A and B. The deflection observed was increased from 77 to 181 
scale-divisions in the same direction as before. The effect of interchanging the two 
halves is the same as if the slider were shifted through a resistance equal to 
their difference. Hence the difference of the two halves is to 10 ohms as 104 is to 
138. The first half of the box is evidently the smaller, as the effect of interchanging 
is the same as that of increasing the reading. The correction to be applied to the 
reading, to reduce to mean ohms of the box, is half the difference of the two halves, 
and is negative, since the first half is the smaller. We have, therefore, 
Correction at reading 50,000 ohms = — 10 X 104/2 X 138 = — 3 - 8 ohms. 
The galvanometer deflections in each case were observed several times and the mean 
taken. The details were also varied by using different resistances for the ratio arms 
in the comparison and different galvanometers. Observations were taken by different 
observers at various temperatures on several occasions, at intervals during five years. 
The greatest divergence of the results from the mean value is less than 1 part in 
100,000 (’4 ohm in 50,000 ohms), which is strong evidence that the relative values 
of the corrections at any part of the box could be relied on at any time to a similar 
order of accuracy. 
The following is a summary of the results of all the tests of which full details 
have been preserved, but several other tests were made from time to time as a 
precaution :— 
VOL. cxcix.—A. 
K 
