DETERMINATION OF THE SPECIFIC HEAT OF WATER. A421 
could be connected to them by means of copper bridges A and B. The standard 
cell being inserted and the two bridges being placed as shown in the figure, the 
Cell II. is opposed to the standard and the difference of their electromotive force is 
measured. Moving the bridges along from one set of cups to the next, the ten 
Cupboard Clarks may be rapidly interchanged. 
The mounting of the cells also allows of their being rapidly placed in series by 
means of copper strips. The figure shows the Cells III., IV., V. thus connected. 
The galvanometer used in these comparisons (G, fig. 5) was one of Lord Kertyin’s 
High Resistance Astatic Galvanometers. Its resistance was 5600 ohms. It 
was placed on paraffin blocks for better insulation, During use its sensitiveness was 
such that one scale division corresponded to 7°3 X 107° volt. A carbon resistance 
(P, fig. 5) could be thrown into series with the galvanometer when it was necessary 
to diminish its sensitiveness. A commutator (K, tig. 5) served to change rapidly the 
circuit in which the galvanometer was inserted. This commutator was made of a 
paraffin block containing seven mercury cups, but two sets of two were permanently 
connected as shown in the figure. Three copper bridges served to connect tem- 
porarily the terminals s to g, g to 7 or k, s’ to p or q; when s was connected to g 
the Standard Clark was opposed to one of the ten Cupboard Clarks. When the ten 
Cupboard Clarks were to be opposed to one half of the battery of twenty cells, 
g—Jj and s’—p were connected ; for the other half g—k and s’—q were joined. In 
the equivalent experiments when the battery had to be opposed to the electro- 
motive force at the end of the heated coil, the three temporary bridges were removed. 
The operation of standardising the electromotive force of our battery took about 
ten minutes and was carried out as follows :— 
1. Reading of the thermometer placed in the Standard Clark, ¢,’. 
2. Comparison of the ten Cupboard Clarks separately with the Standard. The 
comparison of Cell I. with the Standard gives us a relation of electromotive forces 
which may be written 
I. — Standard = a, 
and the addition of all such relations gives us for the ten cells 
(I. + 1. + 111..4+...+ X.) —10 Standards = 4, +0,+...=8,. 
3. Comparison once more of the Cell I. with the Standard gives 
I. — Standard = a,, 
There may be a small difference between @, and a, owing to temperature changes. 
4, Reading of the thermometers placed in the Standard, ¢,". 
5. Comparison of Sets (1) and (2) of the battery of 20 cells with the 10 Cupboard 
Slarks in series, giving by addition the relation 
Seui(byee Sct (2) 125 (Ie-eelly Sine tes) FX.) on, 
