CONTINUOUS ELECTRIC CALORIMETRY. 
63 
The accuracy of the reading x Q for the cells, which enters by its square, is twice as 
important as that of x' or x", but it was also more easy to obtain with certainty, 
since the cells were kept at a constant temperature, and the reading x Q seldom 
changed by more than 1 in 50,000 in the course of an experiment. It will be seen 
that, for the determination of the variation of the specific heat, the most important 
point in the electrical measurements is the question of the accuracy of calibration of 
the potentiometer, which is described in the following sections. The absolute values 
of the units are less important, but I have added a brief account of experiments on 
the absolute value of the Clark cells, and of the tests of the standard resistances, as 
they possess an interest of their own, even apart from the question of the absolute 
value of the “mechanical equivalent.” 
(3.) Description of the Potentiometer. 
The form of potentiometer selected as being most convenient for the purpose was 
the well-known Thomson-Varley Slide-Box, which is described and figured in many 
electrical works (e.g., Munro and Jamieson’s “ Pocket-Book,” p. 150). The annexed 
figure shows the arrangement of the connections, and will be useful for reference in 
explaining the details of the calibration. 
The main dial ABCD contains 101 coils, each of 1000 ohms resistance, connected 
in series, the ends of the series being connected to the terminals AB. The ends of 
each coil are connected to platinized studs, which are indicated by the black dots in 
the diagram. A pair of revolving contact springs, fixed to an ebonite handle, travel 
round the dial. These springs are severally connected to the terminals C and D, and 
the distance between them is adjusted so that they bridge over two of the 1000-ohm 
coils of the main dial. 
The second, or “Vernier” dial, CfD'G, consists of a series of 100 coils of 20 ohms 
each, the ends of which are connected to C' and D', and are thus, by way of C and D 
and the double revolving contact of the main dial, in parallel with two of the 
1000-ohm coils of the main dial. Since two coils of the main dial are always shunted 
in this manner by the vernier dial, the effective resistance between C and D is 
reduced to 1000 ohms, and the whole resistance of the potentiometer between the 
