294 MR. HE. H. GRIFFITHS ON THE LATENT 
Section VIII.—MEAsuREMENT OF THE Heat DEVELOPED BY THE CURRENT (Q,). © 
A sketch of the electrical connections is given in Plate 6, fig. 3, and a full 
description will be found in Paper J., pp. 382-388, therefore I will here give only a 
brief description. 
Measurement of E. 
Leads marked 2 and 4 were connected with the storage cell circuit, leads 1 and 3 
with the Clark cells. A special form of rheochord was used consisting of two long 
barometer tubes (7 feet in length), which were so arranged that they could be raised 
or lowered by the rotation of handles; thus the length of the vacuous space could 
be altered at will. Platinum wires passed down these tubes into the mercury. 
The wires were placed in parallel arc and thus one acted as a shunt to the other. 
The change in resistance depended, therefore, not only on the movement but on the 
ratio of the two resistances, and the instrument could be set so that a large move- 
ment caused but little change, or vice versd. The contact between platinum and 
mercury i vacuo was found to be very satisfactory. The resistance of the storage 
circuit could thus be altered at will. The Clark cell circuit contained a high 
resistance galvanometer G,, with a resistance of about 9000 ohms, and joined the 
storage circuit at the roof of the calorimeter (at M and N, small sketch, Plate 6, 
fig. 3). By adjusting the rheochord the ratio of the external resistance of the 
storage circuit to the coil resistance could be altered at will, so that the D.P. at the 
ends of the coil became equal to that of the Clark cells, and the balance was 
maintained by watching the indications of G,, and, when necessary, adjusting the 
rheochord. The spot of light from G, passed down a tunnel 4 feet in length on to a 
sheet of ground-glass placed opposite the rheochord. In Paper J., p. 384, I have 
given the experimental numbers on which the following statement was based :— 
“Tt thus appears that the variations in E (the potential difference at the ends of 
the coil) were certainly within yo90o00 of the mean value during the experiments, 
and changes in E, consequent on changes in the coil resistance, or the E.M.F. of 
the storages, could be disregarded.” 
The effect of any error in determining E would be serious (the quantity EK? being 
used in the reductions), and hence its accurate measurement was of great importance. 
Tam satisfied, however, that (assuming the value of E for the Cavendish standard 
Clark cell to have been accurately determined) no error in the measurement of Q, is 
due to uncertainty as to the potential difference. The thirty-six Clark cells used 
have been fully described, in Paper J., pp. 385-388, as well as in the Paper by 
Messrs. GLAZEBROOK and SKINNER.* They have been compared with each other at 
regular intervals up to the present time, and their relative changes are small. At 
least three of these cells were always placed in parallel arc. Sometimes when 
* «Phil. Trans.,’ A, 1892, pp. 622-624. 
