370 
MR. E. H. GRIFFITHS ON THE VALUE OF 
calculations, to deduce it from the differences in time caused by differences in the 
mass of the contained water. 
The walls of the chamber containing the calorimeter were maintained at a constant 
temperature throughout each experiment, this being, in general, the mean tempera¬ 
ture of the above-mentioned range. Direct observations of the changes in 
caused by changes in dj, enabled us to ascertain the values of both cr and p. 
(1.) Potential (E).—A detailed account is given in Section YI. of the method by 
which the ends of the calorimeter coil were maintained at a constant potential 
difference. Briefly, the principle on which our method depended was as follows. 
The extremities of the wires CA, CB, will (when the galvanometer G shows no 
Fig. 1. 
deflection) be maintained at a difference of potential equal to that due to the 
cells at C. 
Let K be an adjustable resistance placed in the circuit which communicates with S 
(the storage cells), and let R be the resistance of the wire AB. Whatever variations 
may take place in R and in the E.M.F. at S it is always possible (provided the adjust¬ 
ment of K is sufficiently under control) to maintain, by close attention to the 
indications of the galvanometer, the points A and R at a constant difference of 
potential. 
A special rheostat was designed by means of which it was found possible, in spite 
of variations in the resistance of AB, to maintain the potential-difference unchanged 
throughout an experiment, and we believe that in no case did the variations exceed 
roT^T) of the mean difference of potential during each experiment. 
The Clark cells (which were placed at C) were constructed by us according to the 
directions of Messrs. Glazebrook and Skinner, and have, on several occasions, been 
directly compared with the Cavendish and indirectly with the Berlin standard. 
Their differences from the standard are small, and their mean E.M.F. at 
15° C. = 1'4344 volts (see Section VI.). 
(2.) Resistance (R).—Had it been possible to maintain a constant value for R it 
would have greatly simplified both the experimental work and the calculations. In the 
year 1890 we devoted much time to the examination of the various copper-manganese- 
nickel alloys, and we performed a series of determinations with a coil whose tempera 
