62 
PROFESSOR HUGH L. CALLENDAR ON 
of the current, and does not permit the course of variation of the current to be 
accurately followed throughout an experiment; (2) that with Clark or cadmium cells 
of a suitable pattern it is possible to attain an order of accuracy in the relative values 
of the readings about ten times as good as that attainable with a silver voltameter. 
It was most important for our purpose to obtain accurate relative values, and what¬ 
ever might be the doubt as to the absolute values of the E.M.F. of the cells, there 
could be none as to their constancy, which was easily tested over considerable periods 
of time. 
It therefore appeared most satisfactory to measure the current by passing it 
through a suitable resistance, and observing the P.D. on the terminals with a 
potentiometer in the usual manner. The introduction of the potentiometer may 
appear at first sight to be an additional complication and source of error; but it 
really made the observations much simpler, and I satisfied myself by careful tests of 
the instrument that an accuracy of 1 in 100,000 was readily attainable so far as the 
potentiometer readings were concerned. Besides, it was unnecessary, with the 
potentiometer, to keep the P.D. on the conductor balanced against an integral 
number of cells, and it was, therefore, possible to adjust the electric current to give 
the same rise of temperature with different values of the flow of liquid. This most 
essential adjustment could not be so conveniently or quickly effected by varying the 
How of liquid as by regulating the electric current with a low-resistance rheostat. It 
also proved in practice to be much more convenient to take all the electrical readings 
on a single instrument, instead of having the silver voltameter as well as the 
potential balance to attend to. 
It will be seen that our method is independent of any assumption with regard to 
the electrochemical equivalent of silver, although the contrary is apparently assumed 
in discussing our result both by Ames (‘Report to the Paris Congress of 1900 on the 
Mechanical Equivalent of Heat’), and by Griffiths (‘Thermal Measurement of 
Energy,’ p. 93). The method of measurement is ultimately equivalent to that of 
Griffiths, as it makes the result depend on the International Ohm, and the Clark 
cell. The measurement of the current by observing the P. D. on a known resistance, 
when combined with the observation of the P.D. on the heating conductor itself, is in 
effect equivalent to the measurement of the resistance of the heating conductor 
under the actual conditions of the experiment, in the most direct manner possible. 
The watts expended are derived from the formula E 2 /R, so that an error in the 
absolute value assumed for the standard cell is twice as important as an error in the 
value of the ohm. 
If x 0 is the balance-reading of the potentiometer when the standard cells are 
connected, and e the E.M.F. of the standard cells, and if x , x" are the readings 
corresponding to the P.D. on the heating conductor and the standard resistance S 
respectively, the expression for the heat-supply in watts is evidently 
EC = c*x’ x"/x* S. 
