12 



Mr. E. H. Griffiths. The Value of 



[Feb. 16, 



say, 2 . We then know that S has resumed its former value, and is 

 therefore again at the temperature 6 S ; thus the value of /3 = O s —0 2 . 

 By changing B, the potential difference of the ends of the coil, we 

 can find values of ft corresponding to values of E. By applying to 

 the ends of the coils the potential balance previously described, the 

 potential difference at the extremities of S can, by increasing the 

 E.M.F. at the ends of the bridge, be raised to that of 1, 2, &c, Clark 

 cells. 



The following table gives the results obtained by this method. 

 During our observations, the rate of stirring was the usual one, but 

 we also investigated the effect of small changes in the rate. The last 

 column gives the values deduced from the parabola = 0"00422^ 2 

 (where n is the number of Clark cells), as we found it convenient to 

 express the differences in temperature by the corresponding differences 

 in R. A difference of O'OOIO between the last two columns corre- 

 sponds to a change of 1 in 8600 in R. 



The correction is a most important one, and the neglect of it by 

 previous investigators sufficiently accounts for their failure to obtain 

 satisfactory results by observation of the heat developed in a wire by 

 an electric current. 



Table IX. 



The following numbers were plotted. 













§R deduced from 



No. of cells 



Increase. 



(legal ohms). 



8R = -00422W 2 . 



(»)■ 











— x 











1 







0-0042 



0-0042 



2 



-0120 



-0163 



-0168 



3 



-0333 



-0376 



-0378 



4 



0-0638 



-0681 



0675 



5 



-1023 



-1066 



0-1055 



6 



0-1478 



0-1516 



-1519 



t, Time. — An electrical clock with a seconds pendulum was used as 

 our standard for time. It was carefully compared at intervals with 

 a chronometer by Dent. A chronograph was controlled by this 

 clock. The rate of the clock was a losing one until August 21, but 

 after that date its error was less than 1/25,000, and no correction was 

 necessary. 



w, Mass. — A balance sensitive to a change of 1/100,000 of the least 

 mass measured by us and a set of Oertling's weights* were used in 

 our determination of mass. 



* These weights were re-standardised by Messrs. Oertling in August, 1892. 



