10 



Mr. E. H. Griffiths. The Value of [Feb. 16, 



We have described the manner in which we determined the last 



BO 



term of this equation, and thus, by direct observation of — , we were 



Bt 



able to obtain the value of ( ^L) and 



Bt 



B0,\ E 



\BtJ e 



J.R'M' 



(2), 



where B/ is the resistance of the coil, and M' the capacity for heat of 

 the calorimeter and its contents at a temperature X . 



We have now to indicate the methods by which we ascertained the 

 electrical value of the energy supplied by the current. 



K 



E.— The extremities of the wires CA, CB will (when the galvano- 

 meter Gr shows no deflection) be maintained at a difference of poten- 

 tial 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 calorimeter coil AB. Whatever variations may take place in 

 R and in the potential difference at S, it is always possible (provided 

 the adjustment of K is sufficiently under control) to maintain, by 

 close attention to the indications of the galvanometer, the points A 

 and B at a constant difference of potential. 



A special rheochord 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 each experiment, and 

 we believe that in no case did the variations exceed 1/10,000 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 



