62 Mr. E. H. Griffiths on the Influence of 



mean bridge-wire centimetre) was about 9 2 (i. e. o, 85 C. 

 approximately), and that when the rate was 5*00 the value 

 was about 2*7 ( = o, 25 C. approximately) ; and it is noticeable 

 that the ratio 9-2 : 2*7 is nearly that of (9*1) 2 : (5'0) 2 , thus 

 indicating that the heat developed by the stirrer varied 

 approximately as r 2 , where r is the rate of revolution. 



A modification of this method renders it possible to deter- 

 mine the thermal value of the stirring work. After obtaining 

 fa as above, find what value of X will cause the temperature 

 to remain steady when the heating effect is that due to the 

 stirring and a potential-difference of one Clark cell. Call 

 this temperature fa e ; we have thus two equations, 



and 



<r-p(ex-0 )=0, 



e* 



^ J.R^M + u-O -rt**.-0o) = O ; 



hence a 



, ? _ fa -fl 



a ' J.li 1 (S 1 M + «' 1 ) " fa e -fa 



Whatever method is adopted the experiments involve much 

 labour and time, for a redetermination of fa must be made 

 for each change in fa as it is probable that owing to changes 

 in the viscosity and surface-friction of the contained liquid, 

 fa would be some function of O when the rate of revolution 

 of the stirrer is constant ; and a complete determination of 

 the value of fa for each value of O would have doubled or 

 trebled the length of the investigation. 



It has always appeared to me to be possible that the value 

 of p is a function of the rate of change in the temperature of 

 the calorimeter. The temperature-gradient from the calori- 

 meter to the surrounding walls must alter slightly with the 

 rate of change in fa and the rate of loss or gain in temperature 

 of the calorimeter must as a consequence be affected ; or, in 

 other words. I think it probable that tlie value of fa is a 



function of— r^ } hence the value of fa obtained by experiments 



such as those described above, where the temperature of the 

 calorimeter is constant, may differ appreciably from the value 

 of fa when the temperature is rising. 



Tine method I finally adopted must diminish, although it 

 does not entirely eliminate, any error due to this change in 



fa- 

 Let several experiments be performed similar in all respects 



except that the potential- difference is changed in each case. 



