to the Measurement of Electro-motive Forces. 553 



explained in a previous communication* '^On the Dynamical 

 Theory of Heat/^ will be extended so as to include a mechanical 

 theory of thermo-electric currents. The application to chemical 

 electro-motive forces leads immediately to the expression for the 

 electro-motive force of a galvanic battery, which was obtained by 

 virtually the same reasoning, in another paper published in this 

 Volume of the Magazine f (p. 429) : for if e be the electro-chemi- 

 cal equivalent of one of the substances concerned in the chemical 

 action ; if ^ be the quantity of heat evolved by as much of the 

 chemical action concerned in producing the current as takes place 

 during the consumption of a unit of mass of this substance ; and 

 if J be the mechanical equivalent of the thermal unit, the me- 

 chanical value of the chemical action which goes on in a unit of 

 time will be iOey, and this must therefore be equal to Yy, the 

 work done by the electro-motive force which results. Hence 

 we have 



which is the expression given in the paper referred to above, for 

 the electro-motive force of a galvanic battery in absolute measure. 

 6. In applying this formula to the case of DanielPs battery, I 

 used a value for 6 derived from experiments made by Mr. Joule, 

 the details of which have not yet been published, but which I 

 believe to have consisted of observations of phsenomena depend- 

 ing on the actual working electro-motive forces of the battery. 

 I am now enabled to compare that value of the thermal equiva- 

 lent, with the results of observations made directly on the heat 

 of combination, by Dr. Andrews J, who has kindly communicated 

 to me the following data : — 



(1.) The heat evolved by the (3.) The heat evolved by the 



combination of one grain of zinc combination of the equivalent quan- 



with gaseous oxygen amounts to tity, '9/27 of a grain of copper, with 



1301 units. oxygen, amounts to 



588-6 units. 

 (2.) The heat evolved by the (4.) The heat evolved by the 

 combination of the 1246 grains of combination of the 1'221 grains of 

 oxide thus formed with dilute sul- oxide thus formed, with dilute sul- 

 phuric acid'amounts to phuric acid, amounts to 

 369 units. 293 units. 



Hence the thermal equivalent of the whole chemical action which 

 goes on in a Daniell's battery during the consumption of a grain 



* March 1851. Published in the Transactions, vol. xx. Part II. 



t "On the Mechanical Theory of Electrolysis." 



X Published in his papers "On the Heat disengaged during the Com- 

 bination of Bodies with Oxygen and Chlorine" (Phil, Mag. vol. xxxii.), 

 " On the Heat disengaged during MetaUic Substitutions" (Phil. Transac- 

 tions, Part I. for 1848), "On the Heat developed during the Combination 

 of Acids and Bases" (Trans. Royal Irish Academy, vol. xix. Part II.), &c. 



