Electromotive Forces from Thermochemical Data. 215 



glass beaker as the outer vessel, containing one of the metal 

 electrodes in a solution of one of its salts, and a porous 

 earthenware cell, which contained the other metal in a solu- 

 tion of its corresponding salt. Cells of the ^aoult form 

 could not be employed on account of their inconvenience for 

 thermal measurements and their necessarily high internal 

 resistance. Attempts to reduce resistance by substituting 

 animal membrane, parchment-paper &c. for the porous cell 

 were abandoned owing to the facility with which they allowed 

 small but significant quantities of the solutions to mingle by 

 diffusion, and so permit the deposition of one metal on the 

 more electropositive one, generating local heat and altering 

 the electrical constants. The cell was supported by the rim 

 of the beaker resting on a cork edge which was fitted into the 

 top of a tin C3'Knder, the latter being itself suspended within 

 a larger cylinder of bright tin. With this apparatus, when 

 containing the usual working quantities of solutions, the rate 

 of loss of heat was equal to y^Q- of a gram-degree Centi- 

 grade for each minute-degree above the surrounding air 

 temperature, for the few degrees of the experimental range. 



Thermometers graduated to fractions of a degree were 

 inserted, one in the inner porous cell and another in the outer 

 cell; and in cases in which a slight inequality of heating 

 at the two electrodes occurred, a mean of the rise of tempera- 

 ture of the two thermometers was taken as best expressing 

 the net heat-evolution. 



The external battery, used for sending currents against the 

 E.M.F. of the experimental cell, consisted of either a Daniell 

 cell or a form of constant bichromate battery, in accordance 

 with the E.M.F. of the experimental cell to be overcome. 

 The current was measured by a calibrated galvanometer, 

 which gave a deflexion of 47°*5 with ^q of an ampere, and 

 was regulated by the introduction of the necessary resistance. 



It was first proved in the case of a cell, cadmium-copper 

 sulphate, which gives an E.M.F. almost exactly in accordance 

 with its calculated value, that the heat which is generated by 

 a current passing through the cell depends only upon its 

 resistance, and is independent of the direction in which that 

 current passes. 



This being so, if the permanent experimental value of the 

 E.M.F. of any cell differs by e volts from its calculated 



€ct 



value (E — Ea= +e), an amount of heat equal to -j ought to 



be evolved or absorbed when a current of strength c passes 

 for t seconds through the cell. 



