On the Determination of Chemical Affinity. 265 



produced no effect on the resistance of the carbon rod, or one 

 inappreciably small. When circuit Avas made by pressing 

 pieces of copper and brass against the rough ends of the car- 

 bon rod, contact was only imperfectly obtained, and the resist- 

 ance varied with the pressure because increased pressure 

 brought about better contact, or contact at a greater number 

 of points. 



The bearing of these observations upon the theory of the 

 carbon rheostat, the carbon relay, the carbon transmitting- 

 telephone, and the carbon microphone is obvious. 



University College, Bristol, 

 February 1882. 



XXXIII. On the Determination of Chemical Affinity in terms of 



Electromotive Force. — Part V. By C. E. Alder Weight, 



D.Sc. (Lond.), F.R.S., Lecturer on Chemistry and Physics 



in St. Mary's Hospital Medical School*. 



On the Relationships between the Electromotive Force ofaDaniell 



Cell and the Chemical Affinities involved in its Action. 

 102. TN accordance with the theorem stated in § 61, the 

 -L E.M.F. that would be requisite to break up a given 

 electrolyte under given conditions into the "nascent" products 

 of electrolysis would be a constant amount, were it not that 

 the secondary physical and chemical actions of the electrodes, 

 and of dissolved gases, &c. upon the nascent products give 

 rise to the development of an amount of heat, the energy 

 equivalent to which diminishes the work that would otherwise 

 be done by the current whilst effecting electrolysis ; so that 

 the E.M.F. corresponding to the net electrolytic work actu- 

 ally done is less than the constant amount that would be 

 requisite in the absence of these interfering circumstances ; 

 under certain conditions, the diminution in the work is so 

 great that work is gained instead of spent, when the cell 

 becomes an electromotor. Experiment shows that, cceteris 

 paribus, the amount of diminution is less the more rapid the 

 rate of current-flow; so that in a decomposing cell, in which, 

 on the whole, work is spent during the passage of the current 

 in doing electrolysis (the heating effect due, in accordance 

 with Joule's law, to the resistance proper of the cell being left 

 out of consideration), the counter E.M.F. setup (representing 

 the work so spent) is of + sign, and increases in magnitude 

 with the rate of current-flow ; whilst in an electromotor, in 

 which, on the whole, work is gained during the passage of the 

 current, the counter E.M.F. set up is of — sign (i. e. is a 



* Communicated by the Physical Society, having been read at the 

 Meeting held February 11, 1882, 



