MEASUREMENT OF ELECTROMOTIVE FORCES. 



The ratio represents the quantity of heat which corresponds to the 

 passage of unit electricity, or a coulomb. 



We are indebted to Le Roux* for very careful determinations of 

 the absolute value of H for a great number of metals at the mean 

 temperature 25. For the couple formed by copper and an alloy 

 containing 10 of bismuth and i of antimony, the experiment was 

 made at 25 and 100; the quantities of heat disengaged were in the 

 ratio of 308 to 398. 



The ratio 1*29 of the quantities of heat disengaged at the tem- 

 peratures of 100 and of 25 does not much differ from the ratio 

 1*25 of the absolute temperatures ; the electromotive force of contact 

 H is then proportional to the absolute temperature, and the bismuth- 

 copper couple should have a uniform course, which agrees with 

 experiment (280). 



The electromotive forces of contact are very slight. For the 

 preceding couple, which gives the highest value, Le Roux obtained 

 0*0219 volt. Bellatif found that a coulomb liberates 0*0006065 

 thermal units in the iron -zinc couple at 13*8, which agrees with 

 0*00253 volt. 



1033. SOLIDS AND LIQUIDS. A current cannot traverse the 

 bounding surface of a solid and an electrolysable liquid without 

 producing a chemical action defined by Faraday's law (255), 



If it be assumed that a coulomb decomposes 0*09316 mgr. of 

 water (918), or 1035 x io~ 4 grammes of hydrogen, the action of a 

 coulomb on any body will be represented by the same fraction 

 0*1035 x i o~ 4 of its electrochemical equivalent expressed in grammes. 



Let us again observe that, if the heat of combination of the body 

 in question is q thermal units, the heat relative to a coulomb will be 



q x 0*1035 x io~ 4 c., 



and the corresponding electromotive force, expressed in volts, 



q 

 4*17 x q x 0*1035 x io~~ 4 = 0*432. 



As nothing authorises us to suppose that the Peltier effect can 

 then be neglected, we must in general assume that, concurrently with 

 the chemical operation, there is a disengagement or an absorption of 



* LE Roux. Ann. de Chim. et de Phys. [4], Vol. x., p. 201. 1867. 

 t BELLATI. Atti del R. Inst. Veneto [5], Vol. v. 1879. 



