412 MEASUREMENT OF ELECTROMOTIVE FORCES. 



CHAPTER IV. 

 MEASUREMENT OF ELECTROMOTIVE FORCES. 



1007. STANDARDS OF ELECTROMOTIVE FORCE. When a 

 conductor is traversed by a permanent current, the electromotive 

 force between corresponding elements of two equipotential surfaces, 

 or the difference of potential of two points of these surfaces, is 

 the product of the intensity of the current which traverses one of 

 these elements by the resistance which separates them. The prac- 

 tical unit of electromotive force or the volt, is . the electromotive 

 force which maintains a current of one ampere in the resistance 

 of a legal ohm. (920). The volt thus determined carries into 

 the value, which results from its theoretical determination, the 

 same relative error as that of the legal ohm. 



It has not hitherto been possible to produce a standard of 

 electromotive force so definite and invariable as for resistances, 

 and for want of a better standard it is necessary to have recourse 

 to couples which contain liquid elements. DanielPs element 

 and that of Latimer Clark are those which are most trustworthy. 



Various forms have been proposed for DanielPs element, either 

 with a view of preventing the mixture of the two solutions, or to 

 avoid the use of porous vessels, as for instance in Sir W. Thomson's 

 standard, where the liquids are superposed in the order of their 

 decreasing densities. In any case it is necessary to renew the 

 liquids before proceeding to a new set of measurements. 



The electromotive force depends on the density of the solutions. 

 With saturated copper sulphate, and zinc sulphate of increasing 

 density, Carhart* found that the electromotive force varies from 

 1-04 volt with pure water to a maximum of 1*137 for a 5 per cent, 

 solution of zinc sulphate ; it diminishes slowly to ITII for a solution 

 of 10 per cent, sulphate, and then remains constant. 



* H. S. CARHART. Amer. Journ. of Science, Vol. xxvin. p. 374. 1884. 

 Journal de Physique [2], Vol. IV., p. 98. 



