SCIEXCE AND PRACTICE. 317 



to 4 parts of water, is now usually adopted as the unit of 

 electro-motive force. 



57. Fechner's method of comparing the electro-motive 

 forces of two elements consists in measuring the intensities 

 of the two currents, when the resistances are equal. He 

 prefers to employ a galvanometer with a long thin wire, 

 making many turns round the needle, besides a considerable 

 resistance in order to be able to neglect the resistance of the 

 element itself. 



58. Another and preferable method is mentioned by 

 Wiedemann. The two elements are connected up in the same 

 circuit with a tangent galvanometer, or other apparatus for 

 measuring intensity ; first, so that their currents go in the 

 same direction, and secondly, in contrary directions. 



Let 



The electro-motive forces be =* E and E', 



the resistances of the elements = R and R', 



the interposed resistance = r, and 



the intensities of the sum and difference = I, and I tt 



then 



E 4- E' 



I. = 



R + R' + r 



I E ~ E ' 



= R + R' + r 



whence 



E' = E . * "" Id 



59. Another method, requiring, like the last, two observa- 

 tions for obtaining the necessary data for comparison, is 

 that commonly resorted to when no galvanometer is at hand ; 

 a galvanoscope will then fulfil all that is required, in con- 

 junction with a sufficiently well arranged adjustable resist- 

 ance scale or rheostat. 



One of the elements, E, whose resistance is r, is first con- 

 nected in the circuit of the galvanoscope (resistance g) and of 

 the adjustable resistance R. A deflection of, say degrees, is 

 obtained and noted. The other element, E', whose resistance 



