41 6 MEASUREMENT OF ELECTROMOTIVE FORCES. 



be able to dispose of a series of couples the electromotive force 

 of which is very small compared with that we wish to estimate. 

 M. J. Regnauld* took as unit a bismuth-copper thermoelectric 

 couple, the junctions of which were respectively at and 100. 



In order to avoid the use of too great a number of thermoelectric 

 couples, M. Regnauld used, as an auxiliary multiple, a DanielPs 

 element, in which copper and copper sulphate were replaced by 

 cadmium and cadmium sulphate, and the electromotive force of 

 which was sensibly equal to 55 thermoelectric couples. The experi- 

 ment consists in placing the couple under investigation in a circuit 

 containing a delicate galvanometer, and in which the cadmium and 

 the thermoelectric couples are interposed until the current is nearly 

 zero. This condition is never exactly realised, and in fact we deter- 

 mine the two numbers n and n + 1 of thermoelectric couples, which 

 must be opposed to the electromotive force in question, to obtain 

 deflections in opposite directions in the galvanometer. We thus ob- 

 tain the electromotive force of the couple in question to within a unit. 



The complementary fraction may be determined either by the 

 ratio of the final deflections, or by seeking to what temperature the 

 hot junction of the n + i couple must be lowered to produce null 

 current; but it would be superfluous to push the approximation 

 so far. This method also, besides the practical difficulties which it 

 involves, does not give the degree of accuracy which might be hoped. 

 The cadmium couple is very constant ; but, whatever precaution is 

 taken in constructing thermoelectric couples, they present differences 

 among each other, as Gaugainf found, arising from the crystallisation 

 of the bismuth, and which may often amount to a tenth of the whole 

 value. The gold-copper couple, the electromotive force of which is 

 62*5 times less than that of the bismuth-copper, would give a more 

 satisfactory standard. 



However this may be, Regnauld found that the electromotive 

 force of the Daniell's element is between 175 and 176 units; the 

 thermoelectric couple, bismuth-copper, between the temperatures of 

 o and 1 00, is accordingly about 0*0061 volts, and the Daniell's 

 element, with zinc and cadmium, 0*34 volt. 



1011. COMPENSATION METHODS. By this are understood those 

 methods in which the electromotive force to be determined is 

 compensated by the difference of potential of two points of a 

 circuit traversed by a permanent current. 



* J. REGNAULD. Ann. de Chim. et de Phys. [3], Vol. XLIV., p. 453. 1855. 

 t GAUGAIN. Ann. de Chim. et de Phys. [3], Vol. LXV., p. i. 1862. 



