434 MEASUREMENT OF ELECTROMOTIVE FORCES. 



and this sum would only be equal to the true electromotive force 

 M|M' of the two metals so long as the two electromotive forces AjM 

 and A|M' are zero, which is not at all demonstrated; or, in the 

 opposite case, are equal, which is not probable. We may call A the 

 apparent electromotive force, and this applies to all the preceding 

 methods. Thus, in the method of Bichat and Blondlot, it is clear 

 that the difference observed represents the sum 



L|L' + A|L'-A|L. 



Maxwell assumes that Peltier's phenomenon gives the true 

 electromotive force between two metals. As the results obtained 

 by calorimetric and by electrometric methods are very different 

 (248) the former being incomparably smaller than the latter, and 

 sometimes of contrary signs it would follow from this hypothesis 

 that the electromotive force of contact between the bodies in 

 question and air should form great part of the phenomenon ob- 

 served. 



This point of view is not in contradiction with the experiment of 

 Sir W. Thomson (1022). The liquid L, interposed between the two 

 halves of the ring, comes then into play, and it is seen that we have 



L|M' + M'|A = 0, 

 or 



M[L + L|M' = M|A + A|M', 



which merely proves that the contact electromotive force of metal 

 with air, and with an oxygenated liquid such as water and alcohol, 

 are sensibly equal. 



It would thus be very interesting to measure the electromotive 

 forces in other conditions, and particularly without the intervention 

 of a dielectric. 



1029. The simplest plan would be to repeat the measurements 

 in a vacuum; but the experiments would only be conclusive pro- 

 vided the vacuum were perfect. Pellat, by reducing the pressure 

 to i cm. or 2 cm. of mercury, or by replacing the air by a gas which 

 does not act on the metals, such as nitrogen or hydrogen, only found 

 very slight variations of electromotive force. In the case of copper 

 or of zinc the difference of potential rises when the pressure di- 

 minishes, and, the variation is greater for oxygen than for hydrogen. 



