ELECTROMOTIVE FORCES IX THE VOLTAIC CELL. 499 



Note that the difference of level in this case implies no difference of 

 potential, and as before no work is required to transfer water between A 

 and B. Hence it is not easy to distinguish this case from the former, and 

 this difficulty of distinguishing between the two cases is what has given 

 rise to most of the confusion. The only easy criterion is the non-existence 

 in the second case of any Peltier effect at the junction C. Naturally it is 

 possible and common for the two effects to be superposed, but they are 



•tially independent. 



Since the two vessels in the second case are at the same potential, the 

 way to observe the effect is to cut and seal the pipe at C. and then show 

 that the vessels are differently charged ; which is what Volta did. The 

 model does not indeed represent the gradual change of potential induced 

 as the distance between the condenser plates increases, and it is scarcely 

 worth while to complicate the matter by making a more elaborate model. 

 The thickening of the dielectric layer of a condenser, when its plates arc 

 separated, corresponds exactly to the thickening and strengthening of an 

 elastic membrane ; and rise of potential in the one case is accurately repre- 



table by increase of pressure in the other; but such considerations be- 

 long to general electrostatics, and have no special bearing on our present 

 subject. 



12. This is perhaps the most convenient place to introduce the notes or 

 condensed statements which I drew up and distributed at the meeting 

 before the discussion. They were intended to be critically exact (allowing 

 of course for mistakes and possible slips) so as to bear analysis, and hence 

 it is probably worth while to reproduce them here with notes and com- 

 ments. 



I. — C tfEllENTS RELIEVED BY THE WHITES TO BE TOTE IX 



THE FOIIM HEBE SET DOWN. 



A.— Volta. 



i. Two metals in contact ordinarily acquire opposite charges ; ' for 

 instance, clem zinc receives a positive charge by contact with copper, of 

 Mich a magnitude as would bo otherwise produced under the same cir- 

 cumstances by an E.M.F. of about S volt. 



ii. This apparent contact E.M.F. or ' Volta force 'is independent of 

 all other metallic contacts wheresoever arranged ; hence the metals can be 

 .arranged in a numerical scries such that the ' contact force ' of any two 

 is equal to the difference of the numbers attached to them, whether the 

 contact be direct or through intermediate metals. But whether this series 

 changes when the atmosphere, or medium surrounding the metal, changes 

 is an open question ; on the one side are experiments of De la Rive, 

 Brown, Schnltze-Berge ; on the other side, of Pfaff, Pellat, Thomson, Von 

 Zahn. 2 It certainly changes when the free metallic surfaces are in the 

 ghtest degree oxidised or otherwise dirty. And in general this ' Volta 

 force ' is very dependent on all non-metallic contacts. 



in. In a closed chain of any substances whatever, the resultant E.M.F. 

 is the algebraic sum of the Volta forces measured electrostatically in air 



_ ' Observe that i! is not said that two metals in contact acnuire different poten- 

 3. -uek difference of potential I believe to be only apparent. Compare rigs. 1 ! 

 ^in<_L i.j. 



- I put Von Zahn on that aide because he himself considers himself there, i 

 because the great bulk of his experiments lean decidedly that way. 



K K 2 



