LAW OF SUCCESSIVE CONTACTS. 177 



difference of potential is set up, which depends on their nature, and 

 which is altogether independent of their dimensions, of their shape, 

 of the extent of surface in contact, and of the absolute value of the 

 potential on each of them. 



Volta characterized this property by saying that there is a tension 

 of contact between the two surfaces, but the manner in which he 

 conceived the phenomenon is quite in agreement with the idea of a 

 difference of potential. 



We shall represent this characteristic difference of potential, or 

 electromotive force of two metals A and B, by the symbol A|B, the 

 first letter denoting the metal whose potential is highest. We have 

 then 



We may at once add that this difference is a function of the 

 temperature, and that the contact of two bodies of the same kind, 

 but at different temperatures, also gives rise to an electromotive 

 force. 



In all the questions relative to the electrical equilibrium of 

 conductors, we have hitherto neglected the electromotive forces 

 due to the contact of heterogeneous conductors. All the calcu- 

 lations presuppose that the conductors are identical and at the 

 same temperature, and the results should be modified by allowing 

 for this new circumstance, unless in the case of very high poten- 

 tials, where the effects of contact may be neglected. 



189. LAW OF SUCCESSIVE CONTACTS. After having confirmed 

 the fundamental fact of the electromotive force of contact, Volta 

 compared with each other the results furnished by different metals, 

 and established experimentally a second law, which may be thus 

 enunciated : 



WJien several metals at the same temperature are soldered to each 

 other so as to form a continuous chain, the difference of potentials of the 

 extreme metals is the same as if these two metals are in direct contact. 



Let A, B, C ..... L, M be the metals constituting the chain ; this 

 law, with the symbols adopted above, is represented by the following 

 formula : 



A|B + B|C ..... +L|M = A|M. 



We have, further, 



A|M= -M|A. 



