178 SOURCES OF ELECTRICITY. 



By bringing all the terms within the first member, the equation 

 becomes 



A]B + B|C + +L|M + M|A = 0, 



which amounts to saying that the two ends of any chain which is 

 terminated by identical metals are at the same potential. 



This important proposition is a necessary consequence of the 

 principle of the conservation of energy. If the terminal metals 

 A and A', of the same kind, could be kept at different potentials 

 by intermediate contacts, then if they were joined by a conductor 

 of the same kind, a continuous discharge would take place both in 

 the external conductor and in the chain of intermediate metals 

 that is to say, a permanent flow of electricity. This transference of 

 electricity, analogous to a succession of discharges, would, as a 

 necessary consequence, produce calorific phenomena that is to say, 

 energy which would be a realisation of perpetual motion. 



If even we suppose that the disengagement of heat in certain 

 parts of the circuit, corresponds to an absorption in other parts, there 

 would be a transport of heat from the colder to the hotter parts, 

 without any corresponding work. This result is incompatible with 

 Carnot's principle, which appears as well established in science as the 

 impossibility of perpetual motion. 



190. EXCEPTION TO THE LAW OF SUCCESSIVE CONTACTS. 

 ELECTRICAL BATTERIES. Volta found that the law of tension 

 sometimes ceases to apply. We observe, in fact, that it does not 

 appear necessary as long as there is no source of energy in the 

 circuit ; but if this contains sources of energy of any kind whatever 

 for instance, bodies which may give rise to exothermic reactions 

 correlated to the passage of electricity the energy furnished by 

 these reactions, might assist in keeping up a permanent current. 



Without having very accurate ideas on this point (he was prepared 

 to find metals which did not satisfy the law), Volta had been led to 

 divide bodies into two great classes. The first contains those which 

 obey the law of tensions ; it comprises all the metals and a certain 

 number of solids. The second contains those which do not obey 

 this law : it comprises most liquids and solutions. By associating 

 bodies of the first class with those of the second, a chain can be 

 constructed the ends of which, though formed of the same metal, 

 present a finite difference of potential. 



Experiment shows that in this case also, Volta's fundamental law 

 is verified that is, that the electromotive force corresponding to 



