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Profs. W. E. Ayrton and John Perry. [Mar. 21, 



little diminished by this charging, and we have no right to say that 

 the total amount of possible chemical action at the contact of two 

 metallic surfaces is simply equivalent to the energy of the charges. 

 When A and B are solid their rigidity limits the possible amount of 

 chemical action ; but even when they are both metals if one is a liquid 

 the total amount may be very considerable. With metals there is a 

 limit produced by the want of mobility of the alloy, and when the 

 liquid is not metallic there is a limit produced by the compound being 

 insoluble, and so preventing free contact. After the charges are 

 established the electromotive force of contact is exactly balanced, and 

 there is no further action of any kind. If, however, A or B be a non- 

 conductor, this establishment of charge may take a considerable time. 

 In any case there can only be an exceedingly small amount of chemical 

 action between any two_pwe substances, as for examp]e, pure zinc and 

 pure sulphuric acid. 



If A, B, and G be any three substances insulated from other bodies, 

 A and C being each in contact with B, but not in contact with each 

 other, the electromotive forces of contact AB and BC charge the three 

 bodies in such a way that the difference of potentials between A and B 

 is equal and opposite to AB, and the difference of potentials between 

 B and C is equal and opposite to BC. The energy required to produce 

 this charging of the bodies is due to chemical action at the places of 

 contact. After the three bodies are so charged there cannot be any 

 further chemical action, and the total amount of possible chemical 

 action at these places of contact is very little diminished by the 

 charging. 



Prima facie we cannot compare the present difference of potentials 

 between A and C with the electromotive force of contact that would 

 exist between A and were they brought together. 



The establishment of the above differences of potential takes a shorter 

 or longer time, as the bodies are more or less conducting. 



If now we imagine A and C to be brought into contact, as we do 

 not know the value of AC, we cannot predict the amount of distur- 

 bance produced in the charges of A, B, and C ; but we may assume 

 that some disturbance is produced, that is, a current of electricity flows 

 in the circuit. We can also say that after the possible chemical energy 

 at the various junctions is exhausted, as well as the potential energy 

 equivalent to bringing A and C in contact, then, if the temperature of 

 the circuit is eveiy where the same, there can be no further current, 

 since there is no further supply of energy. 



It is known that in a compound solid metallic circuit, which is every- 

 where at the same temperature, there is no continuous chemical action 

 at the junctions, and consequently by the law of conservation of 

 energy there cannot be a continuous current. Hence it has been usual 



