72 PROCEEDINGS OF THE AMERICAN ACADEMY. 



The Thermodynamic Point of View. 



The greater part of my argument from here on will lie in a thermo- 

 dynamic treatment of thermoelectric action, and in the course of it 

 I shall make free use of the fact, which I have pointed out in previous 10 

 papers, that the ordinary thermoelectric diagram, representing 

 "thermoelectric heights" as functions of temperature, is in reality a 

 temperature-entropy diagram. Following ordinary engineering prac- 

 tice, I shall take the temperature coordinate as vertical and the en- 

 tropy coordinate as extending horizontally toward the right. 



Putting aside for the present the consideration of part (A) of the 

 electric current, I shall discuss part (B) at some length as if it existed 

 alone, returning to the treatment of (A) later. 



Several years ago I discussed the analogy which exists between the 

 cycle described by water in the circuit of a heating system, or in the 

 circuit of a steam engine, and the cycle described by electricity in a 

 thermoelectric circuit. The motion of the water in each of the cases 

 referred to is, of course, due to heat, but heat alone would not main- 

 tain circulation. The application or expenditure of the heat must be 

 managed or maneuvered by agents which do none of the net work of 

 the cycle. In the case of the steam engine this control is given by a 

 system of valves or checks which permit movement in one direction 

 but not in the other. In the heating system, valves and checks may 

 be dispensed with, gravity exercising what in chemistry would be 

 called the catalytic function of maintaining the desired action, circu- 

 lation, at the expense of heat. In the thermoelectric circuit we must 

 look for some agency to perform a like service. Thus, if we have a 

 detached piece of copper with one end at temperature T and the other 

 at a lower temperature T' , a state of equilibrium exists within it such 

 that there is no electric flow along the metal; it is the same with a 

 detached piece of iron wire having its ends at the same temperatures, 

 T and T'; but, if we join the warm end of the copper to the warm end 

 of the iron and the cold end of the copper to the cold end of the iron, 

 we find that a current of electricity flows from copper to iron at one 

 junction and from iron to copper at the other junction. It is quite 

 evident that, if we had to do with electric potential only, in the ordi- 

 nary sense of the term, either there would be no flow on bringing the 

 wires into circuit or there would be flow in the same direction at both 



10 For example, These Proceedings, 46, 649 (1911). 



