[quote="kpw"]In an article published by a respected educational journal the author makes the repeated assertion that the Seebeck effect has "nothing to do with" the contact potentials which exist between dissimilar conductors. (1)

However, informed thermoelectricians recognise that the open circuit (Seebeck) voltage existing in a circuit formed when two dissimilar conductors with Seebeck coefficients SA and SB respectively are joined together at their ends, and the junctions are maintained at temperatures T1 and T2, is given by the equation in the attached document.

Does this equation not also define the difference between (or rather, since they act in opposite directions, the algebraic sum of) the contact potentials at the two junctions?

Keith P Walsh

(1) "Thermoelectric effects and contact potentials", R G Chambers, Physics Education, September 1977.[/quote]

I have found someone else who appears to believe that the thermoelectric potential produced whenever the junctions of a bi-metallic circuit are maintained at different temperatures is not related to the contact potentials at the junctions.

In his paper entitled "The origin of the thermoelectric potential" J Jackle of the Department of Physics at the University of Konstanz in Germany describes in some detail how the conduction electrons in the vicinity of a single isolated junction between two dissimilar metals are able to establish a state of thermodynamic equilibrium when the two metals are brought into contact.

See:

http://www.uni-konstanz.de/physik/Jaeckle/papers/thermopower/node4.html

However, he then argues that the contact potentials cannot be responsible for driving the current in the thermoelectric circuit by apparently assuming that the equilibrium states of the electrons at both junctions are independently maintained when the circuit is formed.

Quote:

"Since the conduction electrons on both sides of a junction are in thermodynamic equilibrium, the contact potentials - more precisely: the difference of contact potentials between two such junctions - cannot drive an electric current. In conclusion, the electric current which flows in a short-circuited thermoelectric circuit cannot be explained by the difference of contact potentials which results from the temperature difference between the junctions."

Is this assumption not flawed?

(For a start, does not the fact that a current always flows in the circuit whenever the junctions are maintained at different temperatures indicate that under these conditions the conduction electrons are no longer in a state of thermodynamic equilibrium anywhere in the circuit?)

I hope that someone can contribute an opinion on this. (How about you Cronin?)

Keith P Walsh

Some other aspects of Jackle's argument don't make sense either.

Consider his following statement:

"However, the voltmeter in the thermoelectric circuit drawn in Fig. 1 measures the difference between the electrochemical potentials at the exits a and b.
In the second expression of eqn. (10), the difference mu(b) - mu(a) vanishes, since the exits of the voltmeter are of the same metal at the same temperature. Therefore the thermoelectric potential measured in the thermoelectric circuit of Fig. 1 is given by the purely electrostatic potential difference between the two exits of the voltmeter!"

eqn. (10) can be found at:

http://www.uni-konstanz.de/physik/Jaeckle/papers/thermopower/node4.html

Fig.1 is at:

http://www.uni-konstanz.de/physik/Jaeckle/papers/thermopower/node1.html

Jackle appears to be offering the above statement as some sort of evidence that the measurement of the electrostatic potential at the voltmeter is completely independent of the contact potentials which arise at the junctions between conductors A and B.

But the measurement recorded by the volmeter is not a property of conductor A. It is a characteristic of the whole circuit, and its value is dependent upon the presence of ANY physical phenomenon in the circuit which acts to produce an electromotive force in the circuit (for example, the contact potentials which arise at any juntion between two dissimilar metals).

When I was in high school we constructed electrical circuits with copper wire and electric cells, and we measured the resultant emfs in the circuits with a voltmeter. If we had four 1.5 volt cells arranged in series and with common polarity our voltmeter, measuring within the limits of its calibrated accuracy, would read about 6 volts. If we turned one of the cells around our voltmeter reading would change to about 3 volts. If we turned two of the cells around the voltmeter reading would fall to about zero volts. And if we turned all of them around the reading would be 6 volts again, but in the opposte direction.

If I had suggested that the measurements we had witnessed could not possibly have had anything to do with the electromotive forces generated by the cells because the voltmeter was not connected directly to them, and that the readings must therefore be measuring some property of the copper wire which just happens to be changing by some inexplicable coincidence to match the sum of the total emf given by the cells, I would have been laughed out of the class - and rightly so.

So shouldn't J Jackle have been laughed out of the University of Konstanz?

And if not, why not?

Has the thermoelectric community been persuaded to abandon the established principles of electrical circuit theory in order to accommodate the "it-cannot-possibly-have-anything-to-do-with-contact-potentials" sect?

And if so, why?

Jackle goes on to say:

"The difference of contact potentials usually agrees with the thermoelectric potential only in its order of magnitude, but not necessarily in its sign."

I disagree with this statement. I think that the sign of the thermoelectric potential will always be consistent with the algenbraic sum of the contact potentials in the circuit.

Does anyone agree with me?

Keith P Walsh

PS, Has anyone ever come across any rational and scientific argument for suggesting that the thermoelectric potential generated in a thermocouple circuit (as exemplified by Jackle's Fig.1) is not related to the difference between the contact potentials at the two junctions?

Can you help me to build a thermoelectric generator of around 4.5 Volts using the heat of kerosene lantern.

I can't see the content of these links.

eqn. (10) can be found at:

http://www.uni-konstanz.de/physik/Jaeckle/papers/thermopower/node4.html

Fig.1 is at:

http://www.uni-konstanz.de/physik/Jaeckle/papers/thermopower/node1.html
Thanks
Hersi