Why Seebeck Needs Two Conductors

Why Seebeck Needs Two Conductors

The celebrated American physicist Richard Feynman took pains to explain the idea that simply knowing the name of any particular natural phenomenon does not necessarily impart any meaningful understanding of what that phenomenon is or does.

In keeping with this philosophy, for those present-day scientists who are engaged in the development of new thermoelectric materials and devices the correct attribution of the fundamental thermoelectric phenomena to the names of the nineteenth century scientists who first identified them may be of little or no consequence.

However, I believe that in the context of enabling the wider public to gain an accurate understanding of the nature of the fundamental thermoelectric effects, it is important that we appreciate how these phenomena were discovered, and by whom.

It may help in explaining this point to restate the definitions of the four fundamental thermoelectric effects as follows:

Effect 1 - If two different conductors are joined and the two junctions are maintained at different temperatures, an electromotive force is developed in the circuit.

Effect 2 - If a current flows in a circuit consisting of two different conductors then one of the junctions is heated and the other is cooled.

Effect 3 - When a temperature difference exists between two points in a single electrical conductor an electrical potential is established between the points.

Effect 4 - If a current passes through a conductor in which a temperature gradient exists, this current causes a flow of heat from one part to the other.

And it may also be instructive in describing the relationship between these four effects if they are arranged in the attached two-way table.

The fact that the effects involving dissimilar conductors in contact with each other (Effects 1 and 2) were discovered appreciably earlier is not without significance. In general, Effects 1 and 2 are more significant (i.e. greater and more noticeable) than their single-conductor counterparts. This explains to some extent how Thomas Johann Seebeck was able to discover, apparently by accident, Effect 1 whilst never appreciating the existence of Effect 3.

Legend has it that Seebeck’s accidental discovery of Effect 1 occurred when he noticed that a compass needle was deflected in the vicinity of a circuit constructed of two dissimilar materials when the two contacts between the two conductors were maintained at different temperatures.

This deflection was of course the result of the electromagnetic field generated by the current which flows continuously in such a circuit. I know of no reports to suggest that Seebeck ever noticed, or even attempted to detect, any similar deflection of the compass needle in the vicinity of a single electrical conductor with its ends maintained at different temperatures.

However I’m certain that thermoelectricians will recognise that the detection any such effect in a single conductor by this means would be much, much, more difficult – as well as being much more unlikely to happen by accident.

This is because in the case of the single conductor (Effect 3), any electric current (movement of electric charge) only occurs momentarily whilst the charge carrying electrons in the conductor are redistributed in order to re-establish equilibrium against the applied temperature differential.

So, unlike Effect 1 where the electric current continues to circulate and influence the local electromagnetic field accordingly for as long as the temperature differential is maintained, any disturbance of the local electromagnetic field caused by Effect 3 is not only smaller, but infinitesimally short-lived.

The electrical potential produced by Effect 3 was first deduced and demonstrated by William Thomson more than 25 years after Thomas Johann Seebeck’s death. To suggest that it was:

“ … Thomas Johann Seebeck in 1821, who found that a voltage existed between two ends of a metal bar when a temperature gradient dT existed in the bar.”

- (as the current entry for “Seebeck effect” at Wikipedia does), not only denies William Thomson the correct attribution for a major part of his own contribution, but it denies the general public of the present day an opportunity to gain a crucial insight in to how the various thermoelectric effects are related to each other (as described above).

I put it to interested thermoelectricians and the members of the International Thermoelectric Society that any definition of the Seebeck effect should always stipulate that this effect requires two (or, more generally, at least two) dissimilar conductors to produce.

Anybody disagree?

Keith P Walsh