The Four Thermoelectric Effects

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Whilst it is generally accepted that there are only three thermoelectric effects, it is in fact possible to describe four.

The four thermoelectric effects, listed in chronological order of their discovery, are:

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.

These effects are very closely related. Indeed, each of them represents a reversible effect whereby effects 1 and 2 are the reverse of each other, and effects 3 and 4 are similarly the reverse of each other.

Thomas Johann Seebeck first identified Effect 1 in 1821. He spent the rest of his scientific career measuring the size of this effect for different pairs of dissimilar conductors in contact with each other. Seebeck died in 1831.

In 1834 Jean Charles Athanase Peltier first identified Effect 2, the reverse of Effect 1. Peltier died in 1845.

Significantly later (around 1854-1855), William Thomson first deduced and demonstrated BOTH of the effects numbered 3 and 4.

Starling and Woodall partly describe Thomson's contribution thus (from "Physics", Longmans, 1950):

"He [Thomson] suggested that there must be other electromotive forces in the circuit and that these exist in the metals themselves, acting between the parts of any one metal at different temperatures. This was found to be correct. Thus if two points in the metal differ in temperature by the amount dT, the electromotive force in this element of the metal is s.dT. The quantity s is called the Thomson coefficient. It is taken to be positive when directed from points of lower to points of higher temperature."

As a result of the above, the four thermoelectric effects are correctly attributed the following names:

Effect 1 is the Seebeck effect.

Effect 2 is the Peltier effect - and is correctly identified as the reverse of the Seebeck effect.

Effects 3 and 4 together comprise both "directions" of the Thomson effect.

My motivation for offering this post is that some recent sources restrict the definition of the Thomson effect to that of Effect 4 only, and that this may be either the cause or the result of a further tendency to prefer that the definition of the Seebeck effect should be satisfied by that of Effect 3. (With the possible consequence of Effect 1 being rendered anonymous.)

Again, it is clear that the relationship between Effect 1 and Effect 3 must be a very close one.

However, it has been demonstrated that during his lifetime Thomas Johann Seebeck could not ever have been explicitly aware of Effect 3.

Furthermore, in the effect which Seebeck spent the greater part of his career measuring, when the junctions between the dissimilar metals are maintained at different temperatures, a net electromotive force exists in the circuit which causes a current to flow around it. Such a circuit cannot be constructed with a single conductor, and therefore the definition of Effect 3 may not serve as an adequate definition for the Seebeck effect.

Apologies if all this is obvious.

Any comments or opinions will be welcome.

I think that it's ok to say that there are only three thermoelectric effects, so as long as people are aware that one of them, the Thomson effect, comprises both "directions" of the reversible effect.

Best regards,

Keith P Walsh

Re:The Four Thermoelectric Effects

Since this posting has attracted no adverse comment with regard to the accuracy or validity of its content after eight weeks and in excess of 350 "views", I have been sufficiently encouraged to publish a version of "The Four Thermoelectric Effects" at the online encyclopedia "Wikipedia" under the title:

"Seebeck-Peltier-Thomson: Who Discovered What"

This article can be found at:

http://en.wikipedia.org/wiki/Peltier%E2%80%93Seebeck_effect

Keith P Walsh

Re:The Four Thermoelectric Effects

I must confess that it is getting a little boring conducting a conversation with myself here.

Nevertheless, this one-man thread is still scoring a significant rate of "Views", so I thought I might up-date readers with news of my submission to the online encyclopedia Wikipedia on the subject of thermoelectric effects.

My piece describing "Who Discovered What" in thermoelectricity was relegated from the "article" page to the less prominent "discussion" page under the topic "Peltier-Seebeck effect.
So it is now to be found under the "discussion" tab at:

http://en.wikipedia.org/wiki/Talk:Peltier%E2%80%93Seebeck_effect

- where, interestingly enough (at least for me), it has attracted a correspondent who appears to be a French scientist specialising in thermoelectric phenomena and who also appears to agree with me that the description of what Seebeck discovered, as presented in the existing Wikipedia "article", is not accurate.

Here's what he says:

"Il you wish (and if you speak french), you can find a detailed description of the thermoelectric effects discoveries there : fr:Thermoélectricité (this article is a brief summary of my thesis introduction). And I can confirm that the statement that Seebeck "... found that a voltage existed between two ends of a metal bar when a temperature gradient dT existed in the bar." is inaccurate : Seebeck found a voltage at the jonctions of of two dissimilar metals. Sincerely, David Berardan"

I've checked out David Berardan's contribution on thermoelectricity at the French language Wikipedia website, and I reckon that French speaking schoolchildren, students, scientists, thermoelectricians, etc., are getting a much more accurate and informed definition of Seebeck's achievements in thermoelectricity from their Wikipedia site than those of us in the English speaking world are getting from ours.

Does anyone at ztforum have an opinion on this?

Could officials at the International Thermoelectric Society help to resolve the apparent discrepancies in the information presented by different Wikipedia sites?

I'd love to hear from you.

Keith P Walsh

PS, Wikipedia is a kind of "democratic" online encyclopedia whereby all entries are contributed, reference, checked, edited and argued by anyone interested enough to bother, or "volunteers". I don't see any reason why such a system shouldn't work quite well. It's already very popular.

Re:The Four Thermoelectric Effects

I think Volta was the first to observe a thermoelectric effect. When he repeated Galvani's frog leg's experiment
to verify the muscle twitch with the bimetallic arc connecting the nerves he also noticed that the effect was influenced by heating the metals. This went unnoticed until some thermoelectric researcher pointed it out some years ago. I don't recall who at the moment.

Seebeck thought he'd found a magnetic effect caused by heat. The other scientists of his time interpreted his results differently, and introduced the thermoelectric argument: Temp diff caused electric current which in turn caused magnetic effect. But, Seebeck argued against this viewpoint.

When Seebeck first discovered the effect in 1821-22 the Voltmeter did not exist. That came later with Ohm 1826 V = IR result. In Seebeck's time potential difference was measured by the static electrometer, which could not measure dynamic voltages, it could only measure the voltage of open circuits.

Re:The Four Thermoelectric Effects

Peter,

Thank you for your reply.

And thank you for reminding me of the Galvani vs Volta frog's leg experiment.

Here's how my high-school physics textbook described their debate:

"Towards the end of the eighteenth century, Luigi Galvani, Professor of Anatomy at Bologna University in Italy, published a book describing a series of investigations he had made on the subject of "animal electricity". In this book he described how a freshly dissected frog's leg could be thrown into muscular convulsions, simply by connecting the foot and the exposed nerves through a length of copper and iron wire (Fig. 34.1 ). It had been known for a long time that the muscles of dead animals could be caused to contract by means of an electric shock from a machine or Leyden jar, and therefore it was suggested that some source of electricity might be responsible for the contractions of the frog's leg.

Galvani himself was of the opinion that the muscle and nerve acted like a kind of charged Leyden jar and that discharge took place when they were joined by the copper and iron wires. His fellow countryman, Alessandro Volta, could not agree with this but believed that the two dissimilar metals were more important than the leg. Subsequently, Volta did some experiments to show that electricity was produced when two different metals were separated by various non-animal liquids and these practically settled the matter in his favour."

(From, ‘Ordinary Level Physics’, Abbott.)

This piece is accompanied by the attached picture.

In spite of the fact that these events took place more than 200 years ago, it appears that there may still exist a degree of unresolved disagreement over the explanation for this phenomenon.

This is because it appears that some 'scientists' automatically presume that the effect is explained by asserting that the two dissimilar metals must become involved in some kind of electrolytic or "electro-chemical" process in order to generate the necessary electrical potential.

However, notwithstanding the fact that there must be certain neurological processes and their and associated "bio-chemical" effects occurring in the muscles of the frog's leg when it twitches, those of us who appreciate the nature of the thermoelectric effect recognise that the necessary conditions exist here for an electrical potential to be established across the frog's leg without there being any requirement for the metal wires to be involved in any kind of electrochemical reaction at all, especially if the frog's leg has been kept in ice and is therefore chilled.

This raises a question; is the thermoelectric emf developed by a single thermocouple junction acting under an ordinary everyday temperature gradient, which might have occurred by chance in a late seventeenth century science laboratory, enough on its own to trigger the neurological synapses in a dissected frog's leg?

Of course, it's always possible that there is a combination of thermoelectricity and electro-chemistry occurring at the same time. (Are the biological fluids in the frog's leg acidic enough to make a significant contribution?)

Personally I would have thought that our metrological capabilities should have advanced sufficiently during the last 200 years for questions such as these to have been answered, and for any debate as to the nature of this phenomenon to have been pretty squarely settled.

I would be most interested to hear if you know of any experimental studies which may have been carried out to this effect.

Best regards,

Keith P Walsh

Re:The Four Thermoelectric Effects

The Volta thermoelectric connection was observed by,

Anatychuk L.I., in "Physics of Thermoelectricity", Institute of Thermoelectricity, Ukraine, 1998. page 11.

Anatychuk reviews a letter written by Alessandro Volta on the 10th February 1794, to the abbot Antonio Mario Vessale on his researches into the Galvani "animal electricity" claims, where Volta writes,

"So, having tried many arcs and on finding one made of iron which was of no use without heating,...I immersed the end of this arc into the boiling water for only about half a minute, extracted it from there and, without letting it cool, resumed the experiment. And only then the frog in the water started contracting, and it even happened two, three, four times when repeating the expriment till the end of the iron which had been earlier immersed into hot water got cold.."

Anatychuk then concludes, "These experiments gave a convincing evidence that A. Volta observed a thermoelectric effect...."

Now it is possible that the electro-chemical action is affected by heating the iron also, so that one would suspect that this effect could be explained entirely by electrochemistry. However, Anatychuk is one of the top Thermoelectric Researchers in the world, so I would presume he carefully considered all the alternative explanations, probably even repeating this experiment in his labs before concluding that it was really thermoelectric in nature. [many chemical reactions require "activation energy" to proceed, so that one guess would be the heat from the iron simply provided the necessary catalytic environment to initiate the electrochemical reactions, etc..but we take Anatychuk's word for it..]

a snapshot from his page 11 is online here
http://ite.cv.ukrtel.net/book/index.html

Note that as early as 1797 Humboldt published his research study of the Galvani-Volta experiments proving that both Galvani and Volta were partly correct. Both "animal electricity" and "chemical electricity" are involved in the frog leg experiment. Is there really a "thermoelectric" part too? Probably. We know that Volta went on to establish without doubt the validity of electrochemical actions by constructing Voltaic Piles. And in the 1830's Carlo Matteucci used Nobeli's static galvanometer, that could measure very small currents, to prove "animal electricity" could be generated by stacking tissue in the same way Volta stacked bimetallic piles, thus establishing without doubt the existance of this biological electricity. But, to answer your question, did anyone ever conduct an experiment to determine, measure, and establish how much of the effect is due to "animal electricity", verses "chemical electricity", verses "thermoelectricity" ? I don't know of any such study. I presume someone has done this. Probably Anatychuk. Maybe someone else can comment on it.

Temperature and heat can influence the generation of electricity in many ways, so experiments have to be carefully designed to isolate the specifically "thermoelectric" contribution. In fact, it's hard to separate electricity from heat, and the influence of one on the other. Even the simple original electric machines, using "friction" to produce charged materials, involve that very mechanical action that produces both "heat" and "electricity" together. [I have a theory that all electric and magnetic phenomena is fundamentally just heat.]

Re:The Four Thermoelectric Effects

Peter,

Thank you again for your contribution.

As I suggested before, I think that anyone who appreciates the nature of thermoelectric phenomena will immediately recognise simply by looking at the picture of the frog's leg experiment that if the points of contact between the copper and iron bars and the extremities of the leg are maintained at a temperature which is significantly different from the temperature at the juction between the two bars, then an electromotive force (e.m.f.) which is purely thermoelectric in origin MUST be established across the frog's leg.

I also believe that it may be possible to demonstrate whether or not this thermoelectric e.m.f. is enough on its own to cause the convulsions in the leg by repeating the experiment with the tips of the copper and iron bars chemically isolated from the frog's leg by being coated with an electrically conductive but corrosion-resistant substance (silver, gold or iridium spring to mind as possible candidates).

If convulsions were still evident then this would demonstrate that it is not necessary for the metal components to become involved in any electrolytic chemical reaction in order for the required electrical stimulus to be generated.

(An alternative approach might be to try and detect the products of any electrolytic activity in the vicinity of the tips of the copper and iron bars in their un-coated state. Given the sensitivity of modern measuring instruments, the absence of evidence of any such products would surely represent a strong indication that no electrolytic reaction involving the metals is taking place.)

I haven't been able to find the results of experiments such as this.

Perhaps these matters have not been considered important enough to warrant investigation.

By the way, I was also interested by the comments you made in your previous post regarding the possible influence of thermoelectric effects in early friction machines used for generating static electrical charges.

The following is a description of the friction surfaces of one such machine:

"The two rubbers, one of which is shewn on the outside, in fig. 48, are triangular pieces of wood, covered with a padding of one or two layers of flannel, enclosed in leather, and they present a flat hard surface to the glass, so that friction between it and them takes place in every part. They are placed in a wooden frame on each side of the plate, and the pressure is regulated by metal springs, fixed to the outside, between them and the frame. Before use, they are covered with an amalgam of mercury, zinc, and tin, which is made to adhere with the aid of a little grease, and which increases immensely the production of electricity."

Ferguson [1873], pp. 75-79

Do you think that the "amalgam of mercury, zinc and tin" may have been employed for its thermoelectric properties?

Keith P Walsh

PS, in my last post I implied that Galvani and Volta made their contributions in the late seventeenth century. It was of course the late eighteenth century.