In 1821, Estonian-German physicist Seebeck demonstrated the electrical potential in the juncture-points of two dissimilar metals when there is a heat difference between the joints. This was the thermoelectric effect and is known as the Seebeck Effect in Physics. It is the basis of the thermocouple and is considered the most accurate measurement of temperature.
Thomas Johann Seebeck was born in Reval (now Tallin), the capital of Estonia, that was then part of East Prussia, on 9 April 1770, into a wealthy merchant family. His father was German descent with ancestral roots in Sweden and perhaps because of this he encouraged Thomas to study medicine in Germany which he did at the Universities of Berlin and Gottingen. He received his medical degree in 1802, but since he preferred research in physics to the practice of medicine he went into education and research as a career.
He is best known as a physicist. After graduation he joined the faculty of the University of Jena where he met and became a good friend of Johann von Goethe. Inspired by the Romantic movement in Germany and the anti Newtonian theory of colors of Goethe he worked with Goethe on the theory of color and the effect of colored light. Seebeck then engaged in research of the solar spectrum.
He uncovered the effects of heating and chemicals on different colors of the solar spectrum in 1806. In 1808, he obtained the first chemical combination of ammonia with mercuric oxide. By 1812, he was studying optical polarization in stressed glass, but his discoveries in this area were discovered earlier, unknown to him, by scientists Brewster and Biot.
Seebeck returned to the University of Berlin around 1818 as faculty where he worked independently on the magnetization of iron and steel when electrical currents were passed through conductors. The magnetic effects of electricity on iron and steel had just been discovered by Arago and Davy. In numerous experiments on the magnetizability of various metals, he observed the anomalous reaction of magnetized red-hot iron, which eventually resulted in the phenomenon now known as hysteresis.
Seebeck made investigations into photoluminescence (the luminescent emission from certain materials excited by light), the heating and chemical effects of different parts of the solar spectrum, polarization, and the magnetic character of electric currents.
In early 1820, Seebeck searched experimentally for a relation between electricity and heat. In 1821, he joined two wires of dissimilar metals (copper wire and bismuth wire) to form a loop or circuit. Two junctions were formed by connecting the ends of the wires to each other. He then accidentally discovered that if he heated one junction to a high temperature, and the other junction remained at a cooler temperature a magnetic field was observed around the circuit of different temperatures.
He did not recognize, believe, or report that an electrical current was being generated when heat was applied to one junction of the two metals. He used the term thermomagnetic currents or thermomagnetism to express his discovery. During the following two years, 1822-1823, he reports on his continuing observations to the Prussian Academy of Sciences, where he describes this observation as “the magnetic polarization of metals and ores produced by a temperature difference.”
Seebeck is credited with the discovery of the thermoelectrical effect, but he used his discovery to incorrectly conclude that the earth’s magnetic field was produced by the temperature differences between the two poles and the equator. This is Seebeck’s theory – the temperature gradient causes the direct magnetization of the metals, and the newly magnetized metal presents a surrounding magnetic field which influences the nearby magnetic needle making it deflect – there is no electric current involved at all.
Seebeck is quite annoyed at the scientific community for suggesting that his temperature gradient causes an electrical current, which then produces a magnetic field around the wire. He attacks their position by complaining that the Oersted experiment has blinded the scientists, who now interpret everything in the light of electrical currents causing magnetic fields, and they are unable to think or reason otherwise. But Seebeck’s argument fails to explain why there is no magnetic field produced by the temperature gradient, when the circuit is electrically broken by materials that are nevertheless good thermal conductors. So the popular thermo-electric viewpoint prevails, and later confirmed, that an electrical current is indeed produced by the temperature difference between the two junctions of a closed bi-metalic curcuit.
He experimented with different metals, different structures (shapes) of the same metal and found the effects of electrical currents or, in his case deflections of his magnetic needle, when their junctions were heated. He even found that electrical currents flowed if one portion of a wire was hammered or twisted while the other portion of the same wire was not reshaped.
Electrical current flowed continuously around the circuit created when two different metal wires were joined together by a soldered junction and then heated.
This continuous flow of current with heat was different than that of voltaic current that he was so familiar with. The flow of current was formed at the soldered junction of the two metals whose temperatures differed at the soldered joint. Seebeck published his findings about thermomagnetism in 1822-1823 as “Magnetische Plarisation der Matalle und Erze durch Temperatur-Differenz. Abhandlungen der Preussischen Akad, Wissenschaften, pp 265-373.”
Experimenting further, he soldered a bar of antimony to a bar of bismuth and joined their ends. The magnetic needle deflected from bismuth to antimony when one soldered joint made by the two different metals was heated. The magnetic needle deflected in the opposite direction (antimony to bismuth) when the soldered joint was cooled.
Later the observation was made that if metals were arranged according to their heating contrasts a series was formed: antimony, iron, zinc, silver, gold, lead, mercury, copper, platinum, and bismuth. The greater the heating contrasts between metals, the greater the electromotive force (EMF). Antimony and bismuth formed the best junction for EMF.
Seebeck also formed a circuit composed of copper and bismuth conductors (wires) in which he held one junction of the metals in one hand, and observed that the needle deflected from the difference in temperature of the metallic junctions caused by the heat of his hand. He experimented more by cooling one of the metallic junctions, and observed the same effect of an electrical current flowing in the circuit.
Dr. Robert Hare, professor of chemistry at the University of Pennsylvania, Philadelphia, in 1822 had the English instrument maker Pepys construct a very large galvanic battery for him to use in electrical experiments. Dr Seebeck constructed a similar battery in 1823 independently of Pepys’ battery for Hare. About the same time Dr. Seebeck showed that the power of multiplication did not increase with the number of windings in the uniting wire of a coil.
The resistance to current passage naturally increased with the length of the wire used, thus the conducting current is reduced rather than proportional to the number of windings as claimed by some at the time. Seebeck’s observation was in response to Schweigger’s newly constructed galvanometer.
In addition to the impact of thermoelectricity on theory, this effect is used in thermocouples to measure temperature. Seebeck’s observation remained rather obscured for a hundred years until Shockley and associates invented the semiconductors.
Seebeck devised thermocouples; used thermoelements to measure temperature, built a polariscope (device to measure polarized light); studied heat radiation, and the rotary effect of sugar solutions on plane polarized light.
He married and had at least one son, Louis Frederick. He became a member of the Berlin Academy of Sciences, and the French Academy of Sciences in 1825. Thomas Seebeck at the age of 61 years died in Berlin, Germany, on 10 December 1831.
The voltage difference, dV, produced across the terminals of an open circuit made up of a pair of dissimilar metals, A and B, whose two junctions are held at different temperatures, is directly proportional to the difference of the hot and cold junction temperatures, Kh – Kc, and does not depend in any way on the distribution of temperature along the metals between the junctions. The factor of proportionality, SAB, is called the relative Seebeck coefficient, thermoelectric power, or just thermopower, of the bi-metalic couple, and in general this coefficient also varies with the level of the temperature at which the temperature difference occurs.
If the circuit is closed, a current will flow in the metals, which can be detected by the magnetic field produced around the wires, or by the joule heating produced by the resistance in the wires, or closing the circuit with a capacitor or condensor of sufficient capacity to accumulate a measurable charge for the transient current which will flow in this case, or by a galvanometer or ammeter placed in the circuit to measure the current, or by measuing the amount of chemical substance deposited at the positive and/or negative electrodes in an electrochemical cell.