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Thermocouples

Dateline: 07/24/00

By Alan Bruzel

Electrical Conduction

Electrical energy generates heat energy in a conductor by following both irreversible and reversible pathways. Irreversible loss of heat occurs through Joule heating (named for the physicist James Prescott Joule, who described this process), and is an inevitable cost of electrical conduction. However, there are three pathways that allow reversible conversions between heat energy and electrical energy: the Seebeck effect, the Peltier effect, and the Thomson effect. Thermocouples exhibit these three phenomena in addition to Joule heating.

What Are Thermocouples?

Thermocouples are devices constructed by forming a loop of two different electrically conducting materials; either metals or semiconductors may be used. At a thermocouple's two junctions, electrons from the electron-rich component move into the electron-poor component as they try to establish electrical equilibrium.

The Seebeck Effect

In 1821, Thomas Johann Seebeck found that when the two junctions of a thermocouple are held at different temperatures, an electromotive force (voltage) propagates through the thermocouple. By noting the voltage produced while holding one junction at a constant temperature and varying the temperature at the other junction, one may calibrate the thermocouple. This forms the basis of the most common application of thermocouples today: quickly measuring temperature, generally over a wide range. A Type J iron and nickel/copper thermocouple, for example, has a usable range of from 0^o to 816^oC (32^o to 1500^oF).

The Peltier Effect

During his study of the Seebeck effect, Jean Charles Athanase Peltier, in 1834, discovered an opposite phenomenon: applying voltage to the thermocouple caused the temperature of one junction of the thermocouple to increase and the temperature of the other junction to decrease. The availability of semiconductors in the mid-twentieth century allowed the Peltier effect to finally become commercially practicable. Here, electron movement removes heat from one side of the thermocouple (the side that is to be kept cool) and transfers the heat to the other side, which is fastened to a heat sink. Simply by changing the polarity of the applied voltage, electrons (and heat) move in the opposite direction. Heating and air conditioning can thus be performed by one appliance.

The Thomson Effect

If a temperature gradient is established along a conductor, and an electric current then flows through that conductor, the temperature distribution is altered over and above what would be caused by Joule heating alone. This phenomenon was discovered in 1854 by William Thomson (later Lord Kelvin), and may be observed in one wire – two dissimilar conductors are not mandatory.

What the Web Has to Say about:
Thermocouples

An Introduction to Thermoelectric Coolers
Background and performance characteristics of semiconductors demonstrating the Peltier effect. From Sara Godfrey, Melcor Corporation.

An Intuitive Introduction to Three Effects in Thermoelectricity
Explanation of the Seebeck, Peltier, and Thomson effects. From Felix Lu, Cables Direct.

Introduction to Thermoelectric Cooling
Circuit diagrams illustrating principles involved in thermocouples. From Ferrotec Corporation.

Peltier-Effekt und Andere Thermoelektrische Phänomene
Insights into Peltier, Seebeck, and Thomson effects. From Stefan Ambühl, Arthur van Dorp, and Christoph Rüegg.

Thermocouples
Specifications and practical considerations in using these devices. From Branom Instrumentation.

Thermocouples
Calculating electromotive forces in connected thermocouples. From Alan Weston, Southern Illinois University.

Thermocouple Types
Chemical compositions and specifications. From Thermometrics Corporation.

Thermo-Electric Technology
Definitions of Joule heating, and Peltier, Seebeck, and Thomson effects. From Steve Spence.

Thermométrie par Thermocouple
Properties and recommended uses of thermocouples. From C. Caron.

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