In 1821, T. J. Seebeck observed the existence of an electromotive force at the junction formed between two dissimilar metals. This phenomenon is named after its discoverer and is known as the Seebeck effect. The Seebeck effect is actually the combined result of two other phenomenon named after their discoverers: Thomson and Peltier. Thomson observed the existence of an electromotive force due to the contact of two dissimilar metals and the junction temperature. Peltier discovered that temperature gradients that exist along the conductors in a circuit generated an electromotive force. The Thomson effect is normally much smaller in magnitude than the Peltier effect and can be minimized and disregarded through correct thermocouple design.
Figure 3A Typical thermocouple circuit
A typical thermocouple circuit is shown in Figure 3. Two conductors and two junctions are required. One of the junctions is kept at a known or reference temperature, typically known as the cold or reference junction. The other junction is known as the hot or measuring junction. The relationships between temperature and voltage are determined empirically and can be expressed as:
V =a(T1 - T2) + g(T12 - T22)
wherea and g are constants for each of the two materials.
The sensitivity of the thermocouple system, also known as the thermoelectric power is obtained by taking the derivative of the voltage with respect to T1:
S =a + 2gT1
Table 2 Properties of selected thermocouples
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Please send questions or comments to Professor Ron Adrezin or Professor Daniel Raichel.