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The Peltier effect, discovered by Jean Peltier in 1834, is an important Thermoelectric Phenomenon that relates to the energy transfer (positive or negative) that occurs, over and above Joule Heating, at the junction of two dissimilar materials when an electric current passes through it. When the junction is maintained at a given temperature, the Peltier—effect results in the equivalent of a heat addition or heat removal the Peltier heat—which is reversible and is proportional to the current. Thus,

where π is the Peltier coefficient and I is the electric current. Note that π depends on the materials forming the junction and the temperature.

The Peltier effect is one of the key phenomena (along with the Thompson effect) determining the emf generated in a thermocouple used for temperature measurement. For a thermocouple of materials A and B, with one junction at a constant temperature and the other at (absolute) temperature T,

where εAB is the thermocouple emf generated at the junction of materials A and B. This equation can be used to calculate the Peltier coefficient for the combination of materials A and B.

The Peltier effect is used in thermoelectric (Peltier) refrigerators or heat pumps. Such devices provide heat removal, sometimes addition, in an easily controlled and reversible device without moving parts. They have been used in space vehicles as well as in a number of small commercial devices for controlling temperature or providing refrigeration, often using semiconductor materials as the thermoelectric elements.

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