The Peltier effect is the reverse phenomenon of the Seebeck effect; the electrical current flowing through the junction connecting two materials will emit or absorb heat per unit time at the junction to balance the difference in the chemical potential of the two materials. Thanks to this effect, an electronic refrigerator can be made, which is known as the Peltier cooler. The Peltier cooler has been applied to niche areas such as infrared detectors, CPU coolers, wine cellars, etc., because the cooling power is lower than that of compressor-based refrigerators. This technique is called thermoelectrics, and is discussed in the next section.Similarly to the Seebeck coefficient, the Peltier coefficient ∏ is defined as the coefficient of the thermal current Q to the electrical current I, given by