Thermocouple

When two dissimilar metals are put into contact with each other, a small voltage in the millivolt range is generated. This junction emf is temperature dependent and can be used as a thermometer. If a loop is made from two lengths of wire of dissimilar metals, and the two junctions placed at different temperatures, a small current will flow around the wire loop. A practical thermometer can be constructed by putting one junction at a standard temperature, say an ice bath, and the other at the temperature to be measured. Iron is often one of the materials used, and a common material with which to pair it is constantan. This combination gives a voltage on the order of 5 millivolts when one junction is at 0 C and the other is at 100 C.

Junction Emfs

When two dissimilar metals are put into contact with each other, a small voltage in the millivolt range is generated. This junction emf is temperature dependent and can be used as a thermometer by making up a thermocouple from two wires of dissimilar metals.

The production of a junction emf is associated with the difference in the Fermi levels of the two metals. If an electron in a metal is at the Fermi level E_F of energy in a metal, then a certain amount of energy is required for the electron to just escape from the metal, and this energy is referred to as the "work function" φ of the metal. Dissimilar metals will in general have different work functions, so if such metals are brought into firm contact where electrons can transfer back and forth between them, there will be an effective emf or voltage between them equal to the difference in the work functions φ_A - φ _B. If there is a conductive path to form a complete circuit, then electrons will flow from the the metal of higher Fermi level to the one with lower Fermi level.

However, if the complete electric circuit is made with wires of the same metals joined together at both ends, and the two junctions are at the same temperature, there will be no net flow of electrons around the circuit since the two junction potentials cancel each other. Useful devices can be made by maintaining the two junctions of such a circuit at different temperatures.

The widespread use of thermocouples arises from the measurement of the junction potential since it is temperature dependent. However, the temperature dependence is nonlinear and must be empirically calibrated to give an accurate indication of temperature. This temperature dependence of the junction potential was discovered by Seebeck in 1821, so is often called the "Seebeck effect". Typical voltages for a single junction are on the order of 10^-6 volts/K, so to get a higher signal voltage a number of them are often combined in series in a temperature probe.

With a large number of junctions and a sufficient amount of heat, a thermoelectric generator can be produced. Since space missions require an extended period of electric power production, the use of the heat from radioactive decay has been used to create Radioisotope Thermoelectric Generators (RTGs). At the time of landing of the Mars Science Laboratory in August of 2012, the United States had launched 45 RTGs as parts of 26 space missions.

The "Peltier effect" is the converse of the Seebeck effect and involves the use of a driving voltage to force current through the junction of dissimilar metals. It results in the cooling of the junction if forced to flow in one direction, and heating of the junction if it is forced in the opposite direction. This is used to advantage to create small thermoelectric refrigerators. Ohmic electric heating will be superimposed on the Peltier effect at the junction, which is an advantage if heating is your intent, but works against your thermoelectric refrigerator.

Another effect with metallic conductors which is related to the Peltier and Seebeck effects is called the "Thomson effect". It is a heating or cooling effect in a metal if current is forced to flow along a thermal gradient that is maintained in the metal. The effective kinetic energy of electrons near the Fermi level is slightly higher for higher temperatures. If an emf causes an electron flow from a cooler to the warmer end of a metal rod, the electrons will need to gain some kinetic energy so they will be in equilibrium at the warmer end. They must get this energy from the thermal reservoir of the crystal lattice, so there will be some cooling effect on the metal lattice at that end.