Thermocouple
In 1821 the German scientist Thomas Seebeck (1770-1831) was experimenting with the thermal properties of certain metals. He constructed a loop of two different metals and applied heat to one of them. To his surprise a magnetic field was formed, as if a current had been generated between the two metals. This phenomenon was named the Seebeck effect, even though Seebeck himself did not fully understand it.
Nearly a century later Seebeck's work became the basis for the field of thermoelectricity, and his metal loop was recognized as the first thermocouple. The Seebeck effect can be observed when an electric circuit is formed from wires of two different metals and the two ends are at different temperatures. A simple galvanometer will then detect the voltage difference between the two metal ends, and the temperature of one end can be determined if the temperature of the other is known. This kind of circuit is called a thermocouple, and for many years after Seebeck's discovery it was used only as a sensitive thermometer.
Thermometers of unsurpassed accuracy and range have been developed using thermocouples. Used primarily in industry, these thermometers have a range from a few degrees above absolute zero to several thousand degrees Fahrenheit. Beginning in the early twentieth century, scientists began to imagine a new use for thermocouples. If a small amount of heat created a small electric current, they thought, then more heat could generate a more powerful current, possibly powerful enough to run machinery. For several years scientists in Europe, the former Soviet Union, and the United States worked to find the best combination of alloys to maximize the output of thermocouples. They developed such devices as an emergency radio that ran on the heat from a kerosene lamp. After World War II, the beginning of the atomic age, physicists experimented with thermocouples that ran on the heat from decaying isotopes. These nuclear thermocouples are used to power satellites and deep space probes, devices that must run unattended for many years. Such devices have become controversial, as with the 1997 launch of the Cassini mission to Saturn, because of the small but nonzero chance of a launch accident of a craft that is carrying radioactive material.
There is another thermoelectric effect, called the Peltier effect, that has found some usefulness in recent years. Essentially the opposite of the Seebeck effect, the Peltier effect (named after French physicist Jean-Charles-Athanase Peltier) shows that when a current is sent through a thermocouple in a certain direction the thermocouple will heat up, and it will cool off when the current is sent in the other direction. The primary application of the Peltier effect is as a refrigerant, since a sizable current can cool a thermocouple to a temperature low enough to liquify nitrogen and helium.
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