Accurately measuring temperatures over a wide range is a challenge to engineers, physicists, and other scientists. Many techniques have been devised to deal with a wide range of conditions and temperatures. One such technique is a thermocouple. A thermocouple makes use of one aspect of the thermoelectric effect to measure temperatures, the voltage produced between two different wires with junctions at different temperatures. Depending on the types of wires chosen, a thermocouple can be used for temperatures ranging from -454°F to 4,172°F (-270°C to 2,300°C).
A thermocouple must consist of two wires of different compositions. A popular combination is copper and constantan. Constantan is an alloy of copper and nickel. The different wires are joined at the ends to make two junctions. One of the wires is then cut so that a voltmeter can be placed in the circuit to measure the voltage between the two junctions. This voltage will depend on the temperature difference between the two junctions. A scientist wanting to use a thermocouple will then place one of the junctions in the object whose temperature is to be measured. Because the voltage depends on the temperature difference between the two junctions, the other junction must be maintained at an accurately known temperature. One way to maintain a known temperature is to place the junction in an ice water bath that will be at the freezing point of water. To find the unknown temperature the scientist must know what temperature difference corresponds to the measured voltage. These figures are determined by careful experiments and then tabulated, so the scientist uses the table to find the unknown temperature.
What causes this voltage difference? The two different types of metal, having different compositions, will have different densities of electrons. The electrons will tend to diffuse from the higher to the lower densities. These electron densities both depend on the temperature, so if the two junctions are at different temperatures the diffusion of electrons will proceed at different rates at each junction. The net result is a motion of the electrons, so there is a voltage between the two junctions.
Thermocouples have the advantage of being accurate over a wide temperature range and of being able to accurately follow rapid temperature changes. They can however be cumbersome to use. The need to keep one junction at an accurately known temperature limits their portability.