What do thermocouples require to function properly?

Thermocouples function by utilizing the principle of thermoelectricity, which involves two dissimilar metals joined at a junction. When this junction is heated or cooled, it generates a voltage that can be correlated to temperature differences. This voltage is the result of the Seebeck effect, which is the basis for the operation of thermocouples.

In practice, the combination of two different metals, such as copper and constantan, allows the thermocouple to be responsive to temperature changes and produce a measurable output. This makes the choice of metals crucial for the accuracy and range of temperature measurements.

The other options do not directly fulfill the fundamental requirement for a thermocouple's operation. A bi-metallic strip or coil relates more to the operation of thermometers or temperature switches rather than thermocouples. A thermopile, while it is an arrangement of multiple thermocouples may increase the output voltage, it is not a requirement for a single thermocouple. A heating element isn't necessary for a thermocouple; instead, it's a device that can generate heat for other applications. Thus, the requirement of two dissimilar metals is the definitive and correct answer.