A thermocouple (or TC) is a form of temperature-sensing apparatus that doesn't rely on an excitation signal to operate. Therefore, they produce very small signals (often microvolts in amplitude). Two wires of two different materials meet where a temperature measurement is to be sampled. Each metal develops a voltage differential independently of each other. This effect is known as the Seebeck electromotive effect, in which the difference between the voltage of the two metals has a nonlinear relationship to the temperature.

The magnitude of the voltage depends on the metal material chosen. It is critical that the ends of the wires are thermally isolated from the system (and the wires need to be at the same controlled temperature). In the following diagram, you will see a thermal block whose temperature is controlled with a sensor. This is usually controlled via a technique called the cold junction compensation, where the temperature can vary, but is measured by the block sensor accurately. 

When sampling the voltage differential, the software will usually have a look-up table to derive the temperature based on the nonlinear relationship of the metals chosen:  

Thermocouples should be used for simple measurements. The system can vary in accuracy as well, since subtle impurities can affect the wire composition and cause a mismatch in the lookup tables. Precision-grade thermocouples may be needed, but come at a higher cost. Another effect is aging. Since thermocouples are often used in industrial settings, high-heat environments can degrade the accuracy sensors over time. Therefore, IoT solutions must account for changes over the life of a sensor.

Thermocouples are good for wide temperature ranges, are color-coded for different metal combinations, and labeled by type (for example, E, M, PT-PD, to name a few). In general, these sensors are good for long distance measurements with long leads, and are often used in industrial and high-temperature environments.

Shown below are various thermocouple metal types and their respective energy linearities across a range of temperatures.