The output of a thermocouple is expressed in which unit?

The output of a thermocouple is expressed in millivolts because thermocouples generate a small voltage that is directly related to the temperature difference between the junctions of the two different metals used in the thermocouple. The typical output voltage range for a thermocouple is in the millivolt range, which makes it suitable for precise temperature measurements.

The millivolt output is a result of the Seebeck effect, where a voltage is produced when there is a temperature difference across the junctions of the two conductors. Because the voltage produced is relatively small compared to other electrical signals, it is most commonly measured in millivolts to provide a practical and manageable scale for instrumentation and data acquisition systems.