A thermocouple is considered an active transducer because it generates an electrical signal in response to a physical phenomenon—in this case, a temperature difference. Active transducers are those that require an external power source to operate and produce an output signal that is proportional to the input stimulus.

In the case of a thermocouple, it operates based on the Seebeck effect, which is the phenomenon where a voltage is generated when two dissimilar metals are joined at two junctions and there is a temperature gradient between the junctions. This generated voltage is directly proportional to the temperature difference between the two junctions, and it can be measured using a voltmeter or converted into a temperature reading using appropriate calibration.

Since the thermocouple generates an electrical voltage signal as its output, it requires an external power source to drive the current flow through the circuit. This could be achieved using a battery, power supply, or loop-powered transmitter depending on the application.

In summary, a thermocouple is an active transducer because it converts a temperature difference into an electrical voltage signal, which requires an external power source to operate and produce an output proportional to the input stimulus.