Why thermocouple is an active transducer?

The output of the thermocouple is voltage difference on its terminals that is proportional to the corresponding temperature .But when we look in to the case of passive transducers or sensors the output will be change in some passive electrical quantity such as capacitance ,resistance or inductance

Another way to find an active and passive device is ,if something requires power supply to operate it’s active otherwise it’s passive

Example for active transducers:Piezoelectric Type,thermopile,LVDT Example for passive transducers:RTD,Resistance Hygrometer,resistance thermometer