What is the principle of cold junction compensation for thermocouples

When measuring temperature with a thermocouple, it is required that the temperature of its cold end (the measuring end is the hot end, and the end connected to the measuring circuit through a lead is called the cold end) remains constant, so that the magnitude of its thermoelectric potential is proportional to the measured temperature. If the temperature changes at the cold end during measurement, it will seriously affect the accuracy of the measurement. Taking certain measures to compensate for the impact caused by temperature changes at the cold end is called cold end compensation of thermocouples.
The cold end compensation of thermocouples usually uses a bridge composed of a thermistor connected in series at the cold end. The three arms of the bridge are standard resistors, and the other arm is composed of a (copper) thermistor. When the temperature at the cold end changes (such as increasing), the thermoelectric potential generated by the thermocouple will also change (decrease), and at this time, the resistance value of the thermoelectric resistor in the series bridge will also change, causing the voltage at both ends of the bridge to change (increase). If the parameters are well selected and the wiring is correct, the voltage generated by the bridge is exactly equal to the amount of change in thermoelectric potential with temperature, and the total output voltage (potential) of the entire thermocouple measurement circuit accurately reflects the measured temperature value. This is the principle of cold junction compensation for thermocouples.
But the thermistor does not require cold end compensation, because the so-called "cold end compensation" refers to the measurement of the thermoelectric potential of the thermocouple based on 0 degrees, and it does not require an excitation source. And the instrument is at the room temperature end, so for thermocouples, it is not measured at 0 degrees, which makes the measurement inaccurate. So in the circuit of the instrument, there is usually a cold end compensation circuit.