There are two thermocouples namely measuring thermocouple and balancing thermocouple. They forms a part of bridge at the input of DC amplifier. The same non linearity effect takes place in both the thermocouple, so this effect is compensated.
There are some advantages of true RMS reading voltmeter which are given below, The true RMS value is measured independently of an input signal waveform. Because of the use of two thermocouples, the non linearity problem is