Wheatstone bridge circuit:

The filament is made of a material whose electrical resistance varies greatly along with temperature. It has a high temperature coefficient of resistance. A constant current is passed by the filament causing its temperature to rise along with pure carrier gas. With pure carrier gas flowing the heat loss is constant and therefore, the filament temperature is also constant. If the gas composition changes, example for while solute peak emerges, the filament temperature changes, causing a corresponding modify in electrical resistance. It is this resistance change that is measured through a Wheatstone bridge circuit.

Figure:  T C Wheatstone bridge circuit

The TCD is universal that is it will respond to any solute which has a thermal conductivity different than the carrier gas. It is majorly used for analyzing inorganic gases like as the elements of air and its impurities. The sensitivity ranges from huge percentages of components down to a few parts per million.