Thermal Conductivity Detector (Katharometer) TCD:

The thermal conductivity detector is based on the principle in which a hot body will lose heat at a rate that depends upon the composition of the surrounding gas. Therefore, the rate of heat loss could be used as a measure of the gas composition. The TCD operates through comparing the thermal conductivity of sample elements as reference against the carrier gas. Figure shows a classical TC cell consisting of a spiral metal filament supported within a cavity inside a metal block.

Figure:  Schematic of TC cell