A condition way of measuring the thermal conductivity of a material is to sandwich an electric thermo foil heater between two identical samples of the material, as shown in Fig. 1-30. The thickness of the resistance heater, including its cover, which id made of thin silicon rubber, is usually less than 0.5 mm. a circulating fluid such as tap water keeps the exposed ends of the samples at constant temperature. the lateral surfaces of the samples are well insulated to ensure that heat transfer through the samples is one-dimensional. two thermocouples are embedded into each sample some distance L apart, and a differential reads the temperature drop T across this distance along each samples. When steady operating conditions are reached, the total rate of heat transfer through both samples becomes equal to the electric power drown by the heater.