A gas filled tube has 2 mm inside dia and 25 cm length. The gas is heated by an electric wire of dia 0.05 mm located along the axis of the tube. Current and the voltage drop across the heating element are 0.5 amps and 4 volts, respectively. If measured wire and inside tube wall temperatures are 175°C and 150°C respectively, find the thermal conductivity of the gas filling tube.