The thermal conductivity of an insulating material is to be determined using a guarded hot-plate apparatus. A 30 × 30-cm sample is used, and the approximate thermal conductivity is about 1.7 W/m · ?C. A differential-thermocouple arrangement is used to measure the temperature drop across the sample with an uncertainty of ±0.3?C. A heat input of 5 kW ± 1 percent is supplied, with a resulting temperature drop of 55?C. The sample thickness is 2.0 mm. What is the thermal conductivity and its uncertainty, assuming no heat losses and that the sample dimensions are known exactly?