A 1.0-mm-diameter wire is maintained at a temperature of 400°C and exposed to a convection environment at 40"C with h=120 W/m. "C. Calculate the thermal conductivity that will just cause an insulation thickness of 0.2 mm to produce a critical radius." How much of this insulation must be added to reduce the heat transfer by 75 percent from that which would be experienced by the bare wire?