A 1.0-mm diameter wire is maintained at a temperature of 400oC and exposed to a convection environment at 40oC with h = 120 W/m2-oC. Calculate the thermal conductivity which 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?