A 2-mm-diameter electrical wire is insulated by a 2-mm-thick rubberized sheath (k = 0.13 W/m ∙ K), and the wire/sheath interface is characterized by a thermal contact resistance of R c = 3 X 10^-4 m² ∙ K/W. The convection heat transfer coefficient at the outer surface of the sheath is 10 W/m² ∙ K, and the temperature of the ambient air is 20°C. If the temperature of the insulation may not exceed 50°C, what is the maximum allowable electrical power that may be dissipated per unit length of the conductor? What is the critical radius of the insulation?