A thick walled, hollow metallic cylinder has height h and inner and outer radii r₁ and r₂ , respectively. It contains water which is heated with a constant power source so that its temperature is maintained at T₁ . In the steady state, the temperature of the inner and outer isothermal surfaces of the cylinder are T₁ and T₂ , respectively. The temperature difference (T₁-T₂ ) is proportional to