What is the critical radius of insulation $($cms$)$ for a metallic cylinder, if the convective heat transfer co$-$efficient with the ambient atmosphere is $5$ W$/$m$2.$ K$?$ Thermal conductivity of metal and insulation material are $40$ and $0\mathrm{.}1$ W$/$m$.$K respectively.

$2$

$8$

$10$

$40$