Given:

Cylindrical graphite resistor, k $=0\mathrm{.}1$W$/$m$.\backslash$deg C of $1$mm diameter and $20$mm

length, encapsulated in micanite in an environment of h $=16$W$/$m$2.\backslash$deg C and

temperature of the environment is $27\backslash$deg C$.$ Calculate:

a$)$ Critical radius of micanite insulation, and

b$)$ The resistor temperature if heat transfer of the total system is

$0\mathrm{.}25$W