A solid sphere of diameter D $=12$ m m $$and thermal conductivity k $=120$ W m $*$ K $$is suspended in a large vacuum oven with internal wall temperatures of T s u r $=20\backslash$deg C $.$The initial sphere temperature is T i $=100\backslash$deg C $,$and its emissivity is $\backslash$epsi $=0.73.$

Determine an appropriate characteristic length $L_c,$ in mm$,$ and the corresponding Biot number $B_i$ that is associated with the

transient thermal response of the solid object in case $($e$).$

$L_{c1}=2mm$

$B_{i1}=\mathrm{.}0001$

$L_{c2}=$

$B_{i2}=$