A solid coper sphere of density $\rho ,$specific heat ${ }^{\text{'}}{C}^{\text{'}}$ and radius ${ }^{\text{'}}{r}^{\text{'}}$ is at temperature $T_1.$ It is suspended inside a chamber whose walls are at temperature $T_2$. The time required for the temperature of sphere to drop to $T_2$ is $\frac{r\rho C}{Xe\sigma }(\frac{1}{T_2^3}-\frac{1}{T_1^3}).$ Find the value of ${ }^{\text{'}}{X}^{\text{'}}$ $(e=$emissivity of the sphere)