A certain elastic conducting material is stretched into a circular loop. It is placed with its plane perpendicular to a uniform magnetic field $\mathrm{B}=0.8 \mathrm{~T}$. When released the radius of the loop starts shrinking at a constant rate of $2 \mathrm{~cm}^{-1}$. The induced emf in the loop at an instant when the radius of the loop is 10 cm will be $\_\_\_\_$ mV .