There is a uniform magnetic field B in a circular region of radius R as shown in figure whose magnitude changes at the rate of $\frac{\mathrm{dB}}{\mathrm{dt}}$ . The e.m.f. induced across the ends of a circular concentric conducting arc of radius R1 having an angle $\mathrm{\theta }$ as shown$\left(\mathrm{\angle }{\mathrm{OAO}}^{\mathrm{\prime }}=\mathrm{\theta }\right)$ is