A coil has $1,000$ turns and $500 cm^2$ as its area. The plane of the coil is placed at right angles to a magnetic induction field of $2 \times {10^{ - 5}}\,Wb/{m^2}$. The coil is rotated through ${180^o}$ in 0.2 seconds. The average e.m.f. induced in the coil, in milli-volts, is