Problem 3.253

A copper wire with cross-sectional area $S=2.5 \mathrm{~mm}^{2}$ bent to make three sides of a square can turn about a horizontal axis $O O^{\prime}$ (Fig. 3.69). The wire is located in uniform vertical magnetic field. Find the magnetic induction if on passing a current $I=16 \mathrm{~A}$ through the wire the latter deflects by an angle $\theta=20^{\circ}$