Problem 3.281

A permanent magnet is shaped as a ring with a narrow gap between the poles. The mean diameter of the ring equals $d=20 \mathrm{~cm}$. The width of the gap is equal to $b=2.0 \mathrm{~mm}$ and the magnetic induction in the gap is equal to $B=40 \mathrm{mT}$. Assuming that the scattering of the magnetic flux at the gap edges is negligible, find the modulus of the magnetic field strength vector inside the magnet.