Solving a problem in physics section 4 paragraph 86 ele

85. Coil diameter d = 20 cm can be rotated about a vertical axis coinciding with one of the diameter of the coil. Revolution installed in the plane of the magnetic meridian, and let him on the current of I = 10 A. Find mechanical moment M, which must be applied to a coil, to keep him in the starting position. (Horizontal component Bz of the magnetic induction field of the Earth taken equal to 20 mT).