a. Inside a large square circuit with current, many square micro-circuits with current are evenly distributed. The magnetic moment of each micro-circuit is M₀. Prove that at a distance much greater than the distance between the micro-contours, the induction of their magnetic field coincides with the induction of the magnetic field of a large contour, the magnetic moment of which is nM₀, where n is the number of micro-contours inside the large contour.
b. A thin square plate with dimensions a × a × h (h ≪ a) is magnetized in a direction perpendicular to its plane. Magnetic field induction in the center of plate B. Determine the magnetic moment of the unit volume of the plate substance.