A conducting bar slides without friction on two parallel horizontal rails that are $20$ cm apart and connected by a wire at one end. The resistance of the bar and the rails is constant and equal to $0\mathrm{.}50.$ A uniform magnetic field is perpendicular to the plane of the rails. A $0\mathrm{.}120-$N force parallel to the rails is required to keep the bar moving at a constant speed of $0\mathrm{.}75$ m$/$s$.$ What is the magnitude of the magnetic field?