A laboratory technician designs a prototype electromagnetic rail launcher, as shown in the diagram below. A light, movable conducting armature is placed horizontally across two parallel rails. A uniform vertical magnetic field, BBB, points upwards across the region of the armature.
When a direct current, III, is passed through the rails and the armature, an electromagnetic force, FFF, accelerates the armature to the right.
State one modification to the setup that would cause the armature to accelerate in the opposite direction (to the left).