Two uncharged parallel plates A and B are at the same electrical potential, and separated by a distance d. A uniform magnetic induction field B acts perpendicular to plane of paper (and parallel to plates) between the plates. Particles, each with charge q, mass m and kinetic energyK are shot through a hole in A directly towards B, particle entering the hole perpendicular to plate A. An ammeter Am connected to B and measures the current collected by the plate B, as the particles hit it. The limiting value of the field B for which the ammeter will stop showing a current is