in experimental attempts to control positively tribocharged toner particles on the surface of applicator roll 55 applicant has tried using a corona wire to overcome the air velocity and push toner back to the surface of roll 55 such a corona wire has been observed to overcharge the toner particles causing excessive buildup of toner on the surface of the toner roll and more prominent background images see example 2 best results have been achieved using a field electrode 70 placed near the nip between charging roll 60 and applicator roll 55 maintained at a direct current potential of like polarity as the intended charge on the toner particles in the preferred embodiment positive by maintaining the field electrode substantially below the corona threshold potential the overcharging problem is avoided in an illustrative embodiment a 10 mil copper wire field electrode was operated at various potentials between 2 and 6 kilovolts well below the corona threshold potential for a wire this size toner yields were dramatically improved with less than 5 percent of toner by weight being collected by the vacuum housing as compared with about 30 percent in the absence of the field electrode at an operating speed of 300 feet per minute 1 5 meters per.