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The surface-potential-decay measurement techniques are commonly employed for the adjustment of Xerox photography processes, development of electrets, and assessment of polyethylene films for cable insulation. In all these situations, it is important to evaluate the surface charge density and compare it to the limit imposed by the dielectric rigidity of atmospheric air. The aim of the experiments reported in this paper is to enhance the efficiency of the corona charging of nonwoven filter media for heat, ventilation, and air-conditioning applications. Samples of nonwoven polypropylene fibrous media were exposed to positive corona discharges from a wire electrode, as the efficiency of the filter can be enhanced when the insulating fibers are electrically charged. The observed surface potential levels and the potential decay characteristics indicate that the charge of the filter is limited by the local discharges that occur inside the fibrous dielectric. The performance of the filter can be further improved by using a triode-type electrode system that assures a uniform distribution of the electric charge at the surface of the filtering media.