An air filter made from an electret and a conductor

Air filters constructed from electrets must have an electric field external to the electret material. This can be achieved by placing a corrugated conductor close to the surface of a planar electret, resulting in a system of parallel pores, with a low resistance to airflow, and an electric field perpecidicular to the airflow. The charge on such a filter has been measured by X-irradiation, and that on its component material with a probe, the two methods giving similar results. The electric field inside the filter has been calculated by a self-consistent method. Specimen results for filtration efficiency are given.