Charging of insulating spheres in contact with an electrode affected by a mono-ionized field

The charge acquired by an insulating sphere in an uniform mono-ionized electric field has been accurately evaluated by Pauthenier. In certain electrostatic applications, such as the electroseparation of mixed granular solids, the particles to be charged are in contact with an electrode. Under these circumstances, Pauthenier's formula is no longer valid, because the field is nonuniform. The present paper addresses this problem from both a computational and an experimental point of view. A numerical method of electrostatic field analysis was employed for the evaluation of the charge acquired by spheres of various dielectric constants, in contact with plate electrode. The experiments were carried out on a laboratory equipment provided with various types of corona electrodes. An electrometer was used to measure the charge acquired by calibrated spheres of polyamide (diameter: 3 mm) when subjected to the positive or negative corona discharge generated between these electrodes and a metallic rotating roll electrode (diameter: 150 mm) connected to the ground. The experimental data were in good agreement with the theoretical predictions. The charge of the particles was found to be maximum for a model of electrode, which was then proposed as the standard for the industrial corona-electrostatic separators.