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CHARACTERIZATION OF THE ELECTRIC FIELD IN ELECTROPHOTOGRAPHIC GRANULAR SYSTEMS L. B. SCHEIN IBM Research Division, Almaden Research Center, 650 Harry Road, San Jose, California 95120-6099 ABSTRACT: Within all electrophotographic (or xerographic) copiers and laser printers are development systems where the image is reproduced. Usually these development systems contain 200 um diameter metallic, magnetic balls called carrier particles. Attached electrostatically to these balls are 10ipm diameter polymer particles called toner which eventually end up on the paper forming the black images given to customers. The process by which the toner leaves the carrier and ends up on the paper is driven by electric fields in the development system. These fields are determined by the applied voltage and the dielectric properties of the granular mixture of metal carrier balls, polymeric toner, and air. The average electric field is determined by the dielectric constant of this granular mixture. But on a microscopic scale that a toner would experience, i.e. 10 pm, the electric field has significant structure. This structure has been determined both experimentally and theoretically. For example, the electric field peaks underneath a chain of carrier particles (which are chained up by a magnetic field). Models of the development process clearly indicate that toner particles transfer from the carrier particles only when the toner particles are directly under a carrier chain, where one would expect the field to be a maximum. However, recently we have succeeded in showing experimentally that the value of the electric field that the toner particles experience is the average field, described by the dielectric constant. Theoretical questions concerning the calculation of the electric field in a granular system raised by this unexpected result will be discussed. I. INTRODUCTION Electrophotography (also called xerography), the technology in almost all copiers and laser-printers, is driven by electric fields. Electric fields play a role in almost all of the process steps [1]. Calculations of the magnitude and determination of the effects of the electric fields have proven to be challenging problems. It is the purpose of this paper to discuss one of these problems, the calculation and measurement of the electric field in the development system [2-5]. As 200 pm diameter metals balls are mixed with 10 pm diameter plastic toner particles [1], this is a granular system. Further, it is a granular system which is macroscopic enough that complete characterization of the electric field and its effects on toner particles is possible and has recently been accomplished. The unexpected results of this work raise interesting questions about the physics of granular systems. To better understand the nature and significance of this unexpected result, a short tutorial on electrophotography is given in Section II. Determinations of the electric field in the development system by two different methods which give inconsistent results are presented in Sections III an