Abstract

Novel apparatus (probe) for sensing and controlling the surface charge on an electro-photographic copier drum in which a hollow, cylindrical, tubular shield member provided with a single elongated aperture therein houses a rotatable charge receiving, insulatingly segmented, rod-like fluted member, the latter extending centrally through the shield member. The elongated slot of the outer shield member extends from edge-to-edge of the drum and in close, spaced proximity thereto. One end of the probe apparatus carries an electromagnetic, Hall effect transducer including a phasing paddle wheel member rotatable with the segmented member for timing and synchronizing the rotation of the probe with the drum rotation. The probe is disposed between two spaced apart drum charging corotrons. The probe senses the surface charge on the drum and compares this charge with a known standard charge level. An output potential level derived from this comparison is incremented or decremented in accordance with the comparison level and the derived charge is automatically applied to the drum surface via the charging corotrons.