Electrostatic printing with density control provided by charge measuring means

Claim 1

IN AN APPARATUS FOR APPLYING AN IMAGE-SHAPED LAYER OF PRINTING POWDER PARTICLES TO AN ARTICLE SURFACE INCLUDING A STENCIL SCREEN HAVING AN IMAGE DEFINING APERTURE THERETHROUGH, MEANS FOR SUPPORTING A SUPPLY OF PRINTING POWDER PARTICLES AT ONE SIDE OF SAID SCREEN, MEANS FOR SUPPORTING AN ARTICLE TO WHICH THE PARTICLES ARE TO BE SUPPLIED AT THE OPPOSITE SIDE OF SAID SCREEN, ELECTRIC POTENTIAL SOURCE MEANS OPERABLE WHEN ENERGIZED TO ESTABLISH AN ELECTRIC FIELD BETWEEN SAID SUPPLY AND ARTICLE CAPABLE OF ELECTRICALLY CHARGING POWDER PARTICLES IN SAID SUPPLY AND IMPELLING THE CHARGED PARTICLES FROM SAID SUPPLY THROUGH THE APERTURE OF SAID SCREEN TO THE SURFACE OF THE ARTICLE, AND MEANS FOR ENERGIZING SAID ELECTRIC POTENTIAL SOURCE MEANS; THE IMPROVEMENT COMPRISING MEANS FOR MEASURING THE VARIATION OF ELECTRIC POTENTIAL BETWEEN SAID SCREEN AND SAID ARTICLE DUE TO THE ACCUMULATION OF CHARGED PARTICLES ON SAID ARTICLE WHILE SAID POTENTIAL SOURCE MEANS IS ENERGIZED, AND MEANS OPERABLE BY SAID MEASURING MEANS FOR DE-ENERGIZING SAID POTENTIAL SOURCE MEANS WHEN THE POTENTIAL DIFFERENCE BETWEEN SAID SCREEN AND ARTICLE REACHES A PRESELECTED MAGNITUDE.