Electrostatographic apparatus comprising improved development bias means

Claim 1

In an electrostatographic apparatus having a photoconductive member and a developing means for applying a toner substance to the photoconductive member for development, the combination comprising: a variable bias voltage means for applying a bias voltage across the developing means and the photoconductive member, the bias voltage means comprising a fixed voltage source and a variable resistance means connected between the voltage source and the developing means; sensing means for measuring the electrical resistance of the toner substance, the bias voltage means being responsive to the sensing means in such a manner that the bias voltage is set for development in accordance with said resistance of the toner substance; and a reference surface means provided on the photoconductive member, the sensing means being responsive to the electrical current through the voltage source, variable resistance means, toner substance and reference surface means, the bias voltage means being constructed to automatically adjust the electrical resistance of the variable resistance means to a value such that the sensing means senses a predetermined electrical current and maintains said resistance of the variable resistance means constant at said value for development.