Abstract: An apparatus and method for sensing and controlling the concentration of toner in a developer mix comprising toner and carrier particles used to develop electrostatic images on a photoconductive or the like member includes the provision of a surface which is electrically insulated from the toner and carrier housing and is triboelectrically dissimilar from the developer mix. The developer mix is flowed over the surface and a corresponding direct current voltage is generated triboelectrically due to the interaction thereof. The direct current voltage is connected through resistance to the housing and the direct current flowing through the resistance is related to the concentration of toner in the developer mix. Apparatus responsive to a predetermined current generated is employed to control the replenishment of toner to the developer mix.