Abstract: A magnetic brush developing system for electrostatic duplication using a monocomponent developer controls the thickness of the developer on the brush such that the necessary amount of developer is applied to the photosensitive surface which carries the latent electrostatic image. A capacitive sensor in the form of a rigid plate of conductive material is spaced closely adjacent to the surface of the roll on which the magnetic brush is formed. The dielectric constant depends on the thickness of the developer in the magnetic brush and is detected by the capacitive sensor. The capacitance of the sensor is converted, into a pulse train the duty cycle of which is modulated in accordance with the change in capacitance, by a circuit arranged on a circuit board in close proximity to the sensor to provide a compact structure which is not subject to perturbations which could effect the capacitance presented by the sensor.