This invention provides an electrostatic coating apparatus to avoid a disruptive electrical discharge from the high voltage electrode to an approaching grounded object. This apparatus employs the current of a high voltage electrostatic system to determine incipient grounding of the high voltage electrode and includes a fast-acting grounding device activated in anticipation of sparking conditions to connect the high voltage electrode to ground. The grounding device is held in the ungrounded position during operation of the electrostatic coating system. The electric current in the ground return of the high voltage source is sensed to provide a signal. All alternating current components of the signal above substantially pure direct current are removed to provide a resulting DC signal that increases as a grounded article approaches a charged electrode. The resultant DC signal is analyzed to determine the approach of a grounded article to the charged electrode. When the level of the signal indicates incipient grounding of the charged high voltage electrode, the grounding device is released from its ungrounded position and quickly connects the high voltage electrode to ground.

Improved cascade apparatus for developing electrostatic images wherein a latent electrostatic image supported on a moving member is developed by cascading a flow of developer material including a magnetic carrier component and an electroscopic toner component through a development zone. A toner dispenser dispenses toner material to supplement that portion consumed during development. The dispensed toner material is received onto a chute which directs the toner material into a predetermined location in the sump of the developer housing. A bucket conveyor moves developer material upwardly in the housing and spillage from the buckets is directed along the chute to combine with the dispensed toner material and also to prevent the dispensed toner material from clinging on the chute. A magnetic field producing member is positioned on the chute adjacent the path of the bucket conveyor to control the direction of flow of the spillage and to pull excess developer material from the buckets.

A simple electrometer system particularly adapted for non-contact measurement of electrostatic charges in electrostatography, such as the charge level on photoreceptor surface areas in commercial xerographic machines. Structure for readily mounting the electrometer as a desired measurement position immediately prior to image development in such machines is disclosed. Stable, long term and very high resistance induced electrostatic charge retention and accurate measurement thereof is provided by a simple disclosed electrometer circuit utilizing only a few commercially available electronic components including a single semi-conductive active element. The circuit output can be read by a conventional test meter, or utilized for various machine controls. The electrometer is controlled by a simple rotary switching arrangement.

An electrostatographic machine has both (1) feedback means for adjusting process control parameters in response to long term variations in the electrostatographic process and (2) feedforward means for adjusting process control parameters in response to short term variations in the electrostatographic process without negating the effect of the feedback means. The average of a predetermined number of sensed recording member reference voltages associated with image areas is compared to the reference voltage output of a single image area, producing a difference signal for controlling at least one process control parameter for the single image area, whereby the control of the parameter is responsive to short term variations in the output signal and is substantially non-responsive to long term variations in the output signal.

A magnetic brush developing apparatus for use in a toner powder image transfer type electrophotographic copying machine which includes an outer cylinder fixedly disposed at a position to confront a photosensitive member or photoreceptor, a magnet roller rotatably accommodated in the outer cylinder for being rotatably driven, a toner tank or hopper containing therein electrically conductive and magnetically attractive toner particles having high resistance and disposed above the outer cylinder for supplying the toner particles onto the outer cylinder, an excessive developing region for developing an image portion of an electrostatic latent image formed on said photosensitive member with sufficient density, and a toner removing region for removing toner particles adhering at least to non-image portion of the electrostatic latent image.