Abstract: A toner of toner particles containing at least one binder, at least one color ant, and an external additive package comprised of zinc stearate and at. least one of silicon dioxide or titanium dioxide, wherein the amount of zinc stearate is limited to about 0.10 percent by weight or less of the toner. It has been surprisingly found that when the amount of zinc stearate is so limited, a developer formed from the toner exhibits excellent triboelectric charging and stability and excellent developer flow. When the developer is used in a magnetic brush development system, consistent, high quality copy images are formed substantially without any depletion defects over time.

Abstract: A carrier core for use in an electrostatographic developer composition is a ferrite substantially free of copper and zinc. By being substantially free of copper and zinc, the carrier core is environmentally friendly and eliminates the need for use of conventional copper zinc carrier cores which must be handled as hazardous waste. Developer compositions employing the ferrite carrier core possess suitable properties.

Abstract: An apparatus and method for determining both the breakdown voltage and the conductivity of a particulate material, such as a developer material, is provided. In a preferred embodiment, the apparatus for determining breakdown voltage comprises a breakdown cell for storing a preselected quantity of the particulate material and a voltage breakdown ("VB") autometer, the VB autometer being adapted to apply a first voltage across at least portions of the preselected quantity of the particulate material. The VB autometer is capable of progressively varying the magnitude of the first voltage. A measuring circuit, for measuring a second voltage, is coupled to the voltage breakdown cell. When the second voltage substantially equals a preselected reference voltage, the first voltage substantially equals the breakdown voltage of the particulate material.