Abstract: Carrier of a two-component type developer having a carrier core material and an organic polymer coating which results in a 20 to 45% carrier filling rate and dynamic permittivity which satisfies both Y.gtoreq.-0.05+9 and Y.ltoreq.-0.05+13.5 with Y equal to dynamic permittivity and X equal to the carrier filling rate percentage value. The dynamic permittivity of the carrier is determined with an apparatus which includes an adjustable detector having an electrode in electrical communication with an AC power source by way of a capacitor like gap between the photoreceptor of a developer and the electrode of the detector. A reading device and arithmetic unit are also in electrical communication with the detector and power source to enable determination of the dynamic permittivity of the carrier.