PURPOSE: To measure the toner concentration correctly from the current corresponding to the toner concentration, by disposing a counter electrode closely to the developer conveying means at the downstream side thereof, and making the developer after development to contact between plurality of conductive electrode pieces and insulating electrode pieces.
CONSTITUTION: Closely adjoining to a developer conveying means 104 which rotates the outside of plurality of magnets 102 is disposed a counter electrode 12 in a manner to contact with a developer layer d' after latent image development. The counter electrode 12 consists of a nonmagnetic conductive electrode piece G and a nonmagnetic insulating electrode piece H which are provided on the lower side of an insulating material plate 15, and the electrode pieces G and H are connected to a power source 18 by means of conductive wires 16, 17. The developer, after developing electrostatic latent image, contacts the electrode pieces G and H to compose an electric circut up to the power source 18, so that a current corresponding to the toner concentration is permitted to flow. This current is detected by a comparative control circuit 19 to find the toner concentration. A humidity compensation means 20 is provided between the power source 18 and the electrode piece G. When the resistance of the developer varies with the change in humidity, the own resistance is changed reversibly, so that the potential applied to the electrode piece G is changed accordingly.
YAMAKAWA KOUICHI
TOZAWA KATSUTOSHI
NISHIMURA TAKEO
KASUYA RIYUUHEI
OKAMURA KAZUO
