1. Field of the Invention
This invention relates to a method and an apparatus for developing a magnetic latent image. More particularly, it is concerned with a method and an apparatus for developing a magnetic latent image using magnetic toner which has been imparted an electric charge in a particular polarity.
2. Description of the Prior Art
As the method for developing a magnetic latent image, there have so far been known a method, in which a toner containing a ferromagnetic material in a resin as a carrier (the so-called "magnetic toner") is spread in a thin layer on a toner bearing member, and caused to lightly contact to a magnetic latent image bearing member (as disclosed in Japanese patent publication No. 50-90342); a method, in which a toner brush is formed on a sleeve with a permanent magnet incorporated therewithin and the brush is caused to frictionally slide over the magnetic latent image bearing member (as disclosed in Japanese Patent publication No. 51-100732); and others. However, the so-called insulative toner having electric charge in a particular polarity, when it is contacted on the latent image bearing member, tends to adhere onto even a portion of the latent image bearing member where no magnetic signal is applied, due to the electrostatic adhesive force, and the adhered toner is liable to cause fogging on the white background. Further, since such insulative toner has high agglomerating property, when the toner is formed in a thick layer to facilitate light contact with the magnetic latent image, there tends to produce collected masses of toner, which makes it extermely difficult to obtain a stable thick layer of the toner. In order to prevent such electrostatic adhesion (hence fogging) and agglomeration, it may be contemplated that electric resistance of the toner is reduced to some extent. In this case, however, there arises such a problem that, when the electric resistance becomes low, it becomes difficult to perform transfer of the toner image which is once developed onto a plain paper by the corona discharge, on account of which these method are not practical.