ELECTROSTATIC LATENT IMAGE DEVELOPING APPARATUS

Abstract: A developing device for developing an electrostatic latent image with a two-component developer comprises a developer container (46) including a developer-accumulating chamber (50), and a developer-agitating chamber (56) provided above the developer-accumulating chamber. A communication passage (64) is provided between the developer-agitating chamber and the developer-accumulating chamber, and is opened to the developer-agitating chamber to define an overflow opening for the developer. The device also comprises a developer-carrying body (48) provided in the developer-accumulating chamber of the developer container, and the developer-carrying body is exposed to face an electrostatic latent image carrying body, and brings the developer from the developer-accumulating chamber to a facing zone therebetween for development of an electrostatic latent image formed on the electrostatic latent image carrying body. The device further comprises a developer-lifting means (68, 68', 70) for lifting the developer, brought to the facing zone by the developer-carrying body, to the developer-agitating chamber of the developer container, and a developer-agitating means (58) for agitating the developer in the developer-agitating chamber of the developer container, a part of the developer agitated by the developer-agitating means being fed to the developer-accumulating chamber through the overflow opening and the communication passage.

Description: TECHNICAL FIELD The present invention relates to a developing device for using in an electrostatic recording apparatus, such as an electrophotographic copying machine or an electrophotographic printer, and, more specifically, to a developing device suitable for use in a multicolor electrostatic recording apparatus for multicolor recording and provided with a plurality of recording units arranged in a line. BACKGROUND ART In an electrostatic recording apparatus of the type as mentioned above, an electrostatic latent image is formed on an electrostatic latent image carrying body, such as a photosensitive body or a dielectric body, the electrostatic latent image is developed electrostatically as a charged toner image using a developer, the charged toner image is transferred electrostatically to a recording medium, such as a recording sheet of paper, and then the toner image is fixed to the recording medium by heat, pressure or light. As a multicolor recording apparatus utilizing such electrostatic recording techniques, a single-drum type of multicolor recording apparatus is known. This recording apparatus comprises a single electrostatic image carrying body, i.e., a photosensitive drum, and a plurality of developing devices using different developers containing toner of different colors, respectively, and arranged between an electrostatic image writing location at which an electrostatic latent image is written on the photosensitive drum and a transfer charger. For example, a multicolor recording apparatus for a full-color recording is provided with four developing devices, which use a yellow developer containing yellow toner, a cyan developer containing cyan toner, a magenta developer containing magenta toner, and a black developer containing black toner, respectively. First, an electrostatic latent image is formed on the photosensitive drum on the basis of yellow image data, and is developed as yellow toner image using the yellow toner, and then the yellow toner image is transferred to and fixed on a recording sheet. Subsequently, an electrostatic latent image is formed on the photosensitive drum on the basis of cyan image data, and is developed as a cyan toner image using the cyan toner, and then the cyan toner image is transferred to and fixed on the sheet of paper carrying the yellow toner image previously fixed thereto. A similar process is repeated for each of magenta image data and black image data. Consequently, the four toner images are superposed on the sheet of paper, whereby a full-color recording is made. Although the single-drum multicolor recording apparatus is advantageous in that the same has a relatively compact construction, the single-drum multicolor recording apparatus is incapable of high-speed recording, because the same needs to form the toner images of the different colors on the single photosensitive drum. Also, as a multicolor recording apparatus for the electrostatic recording techniques, the multi-drum type of multicolor recording apparatus is known, which comprises four electrostatic recording units aligned with each other along path for moving a recording sheet of paper. The respective electrostatic recording units include developing devices which use a developer containing yellow toner, a developer containing cyan toner, a developer containing magenta toner, and a developer containing black toner. While a recording sheet of paper is moved along the path, an yellow toner image, a cyan toner image a magenta toner image and a black toner image sequentially is formed on the sheet of paper by the four electrostatic recording devices, whereby a full-color image is obtained on the sheet of paper. Although the multi-drum type multicolor recording apparatus as mentioned above is advantageous in that a high-speed multicolor recording can be carried out, it has the most disadvantage in that a construction thereof is large due to the alignment of the electrostatic recording units. Especially, as a developing device incorporated in a conventional electrostatic latent recording apparatus, a type of developing device comprising a developing roller and a developer-accumulating roller side by side is well known. The developing device of this type requires a relatively large area for installation thereof. Accordingly, the multi-drum multicolor electrostatic recording apparatus utilizing a plurality of developing devices of that type also has a large construction. Patent Abstract of Japan vol. 005, no. 191 (P-092) discloses a dry type developing device in an electrophotographic apparatus wherein a toner replenishing means is provided in a manner as to drop the toner to be replenished to the upward part of a developer recovery route. This publication does not suggest any multi-color electrostatic recording apparatus, not to mention the miniaturization of such an apparatus. JP-A-01/282583 and EP-A-0 373 704 show multi-color electrostatic recording apparatus having large dimensions. Even a combination of the teachings of these documents with the teaching of the described Patent Abstract of Japan vol. 005, no. 191 (P-092) would not yield a miniaturized multi-color recording apparatus. Recently, there is a demand for miniaturization of various apparatuses in the information processing market. Accordingly, as various companies or makers aim at miniaturizing computers, peripheral equipments therefor, and etc., there is a strong demand for miniaturization of even a recording apparatus provided with a monochromatic developing device and occupying a small area for installation thereof. Nevertheless, the above-mentioned developing device having the developing roller and the developer-accumulating roller arranged side by side unavoidably needs a relatively large area for installation thereof. DISCLOSURE OF INVENTION Accordingly, it is a principal object of the present invention to provide a multicolor recording apparatus with a compact construction. Appended claim 1 defines a multi-color electrostatic recording apparatus in accordance with the present invention. As is apparent from the claimed arrangement, the developer container is divided into the developer-accumulating chamber and the developer-agitating chamber, and the developer-agitating chamber is provided above the developer-accumulating chamber. Accordingly, the developer-accumulating chamber of the developer container has small dimensions. BRIEF DESCRIPTION OF DRAWINGS The above and other objects and advantages of the present invention will be better understood from the following description, with reference to the accompanying drawings in which: Figure 1 is a schematic view of an electrostatic recording unit having a conventional developing device incorporated therein; Figure 2 is an enlarged schematic view of the conventional developing device of Fig. 1; Figure 3 is a schematic view of a conventional multicolor recording apparatus provided with four electrostatic recording units of the type shown in Fig. 1; Figure 4 is a schematic cross-sectional view showing a first embodiment of a developing device according to the present invention; Figure 5 is a perspective sectional view of the developing device of Fig. 4; Figure 6 is an enlarged sectional view of a part of the developing device of Fig. 4; Figure 7 is a diagrammatic view for explaining a positional relationship between magnetic poles of a developing roller, i.e., a magnet roller, used in the developing device of Fig. 4; Figure 8 is a diagrammatic view for explaining a positional relationship between magnetic poles of a developer lifting magnet roller used in the developing device of Fig. 4; Figure 9 is a schematic view of an electrostatic recording unit having the developing device of Fig. 4 incorporated therein; Figure 10 is a schematic view of a multicolor recording apparatus provided with four electrostatic recording units of the type shown in Fig. 9; Figure 11 is a schematic cross-sectional view of a second embodiment of a developing device according to the present invention; Figure 12 is a perspective sectional view of the developing device of Fig. 11; Figure 13 is an enlarged view of a part of the developing device of Fig. 11; Figure 14 is a schematic cross-sectional view of a modification of the developing device of Fig. 11; Figure 15 is an enlarged sectional view of a part of the developing device of Fig. 14; Figure 16 is a plan view of the developing device of Fig. 11, in which a top wall and a front wall are removed to show an interior of the developing device; Figure 17 is a schematic cross-sectional view of a third embodiment of a developing device according to the present invention; Figure 18 is a sectional view taken along a line XVIII-XVIII of Fig. 17; Figures 19(a), 19(b), and 19(c) are diagrammatic views showing a distribution of an accumulated developer and an adhesion of developer to a developing roller in the developing device of Fig. 17 when a speed of rotation of developer-agitating screw conveyors is set to be at 300 rpm and when the developer circulating rate is suitably varied; Figures 20(a), 20(b), and 20(c) are diagrammatic views showing a distribution of an accumulated developer and an adhesion of developer to a developing roller in the developing device of Fig. 17 when a speed of rotation of the developer-agitating screw conveyors is set to be at 350 rpm and when the developer circulating rate is suitably varied; Figures 21(a), 21(b), and 21(c) are diagrammatic views showing a distribution of an accumulated developer and an adhesion of developer to a developing roller in the developing device of Fig. 17 when a speed of rotation of the developer-agitating screw conveyors is set to be at 400 rpm and when the developer circulating rate is suitably varied; Figure 22 is a graph showing a relationship between a total quantity of a developer and a quantity of developer accumulated in the developing device of Fig. 17; Figure 23 is a schematic cross-sectional view of a fourth embodiment of a developing device according to the present invention; Figure 24 is a sectional view taken along a line XXIV-XXIV of Fig. 23; Figures 25(a), 25(b), and 25(c) are diagrammatic views showing a distribution of accumulated developer and an adhesion of developer to a developing roller in the developing device of Fig. 23 when the total quantity of developer is set to be 1.4 kg and when the speed of rotation of developer-agitating screw conveyors is suitably varied; Figures 26(a), 26(b), and 26(c) each is a diagrammatic view showing a distribution of an accumulated developer and an adhesion of developer to a developing roller in the developing device of Fig. 23 when the total quantity of developer is set to be 1.6 kg and when the speed of rotation of developer-agitating screw conveyors is suitably varied; Figures 27(a), 27(b), and 27(c) are diagrammatic views showing a distribution of an accumulated developer and an adhesion of developer to a developing roller in the developing device of Fig. 23 when the total quantity of developer is set to be 1.8 kg and when the speed of rotation of developer-agitating screw conveyors is suitably varied; Figure 28 is a plan view similar to Fig. 16, showing a fifth embodiment of a developing device according to the present invention; Figure 29 is a sectional view taken along a line XXIX-XXIX of Fig. 28; Figure 30 is a partial perspective view showing a part of a developer-agitating chamber of the developing device of Fig. 28; Figures 31(a), 31(b), and 31(c) are diagrammatic views of an adhesion of developer to a developing roller in the developing device of Fig. 28 when the length of a second scraper is set to be 140 mm and when the speed of rotation of developer-agitating screw conveyors is suitably varied; Figures 32(a), 32(b), and 32(c) are diagrammatic views of an adhesion of developer to a developing roller in the developing device of Fig. 28 when the length of a second scraper is set to be 175 mm and when the speed of rotation of developer-agitating screw conveyors is suitably varied; Figures 33(a), 33(b), and 33(c) are diagrammatic views of an adhesion of developer to a developing roller in the developing device of Fig. 28 when the length of a second scraper is set to be 210 mm and when the speed of rotation of developer-agitating screw conveyors is suitably varied; Figure 34 is a schematic sectional view of a sixth embodiment of a developing device according to the present invention; Figure 35 is a sectional view taken along a line XXXV-XXXV of Fig. 34; Figures 36(a), 36(b), 36(c), and 36(d) are diagrammatic views showing the distribution of accumulated developer and the adhesion of developer to a developing roller in the developing device of Fig. 34 when the height of a front partition plate is set to be 11 mm and when the total quantity of developer is suitably varied; Figures 37(a), 37(b), 37(c), 37(d) and 37(e) are diagrammatic views showing the distribution of accumulated developer and the adhesion of developer to a developing roller in the developing device of Fig. 34 when the height of a front partition plate is set to be 5 mm and when the total quantity of developer is suitably varied; Figure 38 is a graph showing the relationship between the height of a front partition plate and the slope of the surface of the accumulated developer in the developing device of Fig. 34; Figure 39 is a graph showing the relationship between a front partition plate and the minimum height of the accumulated developer in the developing device of Fig. 34; Figure 40 is a graph showing the relationship between the total quantity of developer and the minimum height of the accumulated developer in the developing device of Fig. 34; Figure 41 is a graph showing the relationship between the total quantity of the developer and the quantity of the accumulated developer in the developing device of Fig. 34; Figure 42(a) is a sectional view, similar to Fig. 35, showing a modification of the developing device of Fig. 35, and Figures 42(b), 42(c), and 42(d) are diagrammatic views showing the distribution of accumulated developer and the adhesion of developer to a developing roller in the developing device of Fig. 42(a) when the total quantity of developer is suitably varied; Figure 43(a) is a sectional view of a comparative example to the developing device of Fig. 42(a), and Figures 43(b), 43(c) and 43(d) are diagrammatic views showing the distribution of accumulated developer and the adhesion of developer to a developing roller in the developing device of Fig. 43(a) when the total quantity of developer is suitably varied; Figure 44 is a graph showing the relationship between the total quantity of developer and the slope of the surface of accumulated developer in each of the developing devices of Figs. 42(a) and 43(a); Figure 45(a) is a front view of a paddle roller provided with stirring-paddle elements for leveling the surface of an accumulated developer; Figure 45(b) is an end view of Fig. 45(a); Figure 46(a) is a front view of another paddle roller provided with stirring-paddle elements for leveling the surface of an accumulated developer; Figure 46(b) is an end view of Fig. 46(a); Figure 47(a) is a front view of yet another paddle roller provided with stirring-paddle elements for leveling the surface of an accumulated developer; Figure 47(b) is an end view of Fig. 47(a); Figure 48(a) is a front view of yet another paddle roller provided with stirring-paddle elements for leveling the surface of an accumulated developer; Figure 48(b) is an end view of Fig. 48(a); Figure 49(a) is a front view of yet another paddle roller provided with stirring-paddle elements for leveling the surface of an accumulated developer; Figure 49(b) is an end view of Fig. 49(a); Figure 50(a) is a front view of yet another paddle roller provided with stirring-paddle elements for leveling the surface of an accumulated developer; Figure 50(b) is an end view of Fig. 50(a); Figure 51(a) is a front view of yet another paddle roller provided with stirring-paddle elements for leveling the surface of an accumulated developer; Figure 51(b) is an end view of Fig. 51(a); Figure 52(a) is a front view of yet another paddle roller provided with stirring-paddle elements for leveling the surface of an accumulated developer; Figure 52(b) is a plane view of Fig. 52(a); Figure 52(c) is an end view of Fig. 52(a); Figure 53(a) is a front view of yet another paddle roller provided with stirring-paddle elements for leveling the surface of an accumulated developer; Figure 53(b) is an end view of Fig. 53(a); Figure 53(c) is an enlarged front view of the stirring-paddle element showing in Fig. 53(a); Figure 53(d) is a plane view of Fig. 53(c); Figure 54 is a schematic plan view of a developer-agitating chamber of a developing device in accordance with another aspect of the present invention; Figure 55 is a plan view of a feed screw conveyor used in the developer-agitating chamber of Fig. 54; and Figure 56 is a graph showing the relationship between a developer-moving speed and the pitch of the screw of the screw conveyor. BEST MODE FOR CARRYING THE INVENTION Referring to Fig. 1, schematically showing a high-speed electrophotographic printer as a representative of an electrostatic recording apparatus, the printer comprises a photosensitive drum 10 as an electrostatic latent image carrying body. During the operation of the printer, the photosensitive drum 10 is rotated in a direction indicated by the arrow in the drawing. The photosensitive drum 10 is charged uniformly by a precharger 12, and an electrostatic latent image is written by an optical writing means 14 on a charged area of the photosensitive drum 10. Although the precharger 12 comprises a corona charger or a scorotron charger, it may be a conductive roller charger or a conductive brush charger. As the optical writing means, a laser beam scanner, an LED (light emitting diode) array, a liquid crystal shutter array etc. are known. The electrostatic latent image formed on the photosensitive drum 10 is electrostatically developed as a charged toner image by a developing device 16, and the charged toner image is electrostatically transferred to a recording medium such as a recording sheet of paper P by a transfer unit 18. Note, the developing device 16 shown in Fig. 1 is of a conventional type. The sheet of paper P is fed from a paper feeder unit not shown in the drawing, and is once sopped at a pair of register rollers 20. Then, the sheet of paper P is fed by the register rollers 20 into a clearance between the photosensitive drum 10 and the transfer unit 18 at a given timing in accordance with the writing of an electrostatic latent image, whereby the charged toner image is transferred from the photosensitive drum 10 to the sheet of paper P. In this example shown in Fig. 1, the transfer unit 18 comprises a transfer device 18a including a corona discharger or the like, and a charge-eliminating device 18b including an AC eliminator or the like. The transfer device 18a gives the sheet of paper P a charge of a polarity opposite that of the charge of the charged toner image, to thereby transfer the charged toner image from the photosensitive drum 10 to the sheet of paper P, and the charge-eliminating device 18b partially eliminates the charge from the sheet of paper P just after the transfer of the charged toner image, whereby the sheet of paper P can be easily separated from the photosensitive drum 10. The sheet of paper P subjected to the transfer process is fed to a fixing device 22, at which the transferred toner image is fixed on the sheet of paper P. Residual toner not transferred to the sheet of paper P and remaining on a surface of the photosensitive drum 10 is removed by a cleaning means 24. Of course, in the high-speed printer, the residual toner must be quickly and completely removed from the surface of the photosensitive drum 10, and a large quantity of the residual toner must be dealt with, before toner images can be recorded on a large number of recording sheets. Accordingly, in the high-speed printer, the cleaning means 24 of the type shown in Fig. 1 is used. In particular, the cleaning means 24 comprises a toner-recovering container 24a having an inlet opening for receiving a part of the surface of the photosensitive drum 10, a fur brush 24b provided within the toner-recovering container 24a so as to be close to the inlet opening, a toner-scraping blade 24c extending along an upper edge of the inlet opening, and a screw conveyor 24d disposed in the bottom of the toner-recovering container 24a. Naturally, the fur brush 26b brushes the residual toner off the surface of the photosensitive drum 10, and the toner scraping blade 24c scrapes off the residual toner which could not be removed by the fur brush 24b. The residual toner scraped off by the toner-scraping blade 24c falls into the toner-recovering container 24a, and then the screw conveyor 24d conveys the recovered toner to a predetermined place. Thus, the cleaning means 24 of this type is relatively bulky. Note, after the surface of the photosensitive drum 10 is cleaned by removing the residual toner therefrom with the cleaning means 24, the cleaned surface is irradiated by a charge-eliminating lamp 26 to eliminate a residual charge therefrom. As the developer used in the above-mentioned developing process, a two-component developer composed of toner component (fine particle of colored resin) and a magnetic carrier component (fine magnetic carriers) is well known. In general, the two-component developer is widely used in multicolor recording. As shown specifically in Fig. 2, the developing device 16 using the two-component developer comprises a developer container 28 for holding the two-component developer, an agitator 30 for agitating the two-component developer to cause the toner component and the magnetic carrier component to be subjected to triboelectrification, and a magnet roller or developing roller 32 for forming a magnetic brush therearound by magnetically attracting a part of the magnetic carrier component. The developing roller 32 is partially exposed from the developer container 28 and faces the photosensitive drum 10. The toner component is attracted electrostatically to the magnetic brush formed around the developing roller 32, and is brought to a facing zone or developing zone between the developing roller and the photosensitive drum due to the rotation of the developing roller, whereby development of an electrostatic latent image can be carried out. Since the density of a toner image obtained by developing an electrostatic latent image is dependent on a quantity of toner brought to the developing zone, the height of the magnetic brush is regulated by a doctor blade 34 to ensure evenness of the density of development. The developer which has passed through the developing zone, i.e., the developer having a reduced toner component, is scraped off the developing roller 32 by a scraper blade 36, and is then returned to the agitating unit 30. Since the toner component is consumed continuously during the developing process, the two-component developer must be properly replenished with a toner component, to thereby maintain a quality of a developed toner image, therefore a recorded toner image, constant. Also, a uniform distribution of the toner component in the magnetic carrier component is an important factor in the quality of the recorded toner image, as well as a sufficient triboelectrification between the toner component and the magnetic carrier component. Further, in a high-speed printer, consumption of the developer in the developing process necessarily becomes larger, and thus the developer must be quickly and efficiently agitated. For this reason, in general, the agitator 30 is constituted as a circulation-type agitator as illustrated. In particular, the agitator 30 comprises a pair of screw conveyors 30a and 30b, and a partition plate 30c disposed therebetween, and the pair of screw conveyors 30a and 30b is disposed in parallel with the developing roller 32 within the developer container 28. The pair of screw conveyors 30a and 30b are extended between the opposite side walls of the developer container 28, and a length of the partition plate 30c is smaller than those of the screw conveyors 30a and 30b such that each of the opposite ends of the partition plate 30c is spaced a predetermined distance apart from a corresponding side wall of the developer container 28. The screw conveyors 30a and 30b are driven so as to move the developer in opposite directions, to thereby produce a circulation path for the developer. In particular, when the screw conveyor 30a thrusts the developer one end thereof, the developer is moved to a corresponding end of the screw conveyor 30b around a corresponding end of the partition plate 30c. When the screw conveyor 30b thrusts the developer to the other end thereof, the developer is moved to the other end of the screw conveyor 30a around the other end of the partition plate 30c. Thus, the developer is circulated along the screw conveyors 30a and 30b. In the developing device provided with this type agitator 30, a large quantity of developer can be efficiently agitated. Nevertheless, an overall structure of the developing device is relatively large. Figure 3 schematically shows an example of a multi-drum type multicolor recording apparatus having electrostatic recording units of the type as shown in Figs. 1 and 2. This multicolor recording apparatus is provided with four electrostatic recording units Y, C, M and B for full-color recording. The electrostatic recording units Y, C, M and B are identical with each other, and are aligned with along an endless conveyor belt means 38 for conveying a recording sheet of paper. In Fig. 3, elements like or corresponding to those shown in Fig. 1 are designated by the same references. Each of the electrostatic recording units or printers Y, C, M and B features a laser beam scanner used as the optical writing means 14, and a conductive transfer roller as the transfer unit 18. The conductive transfer roller 18 is pressed against a photosensitive drum 10 through the intermediary of an upper run of the endless conveyor belt means 38. The respective developing devices 16 of the electrostatic printers Y, C, M and B use a developer containing an yellow toner component, a developer containing a cyan toner component, a developer containing a magenta toner component and a developer containing a black toner component, respectively. The electrostatic printers Y, C, M and B record an yellow toner image, a cyan toner image, a magenta toner image, and a black toner image, respectively. A pair of register rollers 20 is provided near one end or the inlet end of the endless conveyor belt means 38. In a recording-operation, a recording sheet of paper fed from a sheet feeder unit 40 is once stopped to be on standby at the register rollers 20. Electrostatic latent images are written sequentially on the basis of color image data on the photosensitive drums 10 of the electrostatic printers Y, C, M and B, respectively, and the sheet of paper is sequentially passed through the electrostatic printers Y, C, M and B at a given timing, whereby an yellow toner image, a cyan toner image, a magenta toner image, and a black toner image are sequentially transferred to the sheet of paper to thereby form a full-color image. The sheet of paper carrying the full-color image is passed through a fixing device 22 provided near the other end or outlet end of the endless conveyor belt means 38, to thereby fix the full-color image on the sheet of paper. Thereafter, the sheet of paper is delivered by delivery rollers 42 onto a delivery tray 44 provided outside the multicolor recording apparatus. As mentioned hereinbefore, the multi-drum multicolor recording apparatus is capable of high-speed multicolor recording operation, but it has a disadvantage in that the construction thereof is large due to the alignment of the electrostatic recording units. Especially, a multi-drum type multicolor recording apparatus having high-speed electrostatic printers of the type as depicted in Fig. 1 has a very large construction, because the developing devices and the cleaning means are relatively bulky. However, the application of a developing device in accordance with the present invention to a multi-drum multicolor recording apparatus as mentioned above enables the multi-drum type multicolor recording apparatus to be relatively small. Referring now to Figs. 4 and 5, there is shown a first embodiment of a developing device according to the present invention, which comprises a developer container 46 for holding a two-component developer. Note, in Fig. 4, the two-component developer held in the developer container 46 is represented by fine dots. The developer container 46 has a first bottom wall 46a, a first back wall 46b extending upward from a back end of the first bottom wall 46a, a second bottom wall 46c horizontally extending on the upper end of the first back wall 14b, a second back wall 46d extending upward from a back end of the second bottom wall 46c, a top wall 46e extending horizontally forward from an upper end of the second back wall 46d, and a front wall 46f extending downward from a front end of the top wall 46e. Opposite side walls (not shown) are integrally joined to the opposite ends of the walls 46a through 46f, respectively. Note, in this embodiment, the second back wall 46d, the top wall 46e and the front wall 46f are integrally formed as in a single piece, but the top wall 46e may be constituted as a detachable wall, if necessary. As shown in Figs. 4 and 5, the developer container 46 has an opening defined by a front end of the first bottom wall 46a and a lower end of the front wall 46f, and a magnetic roller or a developing roller 48 is disposed in the opening such that a part of a surface of the developing roller 48 is exposed therefrom. In this embodiment, the developing roller 48 comprises a shaft 48a fixedly supported on the opposite side walls of the developer container 46, a core 48b formed of a magnetic material and fixedly mounted on the shaft 48a, and a sleeve 48c formed of a nonmagnetic material such as aluminum and provided on the core 48b to be rotatable therearound. During an operation of the developing device, the sleeve 48c is rotationally driven in a direction indicated by the arrow in the drawing. When the developing device is incorporated in an electrostatic recording unit, the exposed surface of the developing roller 48 and with the exposed surface of the developing roller 48, therefore the exposed surface of the sleeve 48c faces and electrostatic latent image carrying body such as a photosensitive drum. The first bottom wall 46a of the developer container 46 provides a developer-accumulating chamber 50, and a paddle roller 52 is extended in the developer-accumulating chamber 50. The paddle roller 52 is rotatably supported by the opposite side walls of the developer container 46, and is rotationally driven in a direction indicated by the arrow in the drawing during an operation of the developing device. The developer held in the developer-accumulating chamber 50 is fed to the developing roller 48 by the paddle roller 52, and is entrained by the developing roller 48 in the same manner as the developing roller 32 of Fig. 2, whereby the developer is brought to the facing zone or developing zone between the developing roller 48 and the electrostatic latent image carrying body or photosensitive drum. A doctor blade 54 is attached to the front edge of the first bottom wall 48a such that a quantity of developer to be brought to the developing zone by the developing roller 48 is regulated at a given extent. The second bottom wall 46c of the developer container 46 provides a developer-agitating chamber 56 placed above the developer-accumulating chamber 50. A developer-agitating unit 58 is provided in the developer-agitating chamber 56, and comprises a pair of screw conveyors 58a and 58b extended in parallel to each other between the opposite side walls of the developer container 46. As shown in Figs. 4 and 5, the second bottom wall 46c has a pair of curved recesses formed in an upper surface thereof for receiving the helical screws of the screw conveyors 58a and 58b. Note, in Fig. 4, the helical screws of the screw conveyors 58a and 58b are represented by circles in Fig. 4, but, in Fig. 5, only the shafts of the screw conveyors 58a and 58b are shown, the helical screws thereof being omitted for simplicity. During an operation of the developing device, the shafts of the screw conveyors 58a and 58b are supported rotatably on the opposite side walls of the developer container 46, and are rotationally driven in directions indicted by the arrows in the drawing (i.e., in opposite directions), respectively. In this embodiment, both the helical screws of the screw conveyors 58a and 58b are formed as right-hand screws. Accordingly, in Fig. 4, the screw conveyor 58a thrusts the developer away from the viewer, and the screw conveyor 58b thrusts the developer toward the viewer. A partition plate 60 is extended upright from the second bottom wall 46c between the conveyor screws 58a and 58b. The length of the partition plate 60 is shorter than those of the screw conveyors 58a and 58b, and the opposite ends of the partition plate 60 are spaced a predetermined distance apart from the side walls of the developer container 46, respectively. Similarly to the screw conveyors 30a and 30b previously described with reference to Fig. 2, the screw conveyors 58a and 58b form a circulation path for the developer. Namely, when the screw conveyor 58a thrusts the developer to one end thereof, the developer is moved to a corresponding end of the screw conveyor 58b around a corresponding end of the partition plate 60. When the screw conveyor 58b thrusts the developer to the other end thereof, the developer is moved to the other end of the screw conveyor 58a around the other end of the partition plate 60. Thus, the developer is circulated along the screw conveyors 58a and 58b. As shown in Figs. 4 and 5, a vertical partition wall 46g is extended between the opposite side walls of the developer container 46, and is spaced from the front surface 62 of the second bottom wall 46c so as to form a space of a predetermined width therebetween. This space serves as a communication passage 64 for communicating the developer-agitating chamber 56 with the developer-accumulating chamber 56. An upper end of the communication passage 64 forms an overflow opening through which a part of the developer circulated by the developer-agitating unit 58 overflows into the communication passage 64, and the overflowing developer falls by gravity into the developer-accumulating chamber 50, whereby the developer can be suitably fed thereto. A developer-lifting passage 66 is defined by the front wall 46f and the vertical partition wall 46g of the developer container 46 so as to be vertically extended just above the developing roller 48, as shown in Fig. 4. Two magnet rollers 68 and 70, which are substantially the same in construction as the developing roller 48 formed as the magnet roller, are vertically aligned with each other in the developer-lifting passage 64. The respective magnet rollers 68 and 70 comprise shafts 68a and 70a fixedly supported by the opposite side walls of the developer container 46, cores 68b and 70b formed of a magnetic material and fixedly mounted on the shafts 68a and 70a, and sleeves 68c and 70c formed of a nonmagnetic material such as aluminum and rotatably provided on the cores 68b and 70b. During an operation of the developing device, the sleeves 68c and 70c are rotationally driven in the same direction indicated the arrows in the drawing. The respective cores 48c, 68c, and 70c of the developing roller 48 and the magnet roller 68 and the magnet roller 70 are locally magnetized as shown in Fig. 6, and this local magnetization can be performed by locally applying a magnetic field to the cores 48c, 68c, and 70c. As is apparent from Fig. 6, the core 48c has three S-poles S 1 , S 2 , and S 3 arranged at appropriate angular intervals on the surface thereof, and two N-poles N 1 and N 2 arranged between the adjacent ones of the three S-poles. The core 68c has three S-poles S 1 , S 2 and S 3 arranged at appropriate angular intervals on the surface thereof, and two N-poles N 1 and N 2 arranged between the adjacent ones of the three S-poles, and the same is true for the core 70c. Referring to Fig. 7, a positional relationship between the magnetic poles of the developing roller 48 is diagrammatically shown by way of example. As is apparent from this drawing, the N-pole N 1 is at an angular position of 5° measured clockwise from a horizontal plane including the central axis of the developing roller 48; the N-pole N 2 is at an angular position of 120° measured counterclockwise from the N-pole N 1 ; the S-pole S 1 is at an angular position of 160° separated counterclockwise from the N-pole N 2 ; the S-pole S 2 is at an angular position of 60° measured counterclockwise from the N-pole N 2 ; and the S-pole S 3 is at an angular position of 75° measured clockwise from the N-pole N 2 . Referring to Fig. 8, a positional relationship between the magnetic poles of each of the magnet rollers 68 and 70 is diagrammatically shown by way of example. The N-pole N 2 is at an angular position of 35° measured clockwise from a horizontal plane including the central axis of the magnet roller; the N-pole N 1 is at an angular position of 60° measured counterclockwise from the horizontal plane; the S-pole S 1 is at an angular position of 90° counterclockwise from the horizontal plane, the S-pole S 2 is at an angular position of 10° measured from the horizontal plane; and the S-pole S 3 is at an angular position of 80° measured clockwise from the horizontal plane. According to the positional relationship between the magnetic poles of the developing roller 48, a magnetic field is produced between the adjacent magnetic poles of the opposite polarities, and no magnetic field is produced between the S-poles S 1 and S 3 . Similarly, according to the positional relationship between the magnetic poles of the magnetic roller 68, 70, a magnetic field is produced between the adjacent magnetic poles of the opposite polarities, and any magnetic field is not produced between the S-poles S 1 and S 3 . Further, according to the positional relationship between the developing roller 48, the magnet roller 68, and the magnet roller 70, a production of a magnetic field is prevented between the developing roller 48 and the magnet roller 68 because the S-pole S 3 of the developing roller 48 is adjacent to the S-pole S 1 of the magnet roller 68, and also a production of magnetic field is prevented between the magnet rollers 68 and 70 because the S-pole S 3 of the magnet roller 68 is adjacent to the S-pole S 1 of the magnet roller 70. Thus, as is apparent from Figs. 4 and 6, when the respective sleeves 48c, 68c and 70c of the developing roller 48 and the magnet rollers 68 and 70 are rotationally driven, the developer brought to and passed through the developing zone by the developing roller 48 is lifted along the front sides of the magnet rollers 68 and 70 without being returned to the developer-accumulating chamber 50. To ensure a smooth upward lift of the developer from the developing roller 48 to the magnet roller 68, for example, the respective magnetic poles of the developing roller 48 and the respective magnetic poles of the magnet roller 68 may be given magnetic flux densities, as tabulated in the following table: Polarity Roller 48 Roller 68 N 1 590G (Gauss) 750G (Gauss) N 2 850G ( " ) 700G ( " ) S 1 510G ( " ) 600G ( " ) S 2 850G ( " ) 700G ( " ) S 3 650G ( " ) 600G ( " ) In the above-table, it should be noted that the magnetic flux density (750G) of the N-pole N 1 of the magnet roller 68 is greater than the magnetic flux density (650G) of the S-pole S 3 of the developing roller 48, whereby the lift of the developer from the developing roller 48 to the magnet roller 68 can be facilitated. Although the magnetic flux densities of the magnetic poles of the magnet roller 70 are equal to those of the corresponding magnetic poles of the magnet roller 68, it is preferable to make the magnetic flux density of the N-pole N 1 of the magnet roller 70 to be greater than that of the S-pole S 3 of the magnet roller 68, similar to the relationship between the developing roller 48 and the magnet roller 68. As shown in Figs. 4 and 5, a scraper member 72 is attached to an upper end of the vertical partition wall 46g such that a front edge thereof is positioned slightly behind the S-pole S 3 of the magnet roller 70, whereby the developer lifted from the developing roller 48 to the magnet roller 70 can be separated from the surface of the magnet roller 70 so as to be fed to the developer-agitating chamber 56. In short, the developer held in the developer container 46 is supplied from the developer-agitating chamber 56 through the communication passage 64 into the developer-accumulating chamber 50, and is then brought to and is passed through the developing zone by the developing roller 48. Thereafter, the developer is lifted from the developing roller 48 by the magnet rollers 68 and 70, and is then returned to the developer-agitating chamber 56 by the scraper member 72. In this way, during an operation of the developing device, the developer is continuously circulated in the developer container 46 to ensure that the well mixed developer (i.e., the developer in which the toner component and the magnetic carrier component are sufficiently subjected to triboelectrification, and in which the toner component is uniformly distributed in the magnetic carrier component) is continuously supplied to the developer-accumulating chamber 50. Referring to Fig. 9 which is similar to Fig. 1 showing the electrostatic recording unit provided with the conventional developing device, an electrostatic recording unit provided with the developing device according to the present invention is shown. Note, in Fig. 9, elements like or corresponding to those shown in Fig. 1 are designated by the same references, and the developing device according to the present invention is designated by reference 16'. The developing device 16' features a structural arrangement in which a lower structural portion thereof (i.e., a structure below the second bottom wall 46c) has a substantially reduced size, and thus provides a large empty space, which has been occupied by a part of the arrangement of the conventional developing device 16. According to this structural feature, a multicolor recording apparatus as can be substantially miniaturized, as shown in Fig. 3. Figure 7 shows an embodiment of a multicolor recording apparatus, which is similar to the multicolor recording apparatus of Fig. 3, provided with four electrostatic recording units of the type as shown in Fig. 9. In Fig. 10, elements like or corresponding to those shown in Fig. 3 are designated by the same references, and the developing devices according to the present invention are indicated by reference 16'. As is apparent from Fig. 10, due to the fact that each of the developing devices 16' according to the present invention has a lower structure of a substantially reduced size, a cleaning means 24 included in one of the two adjacent electrostatic recording units can be disposed in a space below the second bottom wall 46c of the developer container 46 included in the other electrostatic recording unit, whereby a length of the alignment of the four electrostatic recording units Y, C, M and B can be substantially reduced. As is apparent from the comparison of Fig. 10 with Fig. 3, an overall construction of the multicolor recording apparatus according to the present invention can be substantially miniaturized. It should be understood that, although the polarities of all the magnetic poles of the developing roller 48 and the magnet rollers 68 and 70 are reversed in the above-mentioned first embodiment, the developer can be brought by the developing roller 48, and can be then lifted by the magnet rollers 68 in the same manner. Also, the arrangement of the magnetic poles shown in Figs. 6 to 8 is merely illustrated by way of example, and it should be understood that the magnetic poles may be arranged in another arrangement. Furthermore, although, in the first embodiment, the periphery of the respective cores 48b, 68b, and 70b of the developing roller 48 and magnet rollers 68 and 70 are locally magnetized, a plurality of bar magnets may be fixedly arranged along the inner periphery of the sleeves 48c, 68c and 70c, instead of the cores. Figures 11 and 12 shows a second embodiment of a developing device according to the present invention, which is constituted in substantially the same manner as the first embodiment as mentioned above. Note, in Figs. 11 and 12, elements like or corresponding to those shown in Figs. 4 and 5 are designated by the same references. As is apparent from Fig. 11, in the second embodiment, a communication passage 64 between a developer-agitating chamber 56 and a developer-accumulating chamber 50 is extended through a middle part of a second bottom wall 46c, and screw conveyors 58a and 58b are disposed on the opposite sides of the communication passage 64, respectively. The communication passage 64 is defined by a pair of partition plates 60a and 60b disposed between the screw conveyors 58a and 58b, and a length of the communication passage 64 is equal to those of the partition plates 60a and 60b. Similarly to the above-mentioned partition plate 60, the lengths of the partition plates 60a and 60b are smaller than those of the screw conveyors 58a and 58b, and the opposite ends of the partition plates 60a and 60b are spaced a predetermined distance apart from the corresponding side walls of a developer container 46. Accordingly, similarly to the first embodiment, the screw conveyors 58a and 58b of the second embodiment form a developer-circulating passage in the developer-agitating chamber 56. As is apparent from Fig. 11, one of the partition plates 60a and 60b, i.e., the partition plate 60a is extended to an upper wall 46e, whereas the other partition wall 60b has a height lower than a level of the respective tops of the screw conveyors 58a and 58b. Accordingly, the upper end opening of the communication passage 64 forms an overflow opening for the developer contained in the developer-agitating chamber 56. As is apparent from Figs. 11 and 12, in the second embodiment, a vertical partition wall 46g is formed integrally with a front part 62' of the second bottom wall 46c, and a scraper member 72 is attached to the upper end of the front part 62'. The vertical partition wall 46g and the front part 62' of the second bottom wall 46c define a developer-lifting passage 66 together with a front wall 46f. As shown in Fig. 13, in the second embodiment, the magnetic poles of a developing roller 46 are arranged in substantially the same manner as the first embodiment, but the arrangement of the magnetic poles of the magnetic rollers 68 and 70 is different from that of the first embodiment. In the first embodiment, the magnet rollers 68 and 70 is substantially identical with each other in the arrangement of the magnetic poles. Nevertheless, in the second embodiment, the magnetic rollers 68 and 70 are analogous in the arrangement of the magnetic poles, but they are opposite to each other in the polarities of the magnetic poles. Further, although the developing roller 48, and the magnet rollers 68 and 70 of the first embodiment are rotated in the same direction, only the magnet roller 68 of the second embodiment is rotated in a direction opposite the direction in which the developing roller 48 and the magnet roller 70 thereof are rotated. Thus, as shown in Figs. 12 and 13, in the second embodiment, the developer transported by the developing roller 48 is lifted along an S-shaped route by the magnet rollers 68 and 70. Similarly to the first embodiment, it is obvious that the second embodiment so constituted enables the miniaturization of the multicolor recording apparatus. Since, in the second embodiment, the developer transported by the developing roller 48 is lifted along the S-shaped route by the magnet rollers 68 and 70, the developer can be relatively smoothly moved upward from the developing roller 48 to the magnet roller 68, and then upward from the magnet roller 68 to the magnet roller 70. This developer-lifting means (68 and 70) may be applied to the first embodiment. Nevertheless, before the developing-lifting means (68 and 70) can be incorporated into the first embodiment, the influence of the magnetic fields, produced by the magnet rollers 68 and 70 of the second embodiment, on the developer falling along the front surface of the second bottom wall 46c must be reduced as much as possible. Otherwise, the developer falling along the front surface 62 of the second bottom wall 46c is magnetically attracted to the vertical partition wall 46g, resulting a clogging of the communication passage 64 with the attracted developer. Although, in each of the first and second embodiments, the developer-lifting means including two magnet rollers 68 and 70 is used for lifting the developer from the developing roller 48 and for returning the same to the developer-agitating chamber 56, it should be understood that the developer raising means may be provided with a single magnet roller or more than two magnet rollers, if necessary. Figure 14 shows a modification of the second embodiment, in which elements like or corresponding to those of the second embodiment are designated by the same references. As is apparent from Fig. 14, the developing device is provided with a mechanical developer-lifting means comprising a vane wheel 68', instead of the magnet roller 68. During operation of the developing device, the vane wheel 68' is rotationally supported by the opposite side walls of a developer container 46 so as to rotate in a direction indicated by the arrow in the drawing. A concave circular surface 74 is formed in the surface of a vertical partition wall 46g so as to conform to the shape of the vane wheel 68'. The developer is lifted from a developing roller 48, and it is moved to a magnet roller 70, by the cooperative action of the vane wheel 68' and the concave circular surface 74. The lower edge of the concave circular surface 74 must be located as shown in Fig. 15 in order to facilitate the action of raising the developer. In particular, when the core 48 of the developing roller 48 is locally magnetized, as shown in Fig. 15, so as to produce a magnetic field MF between an N-pole and an S-pole located near the vane wheel 68', as indicated by a broken line, the lower edge of the concave circular surface 74 is extended along a line at which the magnetic field MF disappears, whereby the vane wheel 68' can easily scrape the developer from the developing roller 48 without being affected by the magnetic field MF. In the first embodiment shown in Fig. 4, the mechanical developer-lifting means, i.e., the vane wheel 68' as shown in Fig. 14, can be preferably used instead of the magnet roller 68, because the vane wheel 68' itself does not exert any magnetic influence on the developer falling down the communication passage 64. In the second embodiment shown in Figs. 11 to 13 and the modified embodiment thereof shown in Figs. 14 and 15, the communication passage 64 for communicating the developer-accumulating chamber 50 and the developer-agitating chamber 56 with each other is formed in the second bottom wall 46c between the screw feeders 58a and 58b. With this arrangement, it is not possible to uniformly distribute the developer in the developer-accumulating chamber 50, the developer being supplied from the developer-agitating chamber 56 through the communication passage 64 into the developer-accumulating chamber 50. In particular, as shown in Fig. 16, when the developer circulates in the developer-agitating chamber 56 of the second embodiment, the first screw conveyor 58a placed on the front side thrusts the developer toward the right side wall R of the developer container, and the second screw conveyor 58b placed on the rear side thrusts the developer toward the left side wall L thereof. Accordingly, the developer returned by the scraper member 72 into the first screw conveyor 58a in the developer-agitating chamber 56 is concentrated near the side wall R of the developer container 46. Consequently, the quantity of the developer supplied into the developer-accumulating chamber 50 through a region of the communication passage 64 near the side wall R is relatively large, whereas the quantity of the developer supplied into the developer-accumulating chamber 50 through a region of the communication passage 64 near the side wall L is relatively small, and thus the developer is distributed unevenly in the developer-accumulating chamber 50. An excessively uneven distribution of the developer prevents the uniform formation of a magnetic brush on the developing roller 48, which makes it impossible to carry out the developing process properly. Figures 17 and 18 show a third embodiment of a developing device according to the present invention, which is constituted so as to obviate the uneven distribution of developer in a developer-accumulating chamber 50. In Figs. 17 and 18, elements like or corresponding to those shown in Figs. 4 and 5 are designated by the same references. The third embodiment is substantially identical with the second embodiment, except that the overflow edge of a partition plate 60b of the third embodiment is sloped in the length direction of the communication passage 64 whereas the partition plate 60b of the second embodiment has a uniform height. That is, as shown in Fig. 18, the overflow edge of the partition plate 60b of the third embodiment is gradually sloped down from one end on the side of the side wall R of a developer container 46 toward the other end on the side of the side wall L thereof. Accordingly, the quantity of the developer that overflows into the communication passage 64 in a region, where the height of the overflow edge is large, is reduced, and thus the developer can be distributed uniformly in the developer-accumulating chamber. In fact, the quantity of the developer accumulated in the developer-accumulating chamber 50 and the distribution of developer in the developer-accumulating chamber 50 are dependent on parameters including the total quantity of developer held in the developer container 46, the quantity of developer circulated in unit time in the developer container 46, and the speed of rotation of the screw conveyors 58a and 58b. The inventors actually fabricated a developing device corresponding to the third embodiment, and conducted experiments using the fabricated developing device to examine how the developer was distributed in the developer-accumulating chamber 50 in accordance with variations of the parameters. The details of the experiments will be described hereinafter. The developing device fabricated for the experiments has following particulars: (1) The longitudinal length of the developer container 46, i.e., the axial length of the developing roller 48 was 300 mm. (2) The developing roller 48 was 40 mm in diameter and 300 in length. (3) The screw conveyors 58a and 58b were 28 mm in diameter and 25 mm in screw pitch. (4) The communication passage 64 was 4 mm in width (the ratio of the width to the diameter of the screw conveyors 58a and 58b was 0.14.). (5) The maximum height H (Fig. 18) of the partition plate 60b was 14 mm (the ratio of the maximum height H to the diameter of the screw conveyors 58a and 58b was 0.5), and the minimum height of the same is zero. The angle of slope of the overflow edge of the partition plate 60b was 3.4°. (6) The total quantity of developer held in the developer container 46 was 1.6 kg. (7) The specific gravity of the developer was 1.85 g/cm. The screw conveyors 58a and 58b were rotated at 300 rpm, and the developer was circulated in the developer container 46 at each of developer-circulation rates of 6.3 g/s/cm (3.4 cm 3 /s/cm as a developer circulation volume), 7.4 g/s/cm (4.0 cm 3 /s/cm as a developer circulation volume), and 8.7 g/s/cm (4.7 cm 3 /s/cm as a developer circulation volume) to examine how a distribution of the developer varies in the developer-accumulating chamber 50 in accordance with the variation of the developer-circulation rate. Note, the developer-circulation rate is defined as a quantity of developer returned by the scraper member 72 to the developer-agitating chamber 56 per unit length of the scraper member 72 at unit time or a quantity of developer transported from the developer-accumulating chamber 50 by the developing roller 48 per unit length of the developing roller 48 at unit time. The developer-circulation rate was adjusted by varying the respective speeds of rotation of the developing roller 48 and the magnet rollers 68 and 70. Figures 19(a), 19(b) and 19(c) show the results of experiments, which are as follow: When the developer-circulation rate was 6.3 g/s/cm, the height distribution of an accumulated developer relative to a paddle roller 52 in the developer-accumulating chamber 50 is represented by a curve D shown on the left-hand side of Fig. 19(a). Each of numerical values written under the paddle roller 52 represents a height of an accumulated developer at the location at which the numerical value is appended. Namely, the numerical value of 34 mm, which is appended to a location of a right end R of the paddle roller 52 (corresponding to the right side wall R of the developer container), represents a height of an accumulated developer at that location of the right end thereof; and the numerical value of 27 mm, which is appended to a location near the right end R of the paddle roller 52, represents a height of an accumulated developer at that location near the right end thereof; and the numerical value of 7 mm, which is appended to a location of a left end L of the paddle roller 52 (corresponding to the left side wall R of the developer container), indicates a height of an accumulated developer at that location of the left end thereof. In this case, the adhesion of developer to the developing roller 48, i.e., the formation of magnetic brush on the developing roller 48 is shown in the right-hand side of Fig. 19(a). A developer-adhesion region of the developing roller 48, in which the magnetic brush is formed, is illustrated as a fine-dotted region, whereas a non-developer region, in which no magnetic brush is formed, is illustrated as a blank region. The width of the non-developer region was about 15 mm from the left end L of the developing roller 48. When the developer-circulation rate was 7.4 g/s/cm, a height distribution of an accumulated developer relative to the paddle roller 52 in the developer-accumulating chamber 50 is represented by a curve D shown on the left-hand side of Fig. 19(b). Numerical values written under the paddle roller 52 are similar to those explained with reference to Fig. 19(a). In this case, the adhesion of developer to the developing roller 48 is shown on the right-hand side of Fig. 19(b), and the width of a non-developer region was about 20 mm from the left end L of the developing roller 48. When the developer-circulation rate was 8.7 g/s/cm, a height distribution of an accumulated developer relative to the paddle roller 52 in the developer-accumulating chamber 50 is represented by a curve D shown on the left-hand side of Fig. 19(c). Numerical values written under the paddle roller 52 are similar to those explained with reference to Fig. 19(c). In this case, the adhesion of developer to the developing roller 48 is shown on the right-hand side of Fig. 19(b), and the width of a non-developer region was about 35 mm from the left end L of the developing roller 48. The screw conveyors 58a and 58b were rotated at 350 rpm, and the developer was circulated in the developer container 46 at each of developer-circulation rates of 6.3 g/s/cm (3.4 cm 3 /s/cm as a developer circulation volume), 7.4 g/s/cm (4.0 cm 3 /s/cm as a developer circulation volume), and 8.7 g/s/cm (4.7 cm 3 /s/cm as a developer circulation volume) to examine how a distribution of the developer varies in the developer-accumulating chamber 50 in accordance with the variation of the developer-circulation rate. Figures 20(a), 20(b) and 20(c) show the results of experiments, the details of which are as follows: When the developer-circulation rate was 6.3 g/s/cm, the height distribution of the accumulated developer relative to the paddle roller 52 in the developer-accumulating chamber 50 is represented by a curve D shown on the left-hand side of Fig. 20(a). Similar to the above-mentioned cases, each of the numerical values written under the paddle roller 52 represents the height of accumulated developer at the location at which that numerical value is appended. In this case, the adhesion of developer to the developing roller 48 is shown on the right-hand side of Fig. 20(a), and the developer was adhered to the entire surface of the developing roller 48. When the developer-circulation rate was 7.4 g/s/cm, the height distribution of the accumulated developer relative to the paddle roller 52 in the developer-accumulating chamber 50 is represented by a curve D shown on the left-hand side of Fig. 20(b). Similar to the above-mentioned cases, each of the numerical values written under the paddle roller 52 represents the height of accumulated developer at the location at which that numerical value is appended. In this case, the adhesion of developer to the developing roller 48 is shown on the right-hand side of Fig. 20(b), and the width of a non-developer region was about 15 mm from the left end L of the developing roller 48. When the developer circulating rate was 8.7 g/s/cm, the height distribution of the accumulated developer relative to the paddle roller 52 in the developer-accumulating chamber 50 is represented by a curve D shown on the left-hand side of Fig. 20(c). Similar to the above-mentioned cases, each of the numerical values written under the paddle roller 52 represents the height of accumulated developer at the location at which that numerical value is appended. In this case, the adhesion of developer to the developing roller 48 is shown on the right-hand side of Fig. 20(c), and the width of a non-developer region was about 23 mm from the left end L of the developing roller 48. The screw conveyors 58a and 58b were rotated at 400 rpm, and the developer was circulated in the developer container 46 at each of developer-circulation rates of 6.3 g/s/cm (3.4 cm 3 /s/cm as a developer circulation volume), 7.4 g/s/cm (4.0 cm 3 /s/cm as a developer-circulation volume), and 8.7 g/s/cm (4.7 cm 3 /s/cm as a developer circulation volume) to examine how the distribution of developer varies in the developer-accumulating chamber 50 in accordance with the variation of the developer-circulation rate. Figures 21(a), 21(b) and 21(c) show the results of experiments, the details of which are as follows: When the developer circulating rate was 6.3 g/s/cm, the height distribution of the accumulated developer relative to the paddle roller 52 in the developer-accumulating chamber 50 is represented by a curve D shown on the left-hand side of Fig. 21(a). Similar to the above-mentioned cases, each of the numerical values written under the paddle roller 52 represents the height of accumulated developer at the location at which that numerical value is appended. In this case, the adhesion of developer to the developing roller 48 is shown on the right-hand side of Fig. 21(a), and the developer was adhered to the entire surface of the developing roller 48. When the developer circulating rate was 7.4 g/s/cm, the height distribution of accumulated developer relative to the paddle roller 52 in the developer-accumulating chamber 50 is represented by a curve D shown on the left-hand side of Fig. 21(b). Similar to the above-mentioned cases, each of the numerical values written under the paddle roller 52 represents the height of accumulated developer at the location at which that numerical value is appended. In this case, the adhesion of developer to the developing roller 48 is shown on the right-hand side of Fig. 21(b), and the developer was adhered to the entire surface of the developing roller 48. When the developer circulating rate was 8.7 g/s/cm, the height distribution of the accumulated developer relative to the paddle roller 52 in the developer-accumulating chamber 50 is represented by a curve D shown on the left-hand side of Fig. 21(c). Similar to the above-mentioned cases, each of the numerical values written under the paddle roller 52 represents the height of accumulated developer at the location at which that numerical value is appended. In this case, the adhesion of developer to the developing roller 48 is shown on the right-hand side of Fig. 21(c), and the developer was adhered to the entire surface of the developing roller 48. As is apparent from the experimental results, in the third embodiment, when the maximum height of the sloped overflow edge of the partition plate 60b is 0.5 times the diameter of the screw conveyors 58a and 58b, and when the speed of rotation of the screw conveyors 58a and 58b is set to be low, the developer-circulation rate must be set to be small, before the developer can be adhered to the entire surface of the developing roller 48. For example, when the speed of rotation of the screw conveyor 58a and 58b is set to be less than 350 rpm, the developer-circulation rate must be 6.3 g/s/cm. On the contrary, when the speed of rotation of the screw conveyors 58a and 58b is set to be high, the developer-circulation rate can be increased. Note, in general, it is undesirable to rotate the screw conveyors 58a and 58b at an excessively-high speed, because an increment of rotational speed of the screw conveyors 58a and 58b for movement of the developer at a high speed results in a premature deterioration of the magnetic carrier component of the two-component developer. As the total quantity of developer held in the developer container 46 is increased, the quantity of developer accumulated in the developer-accumulating chamber 50 becomes large. Accordingly, it can be readily expected that an ability to adhere the developer to the surface of the developing roller 48 can be improved due to the increment of the total quantity of developer. Experiments were conducted using the above-mentioned developing device to examine a relationship between a quantity of developer accumulated in the developer-accumulating chamber 50 and a total quantity of developer held in the developer container 46. Conditions for the experiments are as follows: (1) The developer-circulation rate was 8.7 g/s/cm (4.7 cm 3 /s/cm as a developer-circulation volume). (2) The speed of rotation of the screw conveyors 58a and 58b was 400 rpm. (3) The total quantity of developer was changed in the range of from 1.4 kg to 1.82 kg. (4) The specific gravity of the developer is 1.85 g/cm. The results of the experiments are shown in a graph of Fig. 22. As is apparent from this graph, when the total quantity of the developer was 1.4 kg, the quantity of the developer accumulated in the developer-accumulating chamber 50 was 250g. When the total quantity of the developer was 1.82 kg, the quantity of the developer accumulated in the developer-accumulating chamber 50 was 330g. When the quantity of the developer accumulated in the developer-accumulating chamber 50 was 250g, the quantity of developer devoted to the developing roller 48 per unit length was 8 g/cm. When the quantity of the developer accumulated in the developer-accumulating chamber 50 was 330g, the quantity of developer on the developing roller 48 per unit length was 11 g/cm. Figures 23 and 24 show a fourth embodiment of a developing device according to the present invention, which, similar to the third embodiment, is constituted so as to obviate an uneven distribution of the accumulated developer in the developer-accumulating chamber 50. In Figs. 23 and 24, elements like or corresponding to those shown in Figs. 4 and 5 are designated by the same references. As is apparent from Figs. 23 and 24, in the fourth embodiment, overflow edges of both partition plates 60a and 60b are sloped along a length of a communication passage 64, and an upper end portion of the partition plate 60a is horizontally bent and projected above the partition plate 60b so as to define an overflow opening of the communication passage 64 in a vertical plane. As shown in Fig. 24, each of the overflow edges of both the partition plates 60a and 60b is sloped down such that the height thereof is gradually reduced from the right side wall R of the developer container toward the left side wall L thereof. Accordingly, the quantity of developer that overflows into the communication passage 64 in a region, where the height of the overflow edge is large, is reduced, whereby the developer can be uniformly distributed in the developer-accumulating chamber. A developing device according to the fourth embodiment was actually fabricated, and experiments were conducted to examine how the developer was distributed in the developer-accumulating chamber 50 in accordance with variation of relevant parameters. The details of experiments will be explained hereinafter. The developing device fabricated for the experiments has the following particulars: (1) The longitudinal length of the developer container 46, i.e., the size along the axis of the developing roller 48 was 300 mm. (2) The developing roller 48 was 40 mm in diameter and 300 mm in length. (3) The screw conveyors 58a and 58b are 28 mm in diameter and 25 mm in screw pitch. (4) The maximum height of a lower edge of a horizontal projection of the partition plate 60a was 26 mm (the ratio of the maximum height to the diameter of the screw conveyors 58a and 58b was 0.93), and the maximum height of an upper edge of the partition plate 60b was 22 mm (the ratio of the maximum height to the diameter of the screw conveyors 58a and 58b was 0.79). The lower edge of the horizontal projection of the partition plate 60a and the upper edge of the partition plate 60b were extended in parallel with each other, and the minimum height of the lower edge of the partition plate 60b is zero. (5) The specific gravity of the developer was 1.85 g/cm. The total quantity of developer held in the developer container 46 was set to be 1.4 kg, and the developer-circulation rate was set to be 6.3 g/s/cm (3.4 cm 3 /s/cm as the developer-circulation volume). The screw conveyors 58a and 58b were rotated at 150 rpm, 200 rpm, and 250 rpm to examine how a distribution of developer varies in the developer-accumulating chamber 50 in accordance with the variation of the speed of rotation of the screw conveyors 58a and 58b. Figures 25(a), 25(b) and 25(c) show the results of the experiments, the details of which are as follows: When the screw conveyors 58a and 58b were rotated at 150 rpm, a height distribution of the accumulated developer relative to a paddle roller 52 in the developer-accumulating chamber 50 is represented by a curve D shown on the left-hand side of Fig. 25(a). Similar to the above-mentioned case, each of the numerical values written under the paddle roller 52 represents the height of an accumulated developer at the location at which that numerical value is appended. In this case, the adhesion of developer to the developing roller 48 is shown on the right-hand side of Fig. 25(a), and a non-developer region (as shown as a blank region) having the width of about 400 mm from the right end R of the developing roller 48 was produced. When the screw conveyors 58a and 58b were rotated at 200 rpm, the height distribution of an accumulated developer relative to the paddle roller 52 in the developer-accumulating chamber 50 is represented by a curve D shown on the left-hand side of Fig. 25(b). Similar to the above-mentioned case, each of the numerical values written under the paddle roller 52 represents the height of an accumulated developer at the location at which that numerical value is appended. In this case, the adhesion of developer to the developing roller 48 is shown on the right-hand side of Fig. 25(b), and a non-developer region (as shown as a blank region) having the width of about 20 mm from the right end R of the developing roller 48 was produced. When the screw conveyors 58a and 58b were rotated at 250 rpm, the height distribution of an accumulated developer relative to the paddle roller 52 in the developer-accumulating chamber 50 is represented by a curve D shown on the left-hand side of Fig. 25(c). Similar to the above-mentioned case, each of the numerical values written under the paddle roller 52 represents the height of an accumulated developer at the location at which that numerical value is appended. In this case, the adhesion of developer to the developing roller 48 is shown on the right-hand side of Fig. 25(c), and the developer was adhered to the entire surface of the developing roller 48. Then, the total quantity of the developer held in the developer container 46 was set to be 1.6 kg, the developer circulating rate was set to be 8.7 g/s/cm (4.7 cm 3 /s/cm as the developer-circulation volume), and the screw conveyors 58a and 58b were rotated at 150 rpm, 200 rpm or 250 rpm to examine how the distribution of developer varies in the developer-accumulating chamber 50 in accordance with the variation of the speed of rotation of the screw conveyors 58a and 58b. Figures 26(a), 26(b) and 26(c) show the results of the experiments, the details of which are as follows: When the screw conveyors 58a and 58b were rotated at 150 rpm , the height distribution of the accumulated developer relative to a paddle roller 52 in the developer-accumulating chamber 50 is represented by a curve D shown on the left-hand side of Fig. 26(a). Similar to the above-mentioned cases, each of the numerical values written under the paddle roller 52 represents the height of accumulated developer at the location at which that numerical value is appended. In this case, an adhesion of developer to the developing roller 48 is shown on the right-hand side of Fig. 26(a), and the developer was adhered to the entire surface of the developing roller 48. When the screw conveyors 58a and 58b were rotated at 200 rpm, the height distribution of the accumulated developer relative to a paddle roller 52 in the developer-accumulating chamber 50 is represented by a curve D shown on the left-hand side of Fig. 26(b). Similar to the above-mentioned cases, each of the numerical values written under the paddle roller 52 represents the height of accumulated developer at the location at which that numerical value is appended. In this case, the adhesion of developer to the developing roller 48 is shown on the right-hand side of Fig. 26(b), and the developer was adhered to the entire surface of the developing roller 48. When the screw conveyors 58a and 58b were rotated at 250 rpm, the height distribution of the accumulated developer relative to a paddle roller 52 in the developer-accumulating chamber 50 is represented by a curve D shown on the left-hand side of Fig. 26(c). Similar to the above-mentioned cases, each of the numerical values written under the paddle roller 52 represents the height of accumulated developer at the location at which that numerical value is appended. In this case, an adhesion of developer to the developing roller 48 is shown on the right-hand side of Fig. 26(c), and the developer was adhered to the entire surface of the developing roller 48. Further, the total quantity of developer held in the developer container 46 was set to be 1.8 kg, and the developer-circulation rate was set to be 8.7 g/s/cm (4.7 cm 3 /s/cm as the developer-circulation volume). The screw conveyors 58a and 58b were rotated at 150 rpm, 200 rpm or 250 rpm to examine how the distribution of developer varies in the developer-accumulating chamber 50 in accordance with the variation of the speed of rotation of the screw conveyors 58a and 58b. Figures 27(a), 27(b) and 27(c) show the results of the experiments, the details of which are as follows: When the screw conveyors 58a and 58b were rotated at 150 rpm, the height distribution of the accumulated developer relative to the paddle roller 52 in the developer-accumulating chamber 50 is represented by a curve D shown on the left-hand side of Fig. 27(a). Similar to the above-mentioned cases, each of the numerical values written under the paddle roller 52 represents the height of accumulated developer at the location at which that numerical value is appended. In this case, the adhesion of developer to the developing roller 48 is shown on the right-hand side of Fig. 27(a), and the developer was adhered to the entire surface of the developing roller 48. When the screw conveyors 58a and 58b were rotated at 200 rpm, a height distribution of the accumulated developer relative to the paddle roller 52 in the developer-accumulating chamber 50 is represented by a curve D shown on the left-hand side of Fig. 27(b). Similar to the above-mentioned cases, each of the numerical values written under the paddle roller 52 represents the height of accumulated developer at the location at which that numerical value is appended. In this case, an adhesion of developer to the developing roller 48 is shown on the right-hand side of Fig. 27(b), and the developer was adhered to the entire surface of the developing roller 48. When the screw conveyors 58a and 58b were rotated at 250 rpm, the height distribution of the accumulated developer relative to the paddle roller 52 in the developer-accumulating chamber 50 is represented by a curve D shown on the left-hand side of Fig. 27(c). Similar to the above-mentioned cases, each of the numerical values written under the paddle roller 52 represents the height of an accumulated developer at the location at which that numerical value is appended. In this case, the adhesion of developer to the developing roller 48 is shown on the right-hand side of Fig. 27(s), and the developer was adhered to the entire surface of the developing roller 48. As is apparent from the results of the experiments, in the fourth embodiment, even if the speed of rotation of the screw conveyors 58a and 58b is relatively low, for example, as low as 150 rpm, it is possible to adhere the developer over the entire surface of the developing roller 48 by increasing the total quantity of the developer. As is obvious from the foregoing, a suppression of the speed of rotation of the screw conveyors 58a and 58b is preferable for prolongation of the life of the magnetic carrier component of the two-component developer. Figures 28 and 30 show a fifth embodiment of a developing device according to the present invention, which, similar to the third embodiment, is constituted so as to obviate an uneven distribution of the accumulated developer in the developer-accumulating chamber 50. In Figs. 28 and 30, elements like or corresponding to those shown in Figs. 4 and 5 are designated by the same references. As shown in Figs. 28 and 29, in the fifth embodiment, the developing device is provided with two scraper members, i.e., a first scraper member 72a and a second scraper member 72b: the first scraper member 72a is disposed on the side of the left side wall R of a developer container 46, and the second scraper member 72b is disposed on the side of the right side wall L thereof. As is apparent from Figs. 28 and 29, the width of the second scraper 72b is greater than that of the first scraper 72a such that the developer scraped off from a magnet roller 70 by the first scraper member 72a is supplied to a front screw conveyor 58a whereas the developer scraped off from the magnet roller 70 by the second scraper 72b is supplied to a rear screw conveyor 58b. Both the scraper members 72a and 72b are attached to the upper end of a vertical partition wall 62' formed integrally with a second bottom wall 46c. The upper edge of the vertical partition wall 62' is suitably stepped such that the scraper members 72a and 72b can be sloped at different angles of inclination, respectively. As shown in Fig. 30, in the fifth embodiment, one side section of a front partition plate 60a (placed on the side of the side wall L of the developer container 46) has a greater height than that of the other side section thereof, and a length equal to that of the second scraper member 72b. On the other hand, one section of a rear partition plate 60b (placed on the side of the side wall R of the developing container 46) has a greater height than that of the other side section thereof, and a length equal to that of the first scraper member 72a. In other words, the length of the lower section of the front partition plate 60a is equal to that of the first scraper member 72a, and the length of the lower section of the rear partition plate 60b is equal to that of the second scraper member 72b. During the circulation of the developer by the screw conveyors 58a and 58b, the respective upper edges of the lower sections of the front and rear partition plates 60a and 60b serve as overflow edges for the circulating developer. The developer thrust by the front screw conveyor 58a overflows the upper edge of the lower section of the front partition plate 60a into a communication passage 64, and the developer thrust by the rear screw conveyor 50b overflows the upper edge of the lower section of the rear partition plate 60b into the communication passage 64. With this arrangement, the developer recovered from the magnet roller 70 is prevented from being concentrated at a region near one of the side walls R and L of the developer container 46, resulting in a uniform distribution of developer in a developer-accumulating chamber 50. The inventors of the present invention actually fabricated three types of developing devices in accordance with the fifth embodiment, and conducted experiments using these developing devices to examine how the developer adhered to the developing rollers 48 in accordance with variation of relevant parameters. The details of the experiments will be explained hereinafter. The developing device fabricated for the experiments has the following particulars: (1) The length of the developer container 46, i.e., the length along the axis of the developer roller 48, was 300 mm. (2) The developing roller 48 was 40 mm in diameter and 300 mm in length. (3) The screw conveyors 58a and 58b were 28 mm in diameter and 25 mm in screw pitch. (4) The developer-circulation rate, at which the developer is circulated in the developer container 46, was 8.7 g/s/cm (4.7 cm 3 /s/cm as the developer-circulation volume). (5) Although the above items (1) to (4) are common to the three types of developing devices: the second scraper member 60b of the first-type developing device was 140 mm in length (the length ratio of the second scraper 60b to the developing roller 48 was 0.47); the second scraper member 60b of the second-type developing device was 175 mm in length (the length ratio of the second scraper 60b to the developing roller 48 was 0.58); and the second scraper 60b of the third-type developing device was 210 mm in length (the length ratio of the second scraper 60b to the developing roller 48 was 0.7). (6) The specific gravity of the developer was 1.85 g/cm. First, in the first-type developing device (the second scraper 60b of 140 mm in length), the developer-circulation rate was set to be 8.7 g/s/cm (4.7 cm 3 /s/cm as the developer-circulation volume), and the screw conveyors 58a and 58b were rotated at 150 rpm, 200 rpm, and 250 rpm to examine how distribution of developer varies in the developer-accumulating chamber 50 in accordance with the variation of the speed of rotation of the screw conveyors 58a and 58b. Figures 31(a), 31(b) and 31(c) show the results of the experiments, the details of which are as follows: When the screw conveyors 58a and 58b were rotated at 150 rpm, the adhesion of developer to the developing roller 48 was as shown in Fig. 31(a), and non-developer regions (shown as blank regions) of about 40 mm in width at the opposite ends of the developing roller 48 were produced. When the screw conveyors 58a and 58b were rotated at 200 rpm, the adhesion of developer to the developing roller 48 was as shown in Fig. 31(b), and non-developer regions (shown as blank regions) of about 35 mm in width at the opposite ends of the developing roller 48 were produced. When the screw conveyors 58a and 58b were rotated at 250 rpm, the adhesion of developer to the developing roller 48 was as shown in Fig. 31(c), and non-developer regions (shown as blank regions) of about 15 mm in width at the opposite ends of the developing roller 48 were produced. Then, in the second-type developing device (the second scraper 60b of 175 mm in length), the developer-circulation rate was set to be 8.7 g/s/cm (4.7 cm 3 /s/cm as the developer-circulation volume), and the screw conveyors 58a and 58b were rotated at 150 rpm, 200 rpm, and 250 rpm to examine how distribution of developer varies in the developer-accumulating chamber 50 in accordance with the variation of the speed of rotation of the screw conveyors 58a and 58b. Figures 32(a), 32(b) and 32(c) show the results of the experiments, the details of which are as follows: When the screw conveyors 58a and 58b were rotated at 150 rpm, the adhesion of developer to the developing roller 48 was as shown in Fig. 32(a), and non-developer regions (shown as blank regions) of about 15 mm in width at the opposite ends of the developing roller 48 were produced. When the screw conveyors 58a and 58b were rotated at 200 rpm, the adhesion of developer to the developing roller 48 was as shown in Fig. 32(b), and non-developer regions (shown as blank regions) of about 10 mm in width at the opposite ends of the developing roller 48 were produced. When the screw conveyors 58a and 58b were rotated at 250 rpm, the adhesion of developer to the developing roller 48 was as shown in Fig. 32(c), and the developer was adhered to the entire surface of the developing roller 48. Further, in the third-type developing device (the second scraper 60b of 210 mm in length), the developer-circulation rate was set to be 8.7 g/s/cm (4.7 cm 3 /s/cm as the developer-circulation volume), and the screw conveyors 58a and 58b were rotated at 150 rpm, 200 rpm, and 250 rpm to examine how distribution of developer varies in the developer-accumulating chamber 50 in accordance with the variation of the speed of rotation of the screw conveyors 58a and 58b. Figures 33(a), 33(b) and 33(c) show the results of the experiments, the details of which are as follows: When the screw conveyors 58a and 58b were rotated at 150 rpm, the adhesion of developer to the developing roller 48 was as shown in Fig. 33(a), and non-developer regions (shown as blank regions) of about 15 mm in width at the opposite ends of the developing roller 48 were produced. When the screw conveyors 58a and 58b were rotated at 200 rpm, the adhesion of developer to the developing roller 48 was as shown in Fig. 33(b), and non-developer regions (shown as blank regions) of about 12 mm in width at the opposite ends of the developing roller 48 were produced. When the screw conveyors 58a and 58b were rotated at 250 rpm, the adhesion of developer to the developing roller 48 was as shown in Fig. 33(c), and non-developer regions (shown as blank regions) of about 5 mm in width at the opposite ends of the developing roller 48 were produced. As is apparent from the results of the above-mentioned the experiments, in the fifth embodiment, the length of the second scraper 72b must be about half that of the developing roller 48 and the speed of rotation of the screw conveyors 58a and 58b must be more than 250 rpm, before the developer can be adhered to the entire surface of the developing roller 48. Figures 34 and 35 show a sixth embodiment of a developing device according to the present invention, which, similar to the third embodiment, is constituted so as to obviate an uneven distribution of the accumulated developer in the developer-accumulating chamber 50. In Figs. 34 and 35, elements like or corresponding to those shown in Figs. 4 and 5 are designated by the same references. In the sixth embodiment, an upper edge of a front partition plate 60a has a lower level than that of the top of a front screw conveyor 58a, and thus serves as an overflow edge for the developer thrust by a front screw conveyor 58a, and a rear partition plate 60b is extended upward beyond a level of the top of a rear screw conveyor 58b to thereby prevent the developer from overflowing over the rear partition plate 60b. The inventors of the present invention actually fabricated two types of developing devices in accordance with the sixth embodiment, and experiments were conducted to examine how the developer was distributed in the developer-accumulating chamber 50 and how the developer was adhered to the developing roller 48 in accordance with variation of relevant parameters. The details of experiments will be explained hereinafter. The developing device fabricated for the experiments has the following particulars: (1) The longitudinal length of the developer container 46, i.e., the length along the axis of the developing roller 48, was 300 mm. (2) The developing roller 48 was 40 mm in diameter and 300 mm in length. (3) The screw conveyors 58a and 58b were 28 mm in diameter and 25 mm in screw pitch, and were rotated at 200 rpm. (4) The developer-circulation rate, at which the developer was circulated in the developer container 46, was 8.7 g/s/cm (4.7 cm 3 /s/cm as the developer-circulation volume). (5) Although the particulars of Items (1) to (4) were common to the first-type and second-type developing devices, the front partition plate 60a of the first-type developing device was 11 mm in height (the ratio of the height of the front partition plate to the diameter of the screw conveyors 58a and 58b was 0.39), and the front partition plate 60a of the second-type developing device was 5 mm (the ratio of the height of the front partition plate to the diameter of the screw conveyors 58a and 58b was 0.18). (6) The specific gravity of the developer was 1.85 g/cm. In the first-type developing device (the height of the front partition 60a was 11 mm), the total quantity of developer was set to be 1.3 kg, 1.4 kg, 1.6 kg, and 1.8 kg, and it was examined how the developer was distributed in the developer-accumulating chamber 50 and how the developer was adhered to the developing roller 48 in accordance with the variation of the total quantity of developer. Figures 36(a), 36(b), 36(c), and 36(d) show the results of the experiments, the details of which are as follows: When the total quantity of developer held in the developer container 46 was 1.3 kg, the quantity of the developer accumulated in the developer-accumulating chamber 50 was 432g, and the distribution of developer with respect to the developing roller 48 is represented by the curve D shown in Fig. 36(a). Each of numerical values written under the developing roller 48 represents the height of accumulated developer at the location at which that numerical value is appended. In this case, the developer was adhered to the entire surface of the developing roller 48. When the total quantity of developer held in the developer container 46 was 1.4 kg, the quantity of the developer accumulated in the developer-accumulating chamber 50 saw 572g, and the distribution of developer with respect to the developing roller 48 is represented by the curve D shown in Fig. 36(b). Each of numerical values written under the developing roller 48 represents the height of accumulated developer at the location at which that numerical value is appended. In this case, the developer was adhered to the entire surface of the developing roller 48. When the total quantity of developer held in the developer container 46 was 1.6 kg, the quantity of the developer accumulated in the developer-accumulating chamber 50 was 711g, and the distribution of developer with respect to the developing roller 48 is represented by the curve D shown in Fig. 36(c). Each of numerical values written under the developing roller 48 represents the height of accumulated developer at the location at which that numerical value is appended. In this case, the developer was adhered to the entire surface of the developing roller 48. When the total quantity of developer held in the developer container 46 was 1.8 kg, the quantity of the developer accumulated in the developer-accumulating chamber 50 was 925g, and the distribution of developer with respect to the developing roller 48 is represented by the curve D shown in Fig. 36(d). Each of numerical values written under the developing roller 48 represents the height of accumulated developer at the location at which that numerical value is appended. In this case, the developer was adhered to the entire surface of the developing roller 48. Then, in the second-type developing device (the height of the front partition plate 60a was 5 mm), the total quantity of developer was set to be 1.0 kg, 1.23 kg, 1.4 kg, 1.6 kg, and 1.8 kg and it was examined how the developer was distributed in the developer-accumulating chamber 50 and how the developer was adhered to the developing roller 48 in accordance with the variation of the total quantity of developer. Figures 37(a), 37(b), 37(c), 37(d), and 37(e) show the results of the experiments, the details of which are as follows: When the total quantity of developer held in the developer container 46 was 1.0 kg, the distribution of developer with respect to the developing roller 48 is represented by the curve D shown in Fig. 37(a). Each of numerical values written under the developing roller 48 represents the height of accumulated developer at the location at which that numerical value is appended. In this case, non-developer regions (as shown as a blank region) of about 10 mm in width at the opposite ends of the developing roller 48 were produced. When the total quantity of developer held in the developer container 46 was 1.23 kg, the quantity of the developer accumulated in the developer-accumulating chamber 50 was 448g, and the distribution of developer with respect to the developing roller 48 is represented by the curve D shown in Fig. 37(b). Each of numerical values written under the developing roller 48 represents the height of accumulated developer at the location at which that numerical value is appended. In this case, the developer was adhered to the entire surface of the developing roller 48. When the total quantity of developer held in the developer container 46 was 1.4 kg. The quantity of the developer accumulated in the developer-accumulating chamber 50 was 595g, and the distribution of developer with respect to the developing roller 48 is represented by the curve D shown in Fig. 37(c). Each of numerical values written under the developing roller 48 represents the height of accumulated developer at the location at which that numerical value is appended. In this case, the developer was adhered to the entire surface of the developing roller 48. When the total quantity of developer held in the developer container 46 was 1.6 kg, the quantity of the developer accumulated in the developer-accumulating chamber 50 was 805g, and the distribution of developer with respect to the developing roller 48 is represented by the curve D shown in Fig. 37(d). Each of numerical values written under the developing roller 48 represents the height of accumulated developer at the location at which that numerical value is appended. In this case, the developer was adhered to the entire surface of the developing roller 48. When the total quantity of developer held in the developer container 46 was 1.8 kg, the quantity of the developer accumulated in the developer-accumulating chamber 50 was 977g, and the distribution of developer with respect to the developing roller 48 is represented by the curve D shown in Fig. 37(e). Each of numerical values written under the developing roller 48 represents the height of accumulated developer at the location at which that numerical value is appended. In this case, the developer was adhered to the entire surface of the developing roller 48. As is apparent from the results of the above-mentioned the experiments, in the sixth embodiment, when the height of the front partition plate 60a is set to be 11 mm (the ratio of the height of the front partition plate to the diameter of the screw conveyors 58a and 58b is 0.39), the total quantity of developer to be held in the developer container 46 should be at least 1.3 kg before the developer can be adhered to the entire surface of the developing roller 48. In other words, the quantity of developer to be accumulated in the developer-accumulating chamber should be at least about 430g. Note, when the quantity of the accumulated developer is 250g, the quantity of developer on to the developing roller 48 per unit length was 14 g/cm, and this value of the quantity of developer forms a standard for determining a necessary total quantity of developer to be held in a developer container when a different type of developing device is designed. On the other hand, in the sixth embodiment, when the height of the front partition plate 60a is set to be 5 mm (the ratio of the height of the front partition plate to the diameter of the screw conveyors 58a and 58b is 0.18), the total quantity of developer to be held in the developer container 46 should be at least 1.2 kg before the developer can be adhered to the entire surface of the developing roller 48. In this case, the quantity of the accumulated developer is more than about 450g, and the quantity of developer on to the developing roller 48 per unit length is 15 g/cm. Furthermore, it can found from Figs. 36(a) to 36(d) and Figs. 37(a) to 37(e) that the height of the accumulated developer should be more than about 10 mm before the developer can be adhered to the entire surface of the developing roller 48. As is apparent from Figs. 36(a) to 36(d) and 37(a) to 37(e), in the sixth embodiment, the surface of the accumulated developer in the developer-accumulating chamber 50 tends to slope down from one end of the developing roller 48 toward the other end thereof. A graph of Fig. 38 shows a relationship between the slope of the surface of the accumulated developer and the height of the front partition plate 60a. The results shown in Fig. 38 were obtained through experiments in which the height of the front partition plate 60a was varied in the above-mentioned developing device. In the graph of Fig. 38, when the surface of the accumulated developer slopes down from the side R toward the side L, this slope id conveniently defined as a negative slope. Also, when a total quantity of the developer is 1.4 kg, the slopes of the surface of the accumulated developer is indicated by a symbol "○"; when a total quantity of the developer is 1.6 kg, the slopes of the surface of the accumulated developer is indicated by a symbol ""; and when a total quantity of the developer of 1.8 kg, the slopes of the surface of the accumulated developer is indicated by a symbol "Δ". Of course, preferably, the slope of the surface of the accumulated developer should fail to the least possible slope before a uniform toner density of recording can be ensured along the axis of the developing roller 48. As is apparent from the graph of Fig. 38, in the case where the height of the front partition plate 60a is set to be 14 mm within the range of the total quantity of 1.4 to 1.8 kg (the ratio of the height of the front partition plate to the diameter of the screw conveyors 58a and 58b is 0.50), although the slope of the surface of the accumulated developer is as large as about 5°, that slope can be made -0.5° by reducing the height of the front partition plate 60a to 4 mm. Also, when the height of the front partition plate 60a is selected within the range of 1 to 11 mm, the slope of the surface of the accumulated developer can be generally deemed to be zero in the range of the total quantity of 1.4 to 1.8 kg. A graph of Fig. 39 also shows a relationship between the height of the front partition plate 60a and the minimum height of the accumulated developer on the sixth embodiment. The results shown in Fig. 39 were also obtained through experiments in which the height of the front partition plate 60a was varied in the above-mentioned developing device. Similar to the graph of Fig. 38, in the graph of Fig. 39, when a total quantity of the developer is 1.4 kg, the minimum height of the accumulated developer is indicated by a symbol "○"; when a total quantity of the developer is 1.6 kg, the minimum height of the accumulated developer is indicated by a symbol ""; and when a total quantity of the developer of 1.8 kg, the minimum height of the accumulated developer is indicated by a symbol "Δ". As is apparent from Fig. 38, the minimum height of the accumulated developer is substantially in inverse proportion to the height of the front partition plate 60a. Namely, the height the front partition plate 60a, the smaller the minimum height of the accumulated developer. For example, in the case where the total quantity of developer is 1.4 kg, although the minimum height of the accumulated developer is about 9 mm when the height of the front partition plate 60a is 14 mm, the minimum height of the accumulated developer can be increased to about 25 mm by making the height of the front partition plate 60a 5 mm. The graphs in Figs. 40 and 41 show relationship between the total quantity of developer and the minimum height of the accumulated developer and a relationship between the total quantity of developer and the quantity of the accumulated developer, and these graphs were made on the basis of the experimental results shown in Figs. 36 and 37. As is apparent from the two graphs, the total quantity of developer is in proportion to the minimum height of the accumulated developer, and the total quantity of developer is in proportion to the quantity of the accumulated developer, and thus it can be found that the slope of the surface of the accumulated developer can be reduced by increasing the total quantity of developer. In the sixth embodiment shown in Figs. 34 and 35, as the height of the front partition plate 60a is increased, the slope of the surface of the accumulated developer becomes larger. Accordingly, the height of the front partition plate 60a should be at most 11 mm before the height of the accumulated developer can be made uniform. Nevertheless, when the height of the front partition plate 60a is made small, the developer cannot be sufficiently agitated by the screw conveyors 58a and 58b. This is because the developer is prematurely fed to the developer-accumulating chamber 10 as the height of the front partition plate 60a is made smaller. Figure 42(a) shows a modification of the sixth embodiment shown in Fig. 34 and 35, which is constituted so as to resolve the issue discussed above. As shown in Fig. 42(a), the opposite ends of a rear partition plate 60b are partially cut away so as to form rectangular notches 60b'. In this modified embodiment, the height of a front partition plate 60a is 14 mm, and the rectangular notches 60b' have sizes indicated by the values written in the left-hand side of Fig. 42(a). Namely, the opposite ends of the rear partition plate 60b are spaced apart 22 mm from the side walls R and L of a developer container 46; the rectangular notches 60b' have a horizontal depth of 10 mm; and the horizontal edges of the recesses 60b' have a height of 25 mm. With this arrangement, the quantity of developer, which overflows into the opposite end regions of a communication passage 64, is increased to some extent due to the existence of the rectangular notches 60b', which contributes to leveling the height of the accumulated developer. On the other hand, the height of the front partition plate 60b is as large as 14 mm, and thus a sufficient agitation of developer can be ensured. Figure 43(a) shows a part of a developing device constituted in accordance with the sixth embodiment is shown in Fig. 43(a) for comparison with the modified embodiment shown in Fig. 42(a). The developing device shown in Fig. 43(a) is the same in construction as the developing device shown in Fig. 42(a), except that the rear partition plate in Fig. 43(a) has no corresponding rectangular notches 60b'. Experiments similar to those conducted for the sixth embodiment were carried out in the developing devices as shown in Figs. 42(a) and 43(a) to examine how the developer is distributed in the developer-accumulating chambers 50 thereof and how the developer is adhered to the developing rollers 48 thereof. Figures 42(b) to 42(d) and 43(b) to 43(d) show the results of the experiments. First, in the developing device shown in Fig. 42(a), the total quantity of developer held in the developer container 46 was set to be 1.4 kg, 1.6 kg, and 1.8 kg, and it was examined how the developer was distributed in the developer-accumulating chamber 50 and how the developer adhered to the developing roller 48 in accordance with the variation of the total quantity of developer. The details of the experiments are as follows: When the total quantity of the developer was set to be 1.4 kg, the distribution of the accumulated developer with respect to the developing roller 48 is represented by the curve D shown in Fig. 42(b). Each numerical value written under the developing roller 48 represents the height of the accumulated developer at a location at which that numerical value is appended. In this case, a non-developer region (shown as a blank region) of about 25 mm in width at the end L of the developing roller 48 was produced. When the total quantity of the developer was set to be 1.6 kg, the distribution of the accumulated developer with respect to the developing roller 48 is represented by the curve D shown in Fig. 42(c). Each numerical value written under the developing roller 48 represents the height of the accumulated developer at a location at which that numerical value is appended. In this case, the developer was adhered to the entire surface of the developing roller 48. When the total quantity of the developer was set to be 1.8 kg, the distribution of the accumulated developer with respect to the developing roller 48 is represented by the curve D shown in Fig. 42(d). Each numerical value written under the developing roller 48 represents the height of the accumulated developer at a location at which that numerical value is appended. In this case, the developer was adhered to the entire surface of the developing roller 48. Then, in the developing device shown in Fig. 43(a), the total quantity of developer was set to be 1.4 kg, 1.6 kg, and 1.8 kg in the developer container 46, and it was examined how the developer was distributed in the developer-accumulating chamber 50 and how the developer was adhered to the developing roller 48 in accordance with the variation of the total quantity of developer. The details of the experiments are as follows: When the total quantity of the developer was set to be 1.4 kg, the distribution of the accumulated developer with respect to the developing roller 48 is represented by the curve D shown in Fig. 43(b). Each numerical value written under the developing roller 48 represents the height of the accumulated developer at the location at which that numerical value is appended. In this case, a non-developer region (as shown as a blank region) of about 40 mm in width at the end L of the developing roller 48 were produced. When the total quantity of the developer was set to be 1.6 kg, the distribution of the accumulated developer with respect to the developing roller 48 is represented by the curve D shown in Fig. 43(c). Each numerical value written under the developing roller 48 represents the height of the accumulated developer at the location at which that numerical value is appended. In this case, the developer was adhered to the entire surface of the developing roller 48. When the total quantity of the developer was set to be 1.8 kg, the distribution of the accumulated developer with respect to the developing roller 48 is represented by the curve D shown in Fig. 43(d). Each numerical value written under the developing roller 48 represents the height of the accumulated developer at a location at which that numerical value is appended. In this case, the developer was adhered to the entire surface of the developing roller 48. A graph of Fig. 44 is made on the basis of the experimental results shown in Figs. 42(b) to 42(d) and Figs. 43(b) to 43(d), and shows a relationship between the total quantity of developer and the slope of the surface of the accumulated developer. In the graph of Fig. 44, symbols "" indicate the slope of the surface of the accumulated developer on the case of the developing device shown in Fig. 42(a), and symbols "○" indicate the slope of the surface of the accumulated developer on the case of the developing device shown in Fig. 43(a). As is apparent from the graph of Fig. 44, the developing device of Fig. 42(a) is superior to the developing device of Fig. 43(a) in performance for leveling the surface of the accumulated developer. On the other hand, the distribution of the accumulated developer in the developer-accumulating chamber 50 can be uniformized by giving the paddle roller 52 a developer-stirring function. Figures 45(a) and 45(b) show an embodiment of a paddle roller having such a developer-stirring function, generally indicated by reference 76. The paddle roller 76 comprises a roller body 76a 300 mm in length and 8 mm in diameter, and a plurality of stirring-paddle elements 76b attached to the roller body 76a along a longitudinal axis thereof, 10 mm long and 2 mm thick. The stirring-paddle elements are 8 mm high, and thus the overall diameter of the paddle roller 76 is 16 mm. More particularly, the stirring-paddle elements 76b are arranged in four rows along the longitudinal axis of the paddle roller 76, with the rows being angularly offset form each other by an angle of 90°, and the stirring-paddle elements 76b included in each row are spaced from each other at regular intervals of 10 mm. Also, the stirring-paddle elements 76b included in one pair of diametrically-opposite rows are longitudinally offset by 10 mm with respect to the stirring-paddle elements 76b included in the other pair of diametrically-opposite rows. By rotating the paddle roller 76 so constituted, it is possible to even a distribution of the accumulated developer in the developer-accumulating chamber 50, and an ability to even-out the distribution of the accumulated developer is dependent upon a speed of rotation of the paddle roller 76. Namely, the higher the speed of rotation of the paddle roller 76, the more even the distribution of the accumulated developer. Nevertheless, when the paddle roller 76 is rotated at an excessively high speed, the developer prematurely deteriorates, resulting in a drop in a printing quality. Also, the deterioration of the developer entails not only an electrophotographic fog but also an insufficient printing density. On the other hand, when the paddle roller 76 is rotated at an excessively low speed, the distribution of the accumulated developer in the developer-accumulating chamber 50 is not sufficiently even. The relationship between the printing quality and the speed of rotation of the paddle roller 76 as shown in Figs. 45(a) and 45(b) was examined through experiment. The results are as follows: (1) With the speed of rotation at 100 rpm, an even distribution of the accumulated developer in the developer-accumulating chamber 50 could not be obtained, and thus a printing density was not constant along the longitudinal axis of the developing roller 48. (2) With the speed of rotation at 200 rpm, an even distribution of the accumulated developer in the developer-accumulating chamber 50 could be obtained, and thus a printing density was substantially constant along the longitudinal axis of the developing roller 48. When a number of printed sheets was in excess of 120,000, the developer was deteriorated, resulting in a loss of printing quality. (3) With the speed of rotation at 300 rpm, the results were substantially the same as in the case where the speed of rotation was 200 rpm. (4) With the speed of rotation at 400 rpm, the results were also substantially the same as in the case where the speed of rotation of 200 rpm. (5) With the speed of rotation at 500 rpm, the developer was deteriorated when the number of printed sheets was in excess of 70,000, resulting in a loss of printing quality. (6) With the speed of rotation at 600 rpm, the developer was deteriorated with the number of printed sheets was in excess of 50,000 resulting in a loss of printing quality. (7) With the speed of rotation at 700 rpm, the developer was deteriorated when the number of printed sheets was in excess of 20,000, resulting in a loss of printing quality. (8) With the speed of rotation at 800 rpm, the developer was deteriorated when the number of printed sheets was in excess of 10,000, resulting in a loss of printing quality. Figures 46(a) and 46(b) show another type of paddle roller 78 comprising a roller body 78a, 8 mm in diameter, and a plurality of stirring-paddle elements 78b attached to the roller body 78a along a longitudinal axis thereof and being 10 mm wide and 2 mm thick. The stirring-paddle elements were 8 mm high, and thus the overall diameter of the paddle roller 78 is 16 mm. More particularly, the stirring-paddle elements 78b are diametrically arranged in two rows along the longitudinal axis of the paddle roller 78, and the stirring-paddle elements 78b included in each row are spaced from each other at regular intervals of 10 mm. Also, the stirring-paddle elements 78b included in one row are longitudinally offset by 10 mm with respect to the stirring-paddle elements 78b included in the other row. The relationship between printing quality and speed of rotation of the paddle roller 78 was examined through experiment. The results are as follows; (1) With the speed of rotation at 100 rpm, an even distribution of the accumulated developer in the developer-accumulating chamber 50 could not be obtained, and thus a printing density was not constant along the longitudinal axis of the developing roller 48. (2) With the speed of rotation at 200 rpm, an even distribution of the accumulated developer in the developer-accumulating chamber 50 could be obtained, and thus the printing density was substantially along the longitudinal axis of the developing roller 48. When the number of printed sheets was in excess of 120,000, the developer was deteriorated, resulting in a loss of printing quality. (3) With the speed of rotation of 300 rpm, the results were substantially the same as in the case of the speed of rotation at 200 rpm. (4) With the speed of rotation of 400 rpm, the results were also substantially the same as in the case of the speed of rotation at 200 rpm. (5) With the speed of rotation of 500 rpm, the results were also substantially the same as in the case of the speed of rotation at 200 rpm. (6) With the speed of rotation of 600 rpm, the developer was deteriorated when a number of printed sheets was in excess of 70,000, resulting in a loss of printing quality. (7) With the speed of rotation at 700 rpm, the developer was deteriorated when the number of printed sheets was in excess of 50,000, resulting in a loss of printing quality. (8) With the speed of rotation at 800 rpm, the developer was deteriorated when the number of printed sheets was in excess of 20,000, resulting in a loss of printing quality. Figures 47(a) and 47(b) show yet another type of paddle roller 80 comprising a roller body 80a 8 mm in diameter and a plurality of stirring-paddle elements 80b attached to the roller body 80a along a longitudinal axis thereof and being 10 mm wide and 2 mm thick. The stirring-paddle elements 8 mm in height, and thus the overall diameter of the paddle roller 80 is 16 mm. More particularly, the stirring-paddle elements 80b are arranged in four rows along the longitudinal axis of the paddle roller 80, with the rows being angularly offset form each other by an angle of 90°, and the stirring-paddle elements 80b included in each row are spaced from each other at regular intervals of 30 mm. Also, the stirring-paddle elements 80b included in one of the two adjacent rows are longitudinally offset by 10 mm with respect to the stirring-paddle elements 80b included in the other row. In other words, the paddles 80b are helically arranged around the roller body 80a at a pitch of 40 mm. The relationship between printing quality and the speed of rotation of the paddle roller 80 was substantially the same as the case of the paddle 78 shown in Figs. 46(a) and 46(b). Figures 48(a) and 48(b) show yet another type of paddle roller 82 comprising a roller body 82a 8 mm in diameter and a plurality of stirring-paddle elements 82b attached to the roller body 82a along a longitudinal axis thereof and being 10 mm wide and 2 mm thick. The stirring-paddle elements are 8 mm high, and thus the overall diameter of the paddle roller 82 is 16 mm. An arrangement of the stirring-paddle elements 82b is similar to that of the stirring-paddle elements 78b shown in Figs. 47(a) and 47(b), but a helical direction of the stirring-paddle elements 82b ins reversed with respect to that of the stirring-paddle elements 80b shown in Figs. 47(a) and 47(b). The relationship between the printing quality and the speed of rotation of the paddle roller 82 was substantially the same as the case of the paddle 78 shown in Fig. 46(a) and 46(b). Figures 49(a) and 49(b) show yet another type of paddle roller 84 comprising a roller body 84a 8 mm in diameter and a plurality of stirring-paddle elements 84b attached to the roller body 84a along a longitudinal axis thereof and being 4 mm wide and 2 mm thick. The stirring-paddle elements are 8 mm in height, and thus the overall diameter of the paddle roller 84 is 16 mm. More particularly, the stirring-paddle elements 84b are arranged in four rows along the longitudinal axis of the paddle roller 76, with the rows being angularly offset from each other by an angle of 90°, and the stirring-paddle elements 84b included in each row are spaced from each other at regular interval of 10 mm. Also, the stirring-paddle elements 84b included in one pair of diametrically-opposite rows are longitudinally offset by 10 mm with respect to the stirring-paddle elements 84b included in the other pair of diametrically-opposite rows. The stirring-paddle element 84b included in one pair of diametrically-opposite rows are longitudinally offset by 10 mm with respect to the stirring-paddle elements 84b included in the other pair of diametrically-opposite rows. The stirring-paddle elements 84b are arranged in four rows along the longitudinal axis of the paddle roller 84, with the rows being angularly offset from each other by an angle of 90°, and the stirring-paddle elements 84b included in each row are spaced from each other at regular intervals of 10 mm. Also, the stirring-paddle elements 84b included in one pair of diametrically-opposite rows are longitudinally offset by 10 mm with respect to the stirring-paddle elements 84b included in the other pair of diametrically-opposite rows. The relationship between the printing quality and the speed of rotation of the paddle roller 84 were examined through experiments. The results are as follows: (1) With the speed of rotation at 100 rpm, an even distribution of the accumulated developer in the developer-accumulating chamber 50 could not be obtained, and thus the printing density was not constant along the longitudinal axis of the developing roller 48. (2) With the speed of rotation at 200 rpm, an even distribution of the accumulated developer in the developer-accumulating chamber 50 could be obtained, and thus the printing density was substantially constant along the longitudinal axis of the developing roller 48. When a number of printed sheets was in excess of 120,000, the developer was deteriorated, resulting in a loss of printing quality. (3) With the speed of rotation at 300 rpm, the results were substantially the same as when the speed of rotation was 200 rpm. (4) With the speed of rotation at 400 rpm, the results were also substantially the same as when the speed of rotation was 200 rpm. (5) With the speed of rotation at 500 rpm, the results were also substantially the same as when the speed of rotation was 200 rpm. (6) With the speed of rotation at 600 rpm, the results were also substantially the same as when the speed of rotation was 200 rpm. (7) With the speed of rotation at 700 rpm, the results were also substantially the same as when the speed of rotation was 200 rpm. (8) With the speed of rotation at 800 rpm, the developer was deteriorated when the number of printed sheets was in excess of 80,000, resulting in a loss of printing quality. Figure 50(a) and 50(b) show yet another type of paddle roller 86 comprising a roller body 86a 8 mm in diameter, and plurality of stirring-paddle elements 86b attached to the roller body 86a by small screws 86c. Each of the stirring-paddle elements 86b is 10 mm wide and 0.6 mm thick. More particularly, a set of four stirring-paddle elements 86b are tangentially attached to a periphery of the roller body 86a at locations spaced from each other at an angular intervals of 90°. Also, the four stirring-paddle elements 86b are projected in the same rotational direction such that a horizontal distance between the free end edges of the two diametrically-opposite stirring-paddle elements 86b is 15 mm. The plural sets of four stirring-paddle elements 86b are longitudinally spaced from each other at regular intervals of 10 mm. To further enhance the stirring-action of the paddle roller 86, stirring-screws 86d are directly screwed into the roller body 86a at a zone between the two adjacent sets of stirring-paddle elements 86b. The relationship between the printing quality and the speed of the rotation of the paddle roller 86 was substantially the same as in the case of the paddle 76 shown in Figs. 45(a) and 45(b). Figures 51(a) and 51(b) show yet another paddle roller 88 comprising a roller body 88a 8 mm in diameter, and a plurality of triangularly-shaped stirring-paddle elements 88b attached to the roller body 88a along a longitudinal axis thereof. The stirring-paddle elements 88b are diametrically arranged in two rows along the longitudinal axis of the paddle roller 88, and the stirring-paddle elements 88b included in each row are spaced from each other at regular intervals of 10 mm and are oriented in the same direction. Each of the stirring-paddles 88b is 2 mm thick, and the root thereof is 10 mm wide. The stirring-paddle elements 88b have a maximum height of 8 mm, and thus the overall diameter of the paddle roller 88 is 16 mm. The stirring-paddle elements 88b included in one row are longitudinally offset by 10 mm with respect to the stirring-paddle elements 88b included in the other row. The relationship between the printing quality and the speed of rotation of the paddle roller 88 was substantially the same as in the case of the paddle 84 shown in Figs. 49(a) and 49(b). Figures 52(a), 52(b) and 52(c) show yet another type of paddle roller 90 comprising a roller body 90a 8 mm in diameter, and a plurality of stirring-paddle elements 90b, attached to the roller body 90a, having a width of somewhat greater than 20 mm, a thickness of 2 mm, and a height of somewhat greater than 8 mm. More particularly, the siring-paddle elements 90b are diametrically arranged in two rows along the longitudinal axis of the paddle roller 90, and the stirring-paddle elements 90b included in each row are spaced from each other at regular intervals of 10 mm and are obliquely oriented in the same direction with respect to the longitudinal axis of the roller body 90a. Each of the stirring-paddle elements 90b is securely mounted in a groove obliquely formed in the roller body 90a with respect to the longitudinal axis thereof, such that the overall diameter of the paddle roller 88 is 16 mm. The relationship between the printing quality and the speed of rotation of the paddle roller 90 was substantially the same as in the case of the paddle 84 shown in Figs. 49(a) and 49(b). Figures 53(a) and 53(b) show yet another paddle roller 92 comprising a roller body 92a 6 mm in diameter and a plurality of stirring-paddle elements 92b attached to the roller body 92a and diametrically arranged in two rows along a longitudinal axis thereof. Each of the stirring-paddle elements 92b has an appearance as shown in Figs. 53(c) and 53(d) in detail. In particular, the stirring paddle 92b comprises an elongated thin plate 92b 1 79 mm in length. The top of the thin plate 92 1 is perpendicularly bent as a bent portion 92b 2 3.5 mm in width, such that a height thereof is 10.5 mm in height. The thin plate 92 1 has an opening 92 3 longitudinally formed therein just below the bent portion 92b 2 , and the opening 92 3 is 67 mm long and 4.5 mm high. The thin plate 92 1 has a pair of screw holes 92 4 formed therein, and the distance therebetween is 53 mm. As shown in 53(a) and 53(b), stirring-paddle elements 92b are alternately arranged along two diametrically-opposite sides of the paddle roller 92 at a given regular intervals, and each of the stirring-paddle elements 92b is fixed to the roller body 92a by threading two screws 92b 5 into the roller body 92a through the screw holes 92 4 thereof, such that the top portions of the thin plates 92 1 included in the two diametrically-opposite rows are alternately and reversely projected about 4.6 mm from the roller body 92b. The relationship between the printing quality and the speed of rotation of the paddle roller 92 was substantially the same as in the case of the paddle 84 shown in Figs. 49(a) and 49(b). Regarding a service life of the developing device 16', more than 100,000 sheets could be printed as a general standard. Accordingly, when designing a paddle roller to even out the distribution of the accumulated developer in the developer-accumulating chamber 50, the above-mentioned requirements should be taken into account. It is desirable that the screw conveyors 58a and 58b disposed in the developer-agitating chamber 56 are rotated at as low a speed of rotation as possible and enabled to efficiently move the developer. In a high-speed printer, since the consumption of the toner component of the developer per unit time is large, the developer should be rapidly moved in the developer-agitating chamber 56, before a sufficient amount of developer is fed to the developing roller 48. Also, the speed of movement of the developer by the screw conveyors 58a and 58b should be made large before the toner component of the developer can be sufficiently subjected to triboelectrification. When the speed of rotation of the screw conveyors 58a and 58b is fixed, the speed of movement of the developer is dependent upon the screw pitch of the screw conveyers 58a and 58b. Namely, the speed of movement of the developer is raised as the screw pitch becomes larger, whereas the speed of movement of the developer is lowered as the screw pitch becomes smaller. However, as is generally known, an excessively large or small screw pitch reduces the ability to move developer. Although the speed of movement of the developer can be increased by a speed-up of rotation of the screw conveyors 58a and 58b, this approach results in not only shortening of the life of the developer and but also a rapid abrasion of bearings, or the like, used for the screw conveyors 58a and 58b. Accordingly, the present invention is further directed to a developing device in which not only can the developer be efficiently moved by rotation the screw conveyors at as low a speed of rotation as possible, but also the toner component of the developer can be sufficiently subjected to triboelectrification. The inventors of the present invention have elucidated, through experiment, the relationship between the diameter and the pitch of a screw of the screw conveyors or efficiently moving the developer and for sufficiently subjecting the toner component of the developer to triboelectrification. Note, in general, it is natural that the screw of the screw conveyor has a diameter equal to the pitch thereof. Referring to Fig. 54, the developer-agitation chamber 56' of a developing device used in the above-mentioned experiments is schematically shown as a plan view. In the developer-agitating chamber 56', a pair of screw conveyors 58a' and 58b' are disposed in parallel with each other, and partition plate 60' is intervened therebetween. The screw conveyors 58a' and 58b' are rotationally driven such that the developer is circulated in a direction indicated by the arrows. Note, in Fig. 54, reference 72' indicated a scraper member through which the developer lifted from a developing roller is returned to the developer-agitating chamber 56'. Figure 55 shows details of the screw conveyors 58a', 58b' which have a diameter indicated by DM and a screw pitch indicated by P/T, and which are provided with a vane wheel 94 mounted on one end thereof. As shown in Fig. 54, each of the screw conveyors 58a' and 58b' are rotated such that the developer is moved toward the vane wheel 94, and then this vane wheel 94 transfers the developer to the other screw conveyor. For the experiments, four pairs of screw conveyors 30 mm in diameter DM, of 30, 35, 40, and 45 mm screw pitches PT, respectively, were prepared, and each pair of screw conveyors was installed in the developer-agitating chamber 56' as shown in Fig. 54. Each pair of screw conveyors were rotated at 200, 250 or 300 rpm to measure a speed of movement of the developer. The results are shown in a graph of Fig. 56. As is apparent from the graph of Fig. 56, the speed of movement of the developer increases as the screw pitch PT becomes larger. Also, the higher the speed of rotation of the screw conveyors, the larger the speed of movement of the developer. Also, while the developer was moved by each of the pairs of screw conveyors (P = 30, 35, 40, and 45 mm), a given amount of fresh toner component was added to the moving developer at a location AP. Then, when the section of the moving developer, to which the fresh toner component was added, reaches each of locations BP and CP, an amount of electrical charge of the toner component of the developer was measured at the section concerned. Further, an amount of electrical charge of the toner component of the developer was measured at a location DP where was just upstream with respect to the location at which the fresh toner component was added. Note, the speed of rotation of the screw conveyors was 200 rpm. The results are shown in the following table: Speed of Rotation of Scores Conveyors: 200 rpm Screw Pitch PT Loca. AP Loca. BP Loca. CP Loca. DP 30 mm ○ ○ ○ ○ 35 mm ○ ○ ○ ○ 40 mm ○ ○ ○ ○ 45 mm ○ ○ × × In the table, a symbol "O" indicates that the measured electrical charge was 10 µ c/g or more, and "×" indicates that the measured electrical charge was less than 10 µ c/g. Before a proper development can be carried out, the electrical charge of the toner component should be at least 10 µ c/g. As is apparent from the above-mentioned results, when the diameter DT of the screw conveyors is 30 mm, the screw pitch PT of the screw thereof should fall, preferably, in the range of about 30 mm to about 40 mm, more preferably, the range of about 35 mm to about 40 mm. This relationship may be defined as a ratio of PT/DM below: preferably, 1.0 < PT/DM ≦ 1.5 more preferable, 1.3 < PT/DM ≦ 1.5 It should be understood that the relationship between the diameter DT and the screw pitch PT of the screw conveyors can be applied not only to the various embodiments of a developing device according to the present invention but also to the conventional developing device as shown in Fig. 1. Although the invention has been described as applied to a developing device for a multicolor recording apparatus, an application of the present invention is not be limited thereto. For example, the present invention may be applied to a developing device for a monochromatic recording apparatus. In this case, an empty space provided below the second bottom wall 46c of the developer container 46 can be utilized to receive a part of a paper feeder cassette and a paper feeder roller associated therewith. As is apparent from the above description, the developing device according to the present invention also can contribute to the miniaturization of a monochromatic electrostatic recording apparatus. Furthermore, the developing device according to the present invention enables a substantial miniaturization of a multicolor recording apparatus comprising a plurality of electrostatic recording units aligned with each other. List of Reference Numerals and Items 10 photosensitive drum 12 precharger 14 optical writing means 16 developing device 18 transfer unit 20 resist roller 22 fixing unit 24 cleaning means 26 furbrush 28 developer holding container 30 agitator 32 developing roller 34 doctor blade 36 scraper member 38 paper endless conveyor belt 40 sheet feeder unit 42 delivery roller 44 delivery tray 46 developer holding container 46a first bottom wall 46b first rear wall 46c second bottom wall 46d second rear wall 46c top wall 46e front wall 46g vertical partition wall 48 developing roller 48a shaft 48b core part 48c sleeve 50 developer-accumulating chamber 52 paddle roller 54 developer doctor blade 56 developer agitator 58 developer agitator 58a conveyer screw 58b conveyer screw 60 partition plate 60a front partition plate 60b rear partition plate 62 front wall 62' front wall 64 passage 66 developer-lifting passage 68 magnet roller 70 magnet roller 72 scraper member 72a first scraper member 72b second scraper member 76•76•80 paddle roller 82•84•86 paddle roller 88•90•92 paddle roller

Claims: A multi-color electrostatic recording apparatus comprising a plurality of electrostatic recording units (Y,C,M,B), each recording unit (Y,C,M,B) having an electrostatic latent image carrying body (10), a developing device (16') for developing an electrostatic latent image formed on said image carrying body (10) and cleaning means (24) for cleaning said image carrying body (10), whereby in each recording unit (Y,C,M,B) the developing device (16') is located at one side of the image carrying body (10) and the cleaning means (24) is located opposite to the developing device (16') with respect to the image carrying body (10), and whereby the developing devices have different colors, said plurality of recording units (Y,C,M,B) being arranged in series along a recording sheet conveying direction; each of said developing devices (16') for developing an electrostatic latent image with a two-component developer, comprising: developer container (46) including a developer-accumulating chamber (50), and a developer-agitating chamber (56) provided above said developer-accumulating chamber (50), a communication passage (64) formed between said developer-agitating chamber (56) and said developer-accumulating chamber (50), and said communication passage (64) being opened to said developer-agitating chamber (56)to form an overflow opening for the developer; a developer carrying body (48) provided within the developer-accumulating chamber (50) of said developer container (46), said developer carrying body (48) being partially exposed and facing the respective image carrying body (10) to bring the developer from said developer accumulating chamber (56) to a facing zone therebetween for development of an electrostatic latent image formed on said image carrying body (10); a developer-lifting means (68, 68', 70) located over the developer carrying body (10) for lifting the developer, brought to the developing zone by said developer carrying body (48), to the developer-agitating chamber (56) located beside the developer-lifting means (68, 68', 70)of said developer container (46); and a developer-agitating means (58) located in the developer-agitating chamber (56) for agitating the developer in the developer-agitating chamber (56) of said developer container (46), a part of the developer agitated by said developer-agitating means (58) being fed to said developer-accumulating chamber (50) through the overflow opening and the communication passage (64); said recording apparatus being arranged such that when one of the cleaning means (24) of one of the recording units (Y,C,M,B) is adjacent to one of the developing devices (16') of another recording unit, said cleaning means (24) is located at least partially under the developer-agitating chamber (56) of said adjacent developing device (16'). A recording apparatus as set forth in claim 1, wherein the communication passage (64) is disposed adjacent to said developer-lifting means (68, 68', 70). A recording apparatus as set forth in claim 2, wherein the developer-lifting means comprises at least one magnet roller (68, 70) having magnetic poles which are arranged such that the developer is lifted on the side of the exposed surface of said developer-carrying body (48). A recording apparatus as set forth in claim 1, wherein said developer-lifting means comprises a mechanical developer-lifting means (68') provided above said developer-carrying body (48), and a magnet roller (70) provided above the mechanical developer-lifting means. A recording apparatus as set forth in claim 4, wherein said mechanical developer-lifting means (68') comprises a vane wheel (68') which is arranged so as to lift the developer from the developer-carrying body (48) at a location at which said developer-carrying body produces no magnetic field. A recording apparatus as set forth in claim 1, wherein said communication passage (64) is substantially extended through a middle part of the developer-agitating chamber (56) of the developer container (46), the developer-lifting means comprises at least two magnet (68, 70) which have magnetic poles arranged such that the developer is lifted from said developer-carrying body (48) along an S-shaped path. A recording apparatus as set forth in claim 1, wherein the communication passage (64) is substantially extended through a middle part of the developer-agitating chamber (56) of the developer container (46), and said developer-lifting means comprises a mechanical developer raising means (68') provided above said developer-carrying body (48), and a magnet roller (70) provided above said mechanical developer-lifting means, said mechanical developer-lifting means and said magnet roller being arranged such that the developer is lifted from said developer-carrying means (48) along an S-shaped path. A recording apparatus as set forth in claim 7, wherein said mechanical developer-lifting means (68') comprises a vane wheel (68') which is arranged so as to lift the developer from the developer-carrying body (48) at a location at which said developer-carrying body produces no magnetic field. A recording apparatus as set forth in claim 1, wherein said developer-agitating means (58) comprises a first developer-conveying screw conveyor (58a) disposed in the developer-agitating chamber (56) of the developer container (46) so as to be longitudinally in parallel with said developer-carrying body (48), a second developer-conveying screw conveyor (58b) disposed so as to be in parallel with said first developer-conveying screw conveyor (58a) on a side thereof opposite to said developer-carrying body, a first partition plate (60a) disposed between said first and second developer-conveying screw conveyors on a side adjacent to said first developer conveying screw conveyor, and a second partition plate (60a) disposed between said first and second developer-conveying screw conveyors on a side adjacent said developer-conveying screw conveyor; said first and second developer conveying screw conveyors are driven in reverse directions with respect to each other such that the developer is circulated in said developer-agitating chamber, an the overflow opening of said communication passage (64) is defined by said first and second partition plates. A recording apparatus as set forth in claim 9, wherein said second partition plate (69b) defines an overflow edge of said overflow opening having a height gradually decreasing along a direction in which the developer is moved by said second developer-conveying screw conveyor (58b). A recording apparatuss as set forth in claim 10, wherein said first partition plate (60a) has a projected edge which is positioned above the overflow edge of said overflow opening such that an overflow outlet port having a given width is defined therebetween. A recording apparatus as set forth in claim 9, further comprising: a first scraper (72a) for returning the developer from said developer-lifting means to said first developer-conveying screw conveyor (58a), said first scraper member being disposed at a downstream side in a direction in which the developer is moved by said first developer-conveying screw conveyor (58a) said first partition plate (60a) defining an overflow edge of said overflow opening having a length corresponding to that of said first scraper member; and a second scraper member (72b) for returning the developer from said developer-lifting means to said second developer-conveying screw conveyor (58b), said second scraper member being disposed at a downstream side in a direction in which the developer is moved by said second developer-conveying screw (58b), said second partition plate (60b) defining an overflow edge of said overflow opening having a length corresponding to that of said second scraper member. A recording apparatus as set forth in claim 9, wherein said first partition plate (60a) defines an overflow edge of said overflow opening. A recording apparatus as set forth in claim 13, wherein said second partition plate (60b) defines overflow edges of said overflow opening which are formed at the opposite side ends thereof and which have a given width. A recording apparatus as set forth in claim 1, further comprising a paddle roller (76, 78, 80, 82, 84, 86, 88, 80, 92) disposed in said developer-accumulating chamber (50) for feeding the developer from said developer-accumulating chamber to said developer-carrying body (48), said paddle roller having a plurality of stirring-paddle elements (76b, 78b, 80b, 82b, 84b, 86b, 88b, 90b, 92b) for stirring the developer accumulated in said developer-accumulating chamber, during the rotation of said paddle roller, to even-out the distribution of the developer accumulated therein. A recording apparatus as set forth in claim 1, wherein said each developing device further comprises at least two screw conveyors (58a', 58b') disposed in said developer-agitating chamber (56) to carry out the agitation of the developer, wherein an outer diameter DM and a screw pitch PT of said screw conveyors (58a', 58b') satisfy the requirements defined by the formula below: 1.0 < PT/DM ≤ 1.5. A recording apparatus as set forth in claim 1, wherein said each developing device further comprises at least two screw conveyors (58a', 58b') disposed in said developer-agitating chamber (56) to carry out the agitation of the developer, wherein an outer diameter DM and a screw pitch PT of said screw conveyors (58a', 58b') satisfy the requirements defined by the formula below: 1.3 < PT/DM ≤ 1.5.

Inventors: FUKE KENJI, KAWAI TSUTOMU, SATO KUNIHIKO, SUGIMOTO KATSUMI, SUZUKI EIJI, TONAI KEIKO, TONAI SHOZO, TONOMOTO YOSHIHIRO, YOSHII HITOSHI, FUKE, KENJI, KAWAI, TSUTOMU, SATO, KUNIHIKO, SUGIMOTO, KATSUMI, SUZUKI, EIJI, TONAI, KEIKO, TONAI, SHOZO, TONOMOTO, YOSHIHIRO, YOSHII, HITOSHI, FUKE, Kenji Fujitsu Limited, KAWAI, Tsutomu Fujitsu Peripherals Limited, SATO, KUNIHIKO FUJITSU LIMITED, SUGIMOTO, Katsumi Fujitsu Limited, SUZUKI, Eiji Fujitsu Limited, TONAI, Keiko Fujitsu Limited, TONAI, Shozo Fujitsu Limited, TONOMOTO, Yoshihiro Fujitsu Limited, YOSHII, Hitoshi Fujitsu Peripherals Limited
Applicants: FUJITSU LTD, FUJITSU LIMITED
Application year: 1995
Main classification: G03G15
IPC codes: G03G15