Patent Publication Number: US-7907869-B2

Title: Charging brush unit, charging device, and image forming apparatus

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     The present application is based on and claims priority from Japanese Patent Application Nos. 2007-063975, filed on Mar. 13, 2007, and 2007-324814, filed on Dec. 17, 2007 in the Japan Patent Office, the entire contents of each of which are hereby incorporated herein by reference. 
     BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     Exemplary aspects of the present invention relate to a charging brush unit, a charging device, and an image forming apparatus, and more particularly, to a charging brush unit, a charging device, and an image forming apparatus for uniformly charging a latent image carrier. 
     2. Description of the Related Art 
     A related-art image forming apparatus, such as a copier, a facsimile machine, a printer, or a multifunction printer having two or more of copying, printing, scanning, and facsimile functions, forms a toner image on a recording medium (e.g., a recording sheet) according to image data by electrophotography. For example, a charging device charges a surface of a latent image carrier. An optical writer emits a light beam onto the charged surface of the latent image carrier to form an electrostatic latent image on the latent image carrier according to the image data. A development device develops the electrostatic latent image with a developer (e.g., toner) to form a toner image on the latent image carrier. The toner image is transferred from the latent image carrier onto a recording sheet via an intermediate transfer belt. A fixing device applies heat and pressure to the recording sheet bearing the toner image to fix the toner image on the recording sheet. Thus, the toner image is formed on the recording sheet. 
     As the charging device for charging the surface of the latent image carrier, a scorotron charging device is known. The scorotron charging device includes a grid electrode and a wire. The grid electrode has a mesh-like shape and opposes a latent image carrier at a predetermined distance. The wire is stretched so that a circumferential surface thereof opposes the latent image carrier via the grid electrode. When a predetermined bias is applied to the wire, and the grid electrode is supplied with a bias closer to a uniform charging potential of the latent image carrier than the bias applied to the wire, corona discharge occurs between the circumferential surface of the wire and the latent image carrier. Accordingly, the surface of the latent image carrier is uniformly charged with a polarity identical to that of the bias applied to the wire. It is to be noted that in order to generate the corona discharge between the wire and the latent image carrier, a voltage of 5 kV or higher needs to be applied to the wire. 
     One example of a related art charging device includes a so-called sawtooth discharging electrode instead of a wire. The sawtooth discharging electrode includes a plurality of sharp teeth and opposes a latent image carrier via a mesh-like grid electrode. When the discharging electrode is supplied with a bias, electrical charges are concentrated at the plurality of sharp teeth of the discharging electrode opposing the grid electrode, and thus corona discharge occurs at a lower voltage than the voltage applied in the above scorotron charging device including the wire. 
     However, when the corona discharge occurs, an electrical current flows only from a top of a tooth of the sawtooth discharging electrode, not from the whole surface of the sawtooth discharging electrode opposing the grid electrode. As a result, the latent image carrier may not be uniformly charged. Further, although the related-art charging device may generate the corona discharge at a decreased voltage compared to the scorotron charging device, nevertheless it still needs a voltage of at least 4 kV or higher. 
     BRIEF SUMMARY OF THE INVENTION 
     This specification describes a charging brush unit according to exemplary embodiments of the present invention. In one exemplary embodiment of the present invention, the charging brush unit includes a brush and a conductive holder. The brush includes a plurality of flexible conductive fibers. The plurality of flexible conductive fibers is supplied with a charging bias to generate electrical discharge between a top of the plurality of conductive fibers and a latent image carrier across a gap formed between the top of the plurality of conductive fibers and the latent image carrier. An electrode is provided in the gap and includes a plurality of openings opposing the top of the plurality of conductive fibers, and is supplied with a bias different from the charging bias applied to the plurality of conductive fibers. The conductive holder is configured to hold the brush. 
     This specification further describes a charging device according to exemplary embodiments of the present invention. In one exemplary embodiment of the present invention, the charging device includes a charging brush unit and an electrode. The charging brush unit includes a brush and a conductive holder. The brush includes a plurality of flexible conductive fibers. The plurality of flexible conductive fibers is supplied with a charging bias to generate electrical discharge between a top of the plurality of conductive fibers and the latent image carrier across a gap formed between the top of the plurality of conductive fibers and the latent image carrier. The conductive holder is configured to hold the brush. The electrode includes a plurality of openings opposing the top of the plurality of conductive fibers, and is supplied with a bias different from the charging bias applied to the plurality of conductive fibers, so that the electrical discharge is generated between the plurality of conductive fibers and the latent image carrier via the electrode. 
     This specification further describes an image forming apparatus according to exemplary embodiments of the present invention. In one exemplary embodiment of the present invention, the image forming apparatus includes a latent image carrier, a charging device, a latent image forming member, and a development device. The latent image carrier is configured to carry a latent image. The charging device is configured to uniformly charge a surface of the latent image carrier. The charging device includes a charging brush unit and an electrode as described above. The latent image forming member is configured to form a latent image on the uniformly charged surface of the latent image carrier. The development device is configured to develop the latent image. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       A more complete appreciation of the invention and the many attendant advantages thereof will be readily obtained as the same becomes better understood by reference to the following detailed description when considered in connection with the accompanying drawings, wherein: 
         FIG. 1  illustrates one example of a tandem type image forming apparatus according to an exemplary embodiment of the present invention; 
         FIG. 2  is a schematic view of a process unit included in the image forming apparatus shown in  FIG. 1 ; 
         FIG. 3  a perspective view of the process unit shown in  FIG. 2 ; 
         FIG. 4  is a perspective view of a development unit included in the process unit shown in  FIG. 3 ; 
         FIG. 5  is a perspective view of a charging device and a photoconductor included in the process unit shown in  FIG. 3 ; 
         FIG. 6  is an exploded perspective view of the charging device shown in  FIG. 5 ; 
         FIG. 7  is a schematic view of the charging device shown in  FIG. 6 ; 
         FIG. 8  is an exploded plan view of a charging brush included in the charging device shown in  FIG. 6 ; 
         FIG. 9  is a plan view of the charging brush shown in  FIG. 8 ; 
         FIG. 10  is an enlarged view of the charging brush shown in  FIG. 9  when no charging voltage is applied thereto; 
         FIG. 11  is an enlarged view of the charging brush shown in  FIG. 10  when a charging voltage is applied thereto; 
         FIG. 12  is an enlarged view of a conductive fiber included in the charging brush shown in  FIG. 11 ; 
         FIG. 13  is an enlarged view of a conductive fiber according to another exemplary embodiment; 
         FIG. 14  is an exploded plan view of a charging brush according to yet another exemplary embodiment; 
         FIG. 15  is a plan view of the charging brush shown in  FIG. 14 ; 
         FIG. 16  is an enlarged view of the charging brush shown in  FIG. 15 ; 
         FIG. 17  is a schematic view of a charging device according to yet another exemplary embodiment; 
         FIG. 18  is an exploded perspective view of the charging device shown in  FIG. 17 ; 
         FIG. 19  is a schematic view of the charging device shown in  FIG. 17  illustrating a flow of an electrical current; 
         FIG. 20  is a graph illustrating a relation between a discharging effect and a grid voltage; 
         FIG. 21  is a schematic view of the charging device shown in  FIG. 17  illustrating an occurrence of abnormal discharge due to bending of a conductive fiber; 
         FIG. 22  is a schematic view of the charging device shown in  FIG. 17  illustrating a distance between a conductive fiber and a cover; 
         FIG. 23  is a schematic view of a charging device according to yet another exemplary embodiment; 
         FIG. 24  is a schematic view of a charging device according to yet another exemplary embodiment; 
         FIG. 25  is a graph illustrating a relation between a charging effect and a grid voltage; 
         FIG. 26  is a schematic view of a charging device according to yet another exemplary embodiment; 
         FIG. 27  is a schematic view of a charging device not including a directionality improvement member included in the charging device shown in  FIG. 26 ; 
         FIG. 28  is a schematic view of the charging device shown in  FIG. 26  illustrating a large amount of electrons kept on the directionality improvement member; 
         FIG. 29  is a schematic view of a photoconductor included in the image forming apparatus shown in  FIG. 1  and the charging device shown in  FIG. 26 ; 
         FIG. 30A  is a schematic view of a charging device according to yet another exemplary embodiment; 
         FIG. 30B  is a perspective view of the charging device shown in  FIG. 30A ; 
         FIG. 31  is a schematic view of a charging device according to yet another exemplary embodiment; 
         FIG. 32  is a schematic view of a charging device according to yet another exemplary embodiment; 
         FIG. 33  is a perspective view of one modification example of the charging device shown in  FIG. 32 ; 
         FIG. 34  is a sectional view of the charging device shown in  FIG. 33 ; 
         FIG. 35  is a schematic view of a tandem device included in the image forming apparatus shown in  FIG. 1 ; 
         FIG. 36  is a perspective view of a charging device, a development roller, a toner supply roller, and a photoconductor included in the tandem device shown in  FIG. 35 ; 
         FIG. 37  is a perspective view of the charging device shown in  FIG. 36  illustrating a flow of an electrical current entering the charging device; 
         FIG. 38  is a schematic view of a charging device according to yet another exemplary embodiment; 
         FIG. 39  is a perspective view of a charging device according to yet another exemplary embodiment; 
         FIG. 40  is a schematic view of a charging brush included in the charging device shown in  FIG. 39 ; 
         FIG. 41  is a schematic view of a charging device according to yet another exemplary embodiment; 
         FIG. 42  is a schematic view of one modification example of the charging device shown in  FIG. 41 ; 
         FIG. 43  is a schematic view of another modification example of the charging device shown in  FIG. 41 ; 
         FIG. 44  is a partial schematic view of a charging device according to yet another exemplary embodiment; 
         FIG. 45  is a schematic view of a charging device according to yet another exemplary embodiment; and 
         FIG. 46  is a schematic view of a charging device according to yet another exemplary embodiment. 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     In describing exemplary embodiments illustrated in the drawings, specific terminology is employed for the sake of clarity. However, the disclosure of this specification is not intended to be limited to the specific terminology so selected, and it is to be understood that each specific element includes all technical equivalents that operate in a similar manner and achieve a similar result. 
     Referring now to the drawings, wherein like reference numerals designate identical or corresponding parts throughout the several views, in particular to  FIG. 1 , an image forming apparatus  200  according to an exemplary embodiment of the present invention is described. 
       FIG. 1  illustrates one example of the tandem type image forming apparatus  200  (e.g., an electrophotographic printer). The image forming apparatus  200  includes a body  80  and a stacking device  68 . The body  80  includes process units  1 Y,  1 C,  1 M, and  1 K, an optical writer  20 , a first paper tray  31 , a second paper tray  32 , a first feed roller  31 A, a second feed roller  32 A, a feeding path  33 , a plurality of conveyance roller pairs  34 , a registration roller pair  35 , a transfer device  40 , a fixing device  60 , a discharge roller pair  67 , a controller  70 , and toner cartridges  100 Y,  100 C,  100 M, and  100 K. The process units  1 Y,  1 C,  1 M, and  1 K include photoconductor units  2 Y,  2 C,  2 M, and  2 K and development units  7 Y,  7 C,  7 M, and  7 K, respectively. The photoconductor units  2 Y,  2 C,  2 M, and  2 K include photoconductors  3 Y,  3 C,  3 M, and  3 K, respectively. The optical writer  20  includes a polygon mirror  21 . The transfer device  40  includes an intermediate transfer belt  41 , a belt cleaner  42 , a first bracket  43 , a second bracket  44 , first transfer rollers  45 Y,  45 C,  45 M, and  45 K, a second transfer backup roller  46 , a driving roller  47 , a supplementary roller  48 , a tension roller  49 , and a second transfer roller  50 . The belt cleaner  42  includes a cleaning blade  42 A. The fixing device  60  includes a press heating roller  61  and a fixing belt member  62 . The fixing belt member  62  includes a fixing belt  64 , a heating roller  63 , a tension roller  65 , and a driving roller  66 . 
       FIG. 2  is a schematic view of the process unit  1 Y. The photoconductor unit  2 Y further includes a drum cleaner  4 Y and a charging device  5 Y. The development unit  7 Y includes a first developer container  9 Y and a second developer container  14 Y. The first developer container  9 Y includes a first conveyance screw  8 Y. The second developer container  14 Y includes a toner density sensor  10 Y, a second conveyance screw  11 Y, a development roller  12 Y, and a doctor blade  13 Y. The development roller  12 Y includes a development sleeve  15 Y and a magnetic roller  16 Y. 
       FIG. 3  is a perspective view of the process unit  1 Y.  FIG. 4  is a perspective view of the development unit  7 Y. 
     The respective process units  1 Y,  1 C,  1 M, and  1 K (depicted in  FIG. 1 ) correspond to yellow, cyan, magenta, and black toner, respectively, and have a common structure. Therefore, redundant descriptions thereof are omitted here. 
     As illustrated in  FIG. 3 , the photoconductor unit  2 Y and the development unit  7 Y are integrally provided in the process unit  1 Y, and attachable to and detachable from the body  80  of the image forming apparatus  200  (depicted in  FIG. 1 ). However, when the process unit  1 Y including the photoconductor unit  2 Y and the development unit  7 Y is detached from the body  80 , the development unit  7 Y is attachable to and detachable from the photoconductor unit  2 Y, as illustrated in  FIG. 4 . Alternatively, the charging device  5 Y may include the photoconductor  3 Y, and thus a charging brush  507 Y described later and the photoconductor  3 Y may be integrally attached to and detached from the body  80  of the image forming apparatus  200 . 
     As illustrated in  FIG. 2 , the photoconductor  3 Y, serving as a latent image carrier, has a drum-like shape and includes an organic photoconductor with a multi-layered structure in which an aluminum tube is coated with an electrical charge generation layer and an electrical charge transport layer, but may include a single layer structure. 
     The charging device  5 Y uniformly charges a surface of the photoconductor  3 Y driven to rotate clockwise (e.g., a direction A) by a driver, not shown. After the optical writer  20  (depicted in  FIG. 1 ) emits a laser beam to the charged surface of the photoconductor  3 Y to expose and scan the surface of the photoconductor  3 Y, an electrostatic latent image is formed thereon. 
     The first developer container  9 Y and the second developer container  14 Y store a yellow developer including a magnetic carrier and negatively charged yellow toner. The first conveyance screw  8 Y is driven to rotate by a driver, not shown, and conveys the yellow developer in the first developer container  9 Y in a direction perpendicular to a surface of the drawing (e.g., a longitudinal direction of the first conveyance screw  8 Y). The yellow developer passes through a hole, not shown, on a dividing wall provided between the first developer container  9 Y and the second developer container  14 Y, and enters the second developer container  14 Y. 
     The second conveyance screw  11 Y of the second developer container  14 Y is driven to rotate by a driver, not shown, and conveys the yellow developer in the direction perpendicular to the surface of the drawing (e.g., a direction opposite to the direction in which the first conveyance screw  8 Y conveys the yellow developer). The toner density sensor  10 Y (e.g., a permeability sensor) is fixed to a bottom of the second developer container  14 Y and detects a density of the conveyed yellow developer. Above the second conveyance screw  11 Y is provided the development roller  12 Y in parallel with the second conveyance screw  11 Y. The development sleeve  15 Y of the development roller  12 Y includes a nonmagnetic pipe driven to rotate counterclockwise. The magnetic roller  16 Y is provided in the development sleeve  15 Y. Some of the yellow developer conveyed by the second conveyance screw  11 Y is attracted toward a surface of the development sleeve  15 Y by a magnetic force of the magnetic roller  16 Y. The doctor blade  13 Y is provided such that a predetermined space is maintained between the development sleeve  15 Y and the doctor blade  13 Y, so as to control thickness of the yellow developer. Then, the yellow developer is conveyed to a development area opposing the photoconductor  3 Y and adhered to the electrostatic latent image formed on the photoconductor  3 Y, thereby a yellow toner image is formed on the photoconductor  3 Y. After the development, the yellow developer loses the yellow toner and returns to the second conveyance screw  11 Y according to rotation of the development sleeve  15 Y of the development roller  12 Y. Then, the yellow developer is conveyed to a hole, not shown, provided near one end of the second conveyance screw  11 Y in a longitudinal direction of the second conveyance screw  11 Y, and returns to the first developer container  9 Y through the hole. 
     The toner density sensor  10 Y detects magnetic permeability of the yellow developer and transmits a result thereof to the controller  70  (depicted in  FIG. 1 ) as a voltage signal. Since the magnetic permeability of the yellow developer is related to the yellow toner density of the yellow developer, the toner density sensor  10 Y outputs a value of a voltage corresponding to the yellow toner density. The controller  70  includes a data storage device such as a RAM (random access memory) and the like storing data including a Vtref for yellow toner, which is a reference value of an output voltage from the toner density sensor  10 Y, and other Vtrefs for cyan, magenta, and black toner, which are reference values of output voltages from the toner density sensors of the development units  7 C,  7 M, and  7 K. The controller  70  compares the value of the voltage output from the toner density sensor  10 Y with the value of Vtref for yellow toner, and drives a toner supplier, not shown, for a period of time based on the comparison result. The toner supplier supplies an appropriate amount of yellow toner to the first developer container  9 Y so as to compensate for the shortage of yellow toner included in the yellow developer caused by development of the electrostatic latent image. As a result, the yellow toner density inside the second developer container  14 Y is maintained in a predetermined range. Like the process unit  1 Y, the other process units  1 C,  1 M, and  1 K perform an equivalent toner supply control, respectively. 
     The yellow toner image formed on the photoconductor  3 Y is transferred to the intermediate transfer belt  41  (depicted in  FIG. 1 ) described later. The drum cleaner  4 Y of the photoconductor unit  2 Y removes residual toner remaining on the surface of the photoconductor  3 Y. The cleaned surface of the photoconductor  3 Y is discharged by a discharger, not shown, to be initialized, so as to prepare for a subsequent image formation. Like the process unit  1 Y, the other process units  1 C,  1 M, and  1 K form cyan, magenta, and black toner images on the photoconductors  3 C,  3 M, and  3 K, respectively, and the respective toner images are transferred to the intermediate transfer belt  41 . 
     As illustrated in  FIG. 1 , the optical writer  20  is provided below the process units  1 Y,  1 C,  1 M, and  1 K. The optical writer  20 , serving as a latent image forming member, irradiates a laser beam L emitted based on image information on surfaces of the photoconductors  3 Y,  3 C,  3 M, and  3 K of the process units  1 Y,  1 C,  1 M, and  1 K, thereby forming electrostatic latent images for yellow, cyan, magenta, and black toner on the photoconductors  3 Y,  3 C,  3 M, and  3 K, respectively. After emitted from a light source, not shown, of the optical writer  20 , the laser beam L is deflected by the polygon mirror  21  driven to rotate by a motor, not shown, and irradiated to the surfaces of the photoconductors  3 Y,  3 C,  3 M, and  3 K through a pluralities of optical lenses and mirrors, not shown. Alternatively, the optical writer  20  may use a LED (light-emitting diode) array for performing light scanning. 
     The first paper tray  31  and the second paper tray  32  are provided below the optical writer  20  such that the first paper tray  31  and the second paper tray  32  are layered in a vertical direction, and store a plurality of recording materials (e.g., recording sheet P), respectively. The first feed roller  31 A and the second feed roller  32 A contact an uppermost recording sheet P, respectively. When the first feed roller  31 A is driven to rotate counterclockwise by a driver, not shown, the uppermost recording sheet P in the first paper tray  31  is discharged toward the vertically extending feeding path  33 . Also, when the second feed roller  32 A is driven to rotate counterclockwise by a driver, not shown, the uppermost recording sheet P in the second paper tray  32  is discharged toward the feeding path  33 . The recording sheet P fed to the feeding path  33  is sandwiched between the plurality of conveyance roller pairs  34  provided in the feeding path  33  and conveyed upwards through the feeding path  33 . 
     The registration roller pair  35  is provided in an end of the feeding path  33 . When the recording sheet P is fed from the conveyance roller pair  34 , the registration roller pair  35  sandwiches the recording sheet P and temporarily stops rotation. Then, the registration roller pair  35  feeds the recording sheet P toward a second transfer nip described below at a proper time. 
     The transfer unit  40  is provided above the process units  1 Y,  1 C,  1 M, and  1 K. The intermediate transfer belt  41  of the transfer device  40  is looped over the first transfer rollers  45 Y,  45 C,  45 M, and  45 K, the second transfer backup roller  46 , the driving roller  47 , the supplementary roller  48 , and the tension roller  49 . The intermediate transfer belt  41  moves counterclockwise (e.g., a direction B) by rotation of the driving roller  47 . The intermediate transfer belt  41  is sandwiched between the first transfer rollers  45 Y,  45 C,  45 M, and  45 K and the photoconductors  3 Y,  3 C,  3 M, and  3 K to form first transfer nips, respectively. Then, a transfer bias (e.g., a positive bias) with a porality opposite to a polarity of toner is applied to a back surface (e.g., an inner circumferential surface) of the intermediate transfer belt  41 . The yellow, cyan, magenta, and black toner images formed on the photoconductors  3 Y,  3 C,  3 M, and  3 K are first-transferred and superimposed on a front surface of the intermediate transfer belt  41  while sequentially passing through the respective transfer nips formed between the first transfer rollers  45 Y,  45 C,  45 M, and  45 K and the photoconductors  3 Y,  3 C,  3 M, and  3 K. Accordingly, four color toner images are superimposed on the intermediate transfer belt  41 . 
     The intermediate transfer belt  41  is sandwiched between the second transfer backup roller  46  and the second transfer roller  50  provided to face an outer circumferential surface of the intermediate transfer belt  41  to form a second transfer nip. The registration roller pair  35  feeds the recording sheet P toward the second transfer nip when the four color toner images carried by the intermediate transfer belt  41  reach the second transfer nip. Due to effects of a second transfer bias applied to the second transfer roller  50  to form a second transfer electrical field and nip pressure between the second transfer roller  50  and the second transfer backup roller  46 , the four color toner images are second-transferred to the recording sheet P at the second transfer nip. The transferred four color toner images form a full color toner image on the white recording sheet P. 
     The belt cleaner  42  removes residual toner remaining on the intermediate transfer belt  41  after passing through the second transfer nip. The cleaning blade  42 A of the belt cleaner  42  contacts the front surface of the intermediate transfer belt  41 , and removes the residual toner on the intermediate transfer belt  41  by scraping it. 
     Driving force of a solenoid, not shown, causes the first bracket  43  of the transfer device  40  to swing at a predetermined rotation angle around a rotation axis of the supplementary roller  48 . When the image forming apparatus  200  forms a monochrome image, the solenoid slightly rotates the first bracket  43  counterclockwise. The rotation causes the first transfer rollers  45 Y,  45 C, and  45 M to rotate counterclockwise around the rotation axis of the supplementary roller  48 , thereby separating the intermediate transfer belt  41  from the photoconductors  3 Y,  3 C, and  3 M. Meanwhile, the process unit  1 K is activated so as to form the monochrome image. Accordingly, when the monochrome image is formed, the process units  1 Y,  1 C, and  1 M are not redundantly driven, and thereby may be prevented from being worn. 
     The fixing device  60  is provided above the second transfer nip. The press heating roller  61  of the fixing device  60  includes a heat source such as a halogen lump or the like. The heating roller  63  of the fixing belt member  62  also includes a heat source such as a halogen lump or the like. The endless fixing belt  64  is looped over the heating roller  63 , the tension roller  65 , and the driving roller  66 , and moves counterclockwise. The heating roller  63  heats a back surface of the moving fixing belt  64 . The press heating roller  61  is driven to rotate clockwise and contacts a front surface of the fixing belt  64  looped over the heating roller  63 , thereby forming a fixing nip between the press heating roller  61  and the fixing belt  64 . 
     A temperature sensor, not shown, is provided outside a loop of the fixing belt  64 , and faces the front surface of the fixing belt  64  via a predetermined space, and detects a surface temperature of the fixing belt  64  immediately before the fixing belt  64  passes through the fixing nip. A result thereof is transmitted to a power circuit, not shown. Based on the result, the power circuit performs control of supplying power to the heat source of the heating roller  63  or the heat source of the press heating roller  61 , thereby maintaining the surface temperature of the fixing belt  64  at about 140 degrees centigrade. 
     After passing through the second transfer nip, the recording sheet P is conveyed from the intermediate transfer belt  41  to the fixing device  60 . When the recording sheet P is conveyed upwards and passes through the fixing nip between the fixing belt  64  and the press heating roller  61 , the full color toner image is fixed to the recording sheet P by heat and pressure of the fixing belt  64 . 
     The recording sheet P bearing the fixed full color toner image is discharged to an outside of the image forming apparatus  200  via the discharge roller pair  67 . The discharged recording sheet P is sequentially stacked on the stacking device  68  provided on the body  80  of the image forming apparatus  200 . 
     The toner cartridges  100 Y,  100 C,  100 M, and  100 K are provided above the transfer device  40  and respectively store yellow, cyan, magenta, and black toner, which are supplied to the development units  7 Y,  7 C,  7 M, and  7 K of the process units  1 Y,  1 C,  1 M, and  1 K. The toner cartridges  100 Y,  100 C,  100 M, and  100 K are attachable to and detachable from the body  80  separately from the process units  7 Y,  7 C,  7 M, and  7 K. 
     Referring to  FIGS. 5 to 7 , a description is now given of characteristic features of the image forming apparatus  200  according to the exemplary embodiment.  FIG. 5  is a perspective view of the charging device  5 Y and the photoconductor  3 Y.  FIG. 6  is an exploded perspective view of the charging device  5 Y.  FIG. 7  is a schematic view of the charging device  5 Y. 
     As illustrated in  FIG. 5 , the charging device  5 Y is provided immediately below the photoconductor  3 Y, and includes a casing  501 Y and a grid electrode  503 Y. 
     As illustrated in  FIG. 6 , the casing  501 Y includes a charging brush  507 Y. The grid electrode  503 Y includes a plurality of openings  504 Y. 
     As illustrated in  FIG. 7 , the charging device  5 Y further includes a grid power source  510 Y and a charging power source  511 Y. The casing  501 Y further includes a ventilation opening  502 Y. The charging brush  507 Y includes a brush  505 Y and a metal holder  506 Y. 
     The grid electrode  503 Y is made of a metallic material such as stainless steel, copper, iron, and the like, so as to function as an electrode. The grid electrode  503 Y also functions as a cover for covering a maintenance opening of the casing  501 Y. Meanwhile, each of the plurality of openings  504 Y of the grid electrode  503 Y is slit-shaped, and exposes an inside of the casing  501 Y. 
     As illustrated in  FIG. 7 , the casing  501 Y opposes the photoconductor  3 Y with the maintenance opening on which the grid electrode  503 Y is fixed facing upwards, and is fixed to an inside of the body  80  (depicted in  FIG. 1 ). The ventilation opening  502 Y is provided in a bottom of the casing  501 Y vertically facing downwards. 
     The charging brush  507 Y is fixed to an inside of the casing  501 Y. The brush  505 Y includes a plurality of conductive fibers described below and stands on the metal holder  506 Y. The metal holder  506 Y, serving as a conductive holder, is screwed to the inside of the casing  501 Y. The conductive fiber may include, but is not limited to, petroleum pitch carbon fiber including continuous fiber including acrylic fiber as synthetic fiber, PAN (polyacrylonitrile) series carbon fiber including coal tar, and metal fiber including stainless steel. Although there is no substantial difference between them in terms of how they function and the effect they achieve, compared to metal fiber, carbon fiber is more useful since it is available at a reduced cost, thereby decreasing manufacturing costs. 
     As illustrated in  FIG. 7 , the plurality of openings  504 Y of the grid electrode  503 Y opposes a top of the conductive fiber of the charging brush  507 Y in the casing  501 Y. The charging power source  511 Y applies a charging bias having a polarity (e.g., a negative polarity) equal to a polarity of a uniformly charged potential of the photoconductor  3 Y to the metal holder  506 Y of the charging brush  507 Y, while the grid power source  510 Y applies a grid bias having a polarity equal to the polarity of the uniformly charged potential of the photoconductor  3 Y and an absolute value smaller than that of the charging bias to the grid electrode  503 Y. Then, electrical discharge occurs between the top of the conductive fiber of the charging brush  507 Y and the photoconductor  3 Y via the plurality of openings  504 Y of the grid electrode  503 Y serving as an electrode. As a result, the photoconductor  3 Y is uniformly applied with the negative polarity. 
     Referring to  FIGS. 8 to 11 , a description is now given of a structure of the charging brush  507 Y according to the exemplary embodiment.  FIG. 8  is an exploded plan view of the charging brush  507 Y, showing the metal holder  506 Y of the charging brush  507 Y including a first metal plate  506 AY.  FIG. 9  is a plan view of the charging brush  507 Y, showing the metal holder  506 Y of the charging brush  507 Y further including a second metal plate  506 BY.  FIG. 10  is an enlarged view of the charging brush  507 Y applied with no charging voltage.  FIG. 11  is an enlarged view of the charging brush  507 Y applied with a charging voltage. As illustrated in  FIGS. 8 to 11 , the brush  505 Y of the charging brush  507 Y includes a plurality of conductive fibers  505 AY. 
     The plurality of conductive fibers  505 AY of the brush  505 Y of the charging brush  507 Y is flexible, so as to bend in reaction to the electrical discharge from the top thereof. As illustrated in  FIG. 8 , the plurality of conductive fibers  505 AY is planted in the first metal plate  506 AY of the metal holder  506 Y such that the top of the plurality of conductive fibers  505 AY protrudes from a top surface of the metal plate  506 Y. As illustrated in  FIG. 9 , a base of the plurality of conductive fibers  505 AY is sandwiched between the first metal plate  506 AY (depicted in  FIG. 8 ) and the second metal plate  506 BY, so that the plurality of conductive fibers  505 AY is fixed to the metal holder  506 Y. 
     According to the present exemplary embodiment, a pitch of the plurality of conductive fibers  505 AY of the brush  505 Y of the charging device  5 Y in an axial direction of the photoconductor  3 Y depicted in  FIG. 7  (e.g., a longitudinal direction of the photoconductor  3 Y) is smaller than a pitch of teeth of a charging device including a sawtooth discharging electrode. That is, a distance between points of discharge in the brush  505 Y in the axial direction of the photoconductor  3 Y is shorter than that in the charging device including the sawtooth discharging electrode. Therefore, compared to the charging device including the sawtooth discharging electrode, the charging device  5 Y according to the present exemplary embodiment can more reliably charge the photoconductor  3 Y uniformly. Additionally, as illustrated in  FIG. 10 , the plurality of conductive fibers  505 AY of the brush  505 Y is densely arranged to nearly contact each other. However, since application of a charging bias causes electrical charges to concentrate at the top of the conductive fibers  505 AY, the plurality of flexible conductive fibers  505 AY bends and separates from each other due to reaction force of the electrical charges, as illustrated in  FIG. 11 . Since electrical charges are independently concentrated at the top of each conductive fiber of the plurality of conductive fibers  505 AY, electrical discharge occurs at a decreased voltage in each of the plurality of conductive fibers  505 AY densely arranged. Therefore, according to the present exemplary embodiment, the charging device  5 Y may uniformly charge the photoconductor  3 Y with a charging bias lower than a charging bias applied in the charging device using the sawtooth discharging electrode. 
     The inventors conducted an experiment for uniformly charging the photoconductor  3 Y using a prototype of the charging device  5 Y. A distance from a top edge of conductive fibers  505 AY to a grid electrode  503 Y was set to 4 mm, and a distance from the grid electrode  503 Y to the photoconductor  3 Y was set to 2 mm. The conductive fibers  505 AY included carbon fibers and had a diameter of 7 μm. 
     When a grid bias of −2 kV was applied to the grid electrode  503 Y, and a charging bias of −3.2 kV was applied to the charging brush  507 Y, so as to uniformly charge the photoconductor  3 Y, corona discharge occurred at the top of each of the conductive fibers  505 AY of the charging brush  507 Y. As a result, the photoconductor  3 Y was uniformly charged with a voltage of approximately −900 V. 
     By contrast, when a similar experiment using the above-described charging device including the sawtooth discharging electrode was performed, the photoconductor  3 Y was not uniformly charged unless a charging bias of at least −4 kV was applied to the sawtooth discharging electrode. 
     Therefore, these experiments confirm that the charging device  5 Y according to the present exemplary embodiment may uniformly charge the photoconductor  3 Y at a voltage lower than the voltage applied in the charging device including the sawtooth discharging electrode. Moreover, such uniform charging of the photoconductor  3 Y at a decreased voltage may reduce generation of ozone, nitrogen oxides, and sulphur oxides due to the corona discharge. 
     It is to be noted that charging characteristic was evaluated by measuring a surface potential of the photoconductor  3 Y with a known electrostatic voltmeter before and after the photoconductor  3 Y faces close to the charging brush  507 Y and comparing both measurement values. 
     Referring to  FIG. 12 , a description is now given of a structure of the conductive fiber  505 AY.  FIG. 12  is an enlarged view of the conductive fiber  505 AY of the brush  505 Y of the charging brush  507 Y (depicted in  FIG. 7 ). The conductive fiber  505 AY may preferably have a diameter of from about 0.1 μm to about 100 μm. More preferably, the conductive fiber  505 AY may have a diameter of from about 0.1 μm to about 10 μm. A diameter exceeding 100 μm may reduce the flexibility of the conductive fiber  505 AY. 
     The pitch of the conductive fiber  505 AY of the brush  505 Y in the axial direction of the photoconductor  3 Y (depicted in  FIG. 7 ) is preferably from about 10 fibers/mm to about 10,000 fibers/mm. The absolute value of charging voltage may be preferably set to from about 1 kV to about 4 kV. The conductive fiber  505 AY also may preferably has a heat conductivity of from about 1.2×10 4  J/(m/h/K) to about 2.5×10 4  J/(m/h/K), thereby transmitting heat generated by discharge at the top of the conductive fiber  505 AY quickly to the base thereof, and from there to the metal holder  506 Y (depicted in  FIG. 7 ). The metal holder  506 Y may have a heat conductivity of from about 4.1×10 7  J/(m/h/K) to about 5.2×10 8  J/(m/h/K), and a heat capacity of from about 0.3 J/K to about 10 J/K, thereby drawing heat quickly from the conductive fiber  505 AY to prevent a temperature increase of the conductive fiber  505 AY, and also discharging the heat by storing it. Although according to the present exemplary embodiment the metal holder  506 Y is a copper plate, alternatively it may be an aluminum plate or a stainless steel plate. 
     As illustrated in  FIG. 7 , although according to the present exemplary embodiment the grid electrode  503 Y serving as an electrode includes the plurality of openings  504 Y, alternatively it may include lattice-like openings or mesh-like openings. 
     The casing  501 Y includes an insulating material such as an insulating resin, and functions as an insulating cover for covering all surfaces of the brush  505 Y of the charging brush  507 Y other than a top thereof opposing the grid electrode  503 Y together with the metal holder  506 Y. Therefore, an electromagnetic lines of force may be prevented from moving from the charging brush  507 Y to the casing  501 Y, or from moving from the grid electrode  503 Y to the casing  501 Y when the casing  501 Y includes a conductive material. In particular, although use of the flexible conductive fibers  505 AY may cause an electromagnetic lines of force to move toward the casing  501 Y due to bending of the top of the conductive fibers  505 AY at which the electrical charges are concentrated, use of the insulating material for the casing  501 Y may prevent a failure of discharge due to a disordered electrical field caused by the movement of the electric lines of force, and generation of a charging failure of the photoconductor  3 Y. 
     The insulating casing  501 Y includes the ventilation opening  502 Y for externally exposing an end of the metal holder  506 Y, serving as a conductive holder, on a side opposite to the brush  505 Y, thereby generating an airflow from the ventilation opening  502 Y toward the rotating photoconductor  3 Y through the inside of the casing  501 Y and the openings  504 Y so as to help charging from the top edges of the conductive fibers  505 AY to the photoconductor  3 Y. Further, toner particles are prevented from entering the casing  501 Y, and thus do not adhere to the inside of the casing  501 Y. 
     According to the present exemplary embodiment, the charging devices included in the process units  1 C,  1 M, and  1 K have a structure equivalent to that of the charging device  5 Y, and therefore redundant descriptions thereof are omitted hereinafter. 
     Another charging device includes a carbon nanotube for uniformly charging a photoconductor, and uses a method of emitting an electron from a hole with a diameter on the order of nanometers provided in the carbon nanotube supplied with a charging bias toward a photoconductor. However, in order to emit electrons from the holes in the carbon nanotube to the photoconductor, the carbon nanotube and the photoconductor need to be placed under reduced pressure equivalent to a vacuum. Since pressure inside an image forming apparatus for feeding recording sheets can hardly be reduced, the foregoing method may not be practical. Moreover, even when electrons are emitted from the holes in the carbon nanotube, toner particles may scatter inside the image forming apparatus and clog the holes. As a result, stable charging performance may not be maintained. 
       FIG. 13  is an enlarged view of a conductive fiber  505 BY of the charging brush  507 Y of the charging device  5 Y of the image forming apparatus  200  according to another exemplary embodiment. The conductive fiber  505 BY includes a tapered top formed by an oblique cutting process or a grinding process. Since a larger amount of electrical charges is concentrated at the top of the conductive fiber  505 BY than in the conductive fiber  505 AY (depicted in  FIG. 12 ), corona discharge may occur at a lower charging voltage. The conductive fiber  505 BY may be of a material and a size equivalent to those of the conductive fiber  505 AY. Also, conditions for charging voltage in the conductive fiber  505 BY may be equal to those in the above-described exemplary embodiment. 
       FIG. 14  is an exploded plan view of a charging brush  507 XY of the charging device  5 Y of the image forming apparatus  200  according to yet another exemplary embodiment.  FIG. 15  is a plan view of the charging brush  507 XY. 
     As illustrated in  FIGS. 14 and 15 , the charging brush  507 XY includes a plurality of brushes  505 XY and a metal holder  506 XY. The plurality of brushes  505 XY includes a plurality of conductive fibers  505 AXY. The metal holder  506 XY includes a first metal plate  506 AXY and a second metal plate  506 BXY. 
     As illustrated in  FIG. 14 , unlike the plurality of conductive fibers  505 AY (depicted in  FIGS. 8 and 9 ) evenly provided in a longitudinal direction of the brush  505 Y (e.g., the longitudinal direction of the photoconductor  3 Y depicted in  FIG. 7 ), the plurality of conductive fibers  505 AXY is relatively short and is provided in a longitudinal direction of the charging brush  507 XY at a predetermined pitch. A base of each of the plurality of conductive fibers  505 AXY is tied into a bundle by itself and fixed to the first metal plate  506 AXY. As illustrated in  FIG. 15 , the base of the plurality of conductive fibers  505 AXY is sandwiched between the first metal plate  506 AXY (depicted in  FIG. 14 ) and the second metal plate  506 BXY, so that the plurality of conductive fibers  505 AXY is fixed to the metal holder  506 XY. According to the present exemplary embodiment, compared to the charging brush  507 Y including the plurality of conductive fibers  505 AY separately fixed to the metal holder  506 Y, the plurality of conductive fibers  505 AXY may be more securely prevented from falling out of the brush  505 XY. 
       FIG. 16  is an enlarged schematic view of the charging brush  507 XY and the photoconductor  3 Y. A relation between a distance L from a top edge of the conductive fiber  505 AXY of the brush  505 XY to the photoconductor  3 Y and a pitch P of the plurality of brushes  505 XY in the longitudinal direction of the charging brush  507 XY is represented by P≦L. More specifically, the pitch P is set to be equal to the distance L, or smaller than the distance L by several percent. According to the present exemplary embodiment, uniform charging of the photoconductor  3 Y due to an excessive large arrangement pitch of the brushes  505 XY can be more reliably conducted. 
     Structures of charging brushes included in the process units  1 C,  1 M, and  1 K are equivalent to that of the charging brush  507 XY, and therefore redundant descriptions thereof are omitted hereinafter. 
     Referring to  FIGS. 17 to 22 , a description is now given of a structure of a charging device  5 YA of the image forming apparatus  200  according to yet another exemplary embodiment. 
       FIG. 17  is a schematic view of the charging device  5 YA. The charging device  5 YA includes a spacer  512 Y and a casing  513 Y. The other elements of the charging device  5 YA are common to the charging device  5 Y depicted in  FIG. 7 . 
     The plurality of conductive fibers  505 AY of the brush  505 Y may include a carbon fiber, a conductive acrylic fiber (e.g., SA-7), and a copper sulfide mixed fiber (e.g., thunderon (registered trademark)). 
     Unlike the casing  501 Y (depicted in  FIG. 7 ) according to the above-described exemplary embodiment, the casing  513 Y includes metal such as aluminum and stainless. The spacer  512 Y includes an insulating material. The metal holder  506 Y of the charging brush  507 Y is fixed to an inner wall of the casing  513  via the spacer  512 Y with a screw or the like. 
     A top edge of the plurality of conductive fibers  505 AY of the brush  505 Y faces a surface of the photoconductor  3 Y over a predetermined distance (a gap). A large opening is provided in a surface of the casing  513 Y opposing the photoconductor  3 Y. The grid electrode  503 Y is fixed to the casing  513 Y so as to cover the opening. Therefore, the grid electrode  503 Y is provided between the top edge of the plurality of conductive fibers  505 AY of the brush  505 Y and the photoconductor  3 Y. Additionally, an insulator, not shown, is disposed between the grid electrode  503 Y and the casing  513 Y, thereby providing an insulation property therebetween. 
       FIG. 18  is an exploded perspective view of the charging device  5 YA. The grid electrode  503 Y includes a thin metal plate including stainless, copper, and iron. The plurality of openings  504 Y is formed in the grid electrode  503 Y by etching or the like, and each opening has an oblique slit-like shape or a lattice-like shape. 
       FIG. 19  illustrates a flow of an electrical current in the charging device  5 YA. As described above, the charging power source  511 Y applies a charging bias having a polarity (e.g., a negative polarity) equal to a polarity of a uniformly charged potential of the photoconductor  3 Y to the metal holder  506 Y of the charging brush  507 Y. The grid power source  510 Y applies a grid bias having a polarity equal to the polarity of the uniformly charged potential of the photoconductor  3 Y and an absolute value smaller than that of the charging bias to the grid electrode  503 Y. Then, electrical discharge occurs between the top edge of the conductive fibers  505 AY of the charging brush  507 Y and the photoconductor  3 Y via the plurality of openings  504 Y of the grid electrode  503 Y, producing brush currents I 1 , I 2 , and I 3  as illustrated in  FIG. 19 . The electrical discharge causes the surface of the photoconductor  3 Y to be supplied with electrons or ions and uniformly charged. 
     The inventors conducted an experiment for measuring a discharge effect using a prototype of the charging device  5 YA. Specifically, a constant-current power supply including a constant current control circuit capable of constantly controlling an output current was used as the charging power source  511 Y. In addition, a constant-voltage power supply including a constant voltage control circuit capable of constantly controlling an output voltage was used as the grid power source  510 Y. Carbon fiber with a diameter of 7 μm was used for the plurality of conductive fibers  505 AY of the brush  505 Y. A distance between the grid electrode  503 Y and the photoconductor  3 Y was set to 1.5 mm. 
     The charging power source  511 Y applied a charging voltage to the brush  505 Y so as to produce the brush current I 1  of 80 μA through the brush  505 Y, while the grid power source  510 Y applied a predetermined grid voltage to the grid electrode  503 Y. The grid current I 2  flowing from the brush  505 Y to the grid electrode  503 Y via a space between the brush  505 Y and the grid electrode  503 Y was measured using a multi-ammeter. A discharge effect E was obtained based on the measurement result and a following formula (1):
 
 E =( I   1   −I   2 )/ I   1 ×100  (1)
 
where E represents a discharge effect in percent, I 1  represents a brush current, and I 2  represents a grid current.
 
       FIG. 20  is a graph illustrating a relation between the discharging effect and the grid voltage obtained in the above-described experiment. The graph shows that application of a grid voltage of −2.5 kV or smaller can produce a discharge effect of 80% or larger. 
     When a surface potential of the photoconductor  3 Y was measured by using a surface electrostatic voltmeter, specifically a Model 344 electrostatic voltmeter manufactured by TREK, INC., the photoconductor  3 Y was charged with a desired potential by adjusting the grid voltage. Even when the photoconductor  3 Y was charged under conditions designed to produce a discharging effect of about 50% in order to prevent nonuniform charging of the photoconductor  3 Y, the charging device  5 YA may generate an amount of ozone smaller than an amount of ozone generated by a conventional scorotron charging device. 
     When the plurality of conductive fibers  505 AY of the brush  505 Y is supplied with a charging bias, a conductive fiber  505 AY bends and slightly separates from adjacent conductive fiber  505 AY as illustrated in  FIG. 11 . However, as illustrated in  FIG. 21 , when the conductive fiber  505 AY tends to bend substantially after being bent inadvertently during assembly of the charging brush  507 Y (depicted in  FIG. 19 ) or the like, the top of the conductive fiber  505 AY comes close to the inner wall of the metal casing  513 Y, thus generating undesirable discharge (e.g., abnormal discharge) between the top of the conductive fiber  505 AY and the metal casing  513 Y. 
     Therefore, a distance between the base of the conductive fiber  505 AY of the brush  505 Y provided inside the casing  513 Y and the inner wall of the casing  513 Y is set to be longer than a distance obtained by adding a length of the conductive fiber  505 AY to a distance between the conductive fiber  505 AY supplied with a charging bias and the inner wall of the casing  513 Y. 
     To be specific, as illustrated in  FIG. 22 , L 1  shows a distance from the top edge of the plurality of conductive fibers  505 AY to the base thereof fixed to the metal holder  506 Y. The casing  513 Y for covering the charging brush  507 Y includes four side plates opposing the conductive fibers  505 AY and extending in a longitudinal direction of the conductive fibers  505 AY and a base plate opposing the grid electrode  503 Y via the charging brush  507 Y. L 2  shows a distance between a first side plate, which is one of the four side plates, and a base of one of the plurality of conductive fibers  505 AY that is closest to the first side plate. L 3  shows a distance between a second side plate, which is another one of the four side plates, and a base of another one of the plurality of conductive fibers  505 AY that is closest to the second side plate. L 4  shows a distance between the base plate and the bases of the conductive fibers  505 AY. L 5 , not shown, indicates a distance between a third side plate, not shown, and a base of yet another one of the plurality of conductive fibers  505 AY that is closest to the third side plate. L 6 , not shown, indicates a distance between a fourth side plate, not shown, and a base of yet another one of the plurality of conductive fibers  505 AY that is closest to the fourth side plate. 
     When the charging brush  507 Y supplied with a charging bias is moved in the casing  513 Y to a position at which a predetermined distance is provided between the charging brush  507 Y and the inner wall of the casing  513 Y, electrical discharges start to be generated between the top edge of the conductive fiber  505 AY and the inner wall of the casing  513 Y. The above distance indicates a discharge starting distance L 7  between the conductive fibers  505 AY and the inner wall of the casing  513 Y. 
     According to the present exemplary embodiment, the distances L 2 , L 3 , L 4 , L 5 , and L 6 , all of which indicate the distances between the base of the conductive fibers  505 AY and the inner wall of the casing  513 Y, are set to be longer than a distance obtained by adding the distance L 1  (e.g., the length) of the conductive fibers  505 AY to the discharge starting distance L 7 . Therefore, even if the conductive fiber  505 AY substantially bends such that the top edge of the conductive fiber  505 AY comes as close to the inner wall of the casing  513 Y as possible, the distance between the top edge of the conductive fiber  505 AY and the inner wall of the casing  513 Y may be kept longer than the discharge starting distance L 7 , thereby preventing generation of abnormal discharge therebetween. 
     According to the present exemplary embodiment, the casing  513 Y may include a metal material stiffer than an insulating material such as resin or the like, so as to improve structural strength of the charging device  5 YA and prevent abnormal discharge between the top edge of the conductive fiber  505 AY and the inner wall of the casing  513 Y. Further, such prevention of abnormal discharge may lengthen the useful life of the brush  505 Y, thereby maintaining stable discharge performance for an extended period of time. Additionally, when abnormal discharge occurs, electrons or ions move from the brush  505 Y to the casing  513 Y to ground and are thus wasted without being used for charging of the photoconductor  3 Y. Accordingly, prevention of abnormal discharge may prevent such wasteful power consumption. 
     When a constant-voltage power supply is used as the charging power source  511 Y, a discharge starting distance L 7  is measured by applying a charging voltage of a bias value controlled to be constant by the constant-voltage power supply to the brush  505 Y. When a constant-voltage power supply for correcting a bias control value according to environmental changes is used, a discharge starting distance L 7  is measured by applying an upper limit of charging voltage to the brush  505 Y. When a constant-voltage power supply for correcting a bias control value according to environmental changes without setting upper and lower limits to a correction value is used, a discharge starting distance L 7  is measured by applying a charging bias of the maximum output value, which is a designed value, to the brush  505 Y. When a constant-voltage power supply for supplying a charging voltage having an upper limit is used, a discharge starting distance L 7  is measured by applying a charging voltage of the upper limit to the brush  505 Y. When a constant-voltage power supply for supplying a charging voltage having no upper limit is used, a discharge starting distance L 7  is measured by applying a charging bias of the maximum output value, which is a designed value, to the brush  505 Y. 
     Referring to  FIGS. 23 and 24 , a description is now given of charging devices  5 YB and  5 YC of the image forming apparatus  200  according to yet another exemplary embodiment. 
       FIG. 23  is a schematic view of a charging device  5 YB. The charging device  5 YB includes blocking members  514 Y. The other elements of the charging device  5 YB are common to the charging device  5 YA depicted in  FIG. 22 . 
     Like the casing  513 Y of the charging device  5 YA (depicted in  FIG. 17 ), the casing  513 Y of the charging device  5 YB also include a metal material. The metal holder  506 Y has a rectangular parallelepiped shape (e.g., a box-like shape) with six surfaces. The brush  505 Y is fixed to a fixing surface, that is, one surface of the six surfaces thereof. Four blocking members  514 Y are fixed to four side surfaces adjacent to four sides of the fixing surface on which the brush  505 Y is fixed, respectively. Each of the blocking members  514 Y includes an insulating material and has a plate-like shape. Each of the blocking members  514 Y is fixed to the side surface of the metal holder  506 Y in such a manner that the blocking member  514 Y protrudes from the fixing surface, on which the brush  505 Y is fixed toward the top of the brush  505 Y for a length L 8 . 
     When the conductive fibers  505 AY of the brush  505 Y are supplied with a charging bias, a conductive fiber  505 AY bends and is slightly separated from adjacent conductive fibers  505 AY. However, even when an operator, a service engineer, or the like inadvertently touches the brush  505 Y and the conductive fiber  505 AY bends excessively in any direction, the conductive fiber  505 AY hits a protruding portion of one of the four blocking members  514 Y protruding from the fixing surface on which the brush  505 Y is fixed, thus preventing such excessive bending of the conductive fiber  505 AY. 
     According to the present exemplary embodiment, the casing  513 Y may include a metal material stiffer than an insulating material such as resin, or the like, so as to improve structural strength of the charging device  5 Y and prevent abnormal discharge between the top edge of the conductive fiber  505 AY and the inner wall of the casing  513 Y. Further, such prevention of abnormal discharge may lengthen the useful life of the brush  505 Y, thereby maintaining stable discharge performance for an extended period of time. Additionally, prevention of such abnormal discharge may avoid wasteful power consumption. 
     The length L 8  of the protruding portion of the blocking member  514 Y may preferably be set shorter than the distance L 1  (e.g., the length) of the conductive fiber  505 AY, such that the protruding portion of the blocking member  514 Y protruding from the fixing surface of the metal holder  506 Y on which the conductive fiber  505 AY is fixed does not protrude beyond the top of the brush  505 Y. Therefore, since the blocking member  514 Y having an insulating property is not closer to the grid electrode  503 Y than the top edge of the conductive fiber  505 AY, decrease in strength of an electrical field between the top edge of the conductive fiber  505 AY and the grid electrode  503 Y may be prevented. Accordingly, an increase in charging bias due to a decrease in the strength of the electrical field may be prevented. 
     As illustrated in  FIG. 23 , four blocking members  514 Y may be provided to surround the brush  505 Y except for areas opposing the top edge of the brush  505 Y and the base of the brush  505 Y, thereby preventing excessive bending of the conductive fiber  505 AY in any direction. However, an arrangement of the blocking members  514 Y is not limited to an arrangement thereof as illustrated in  FIG. 23 , that is, not all of the blocking members  514  may be provided. For example, one blocking member  514 Y may be provided on one side of the brush  505 Y to prevent the conductive fiber  505 AY from bending in a direction in which the blocking member  514 Y is provided. 
     The blocking member  514 Y may be softer than the conductive fiber  505 AY so as not to damage the conductive fiber  505 AY, so that abnormal discharge due to excessive bending of the conductive fiber  505 AY may be prevented. 
     A top edge of the protruding portion of the blocking member  514 Y may preferably be chamfered or R-chamfered, thereby preventing the conductive fiber  505 AY from being snagged by the top edge of the blocking member  514 Y. 
     The blocking member  514 Y may preferably have a flexural rigidity greater than that of the conductive fiber  505 AY, thereby preventing bending of the blocking member  514 Y caused by hitting of the conductive fiber  505 AY, and thus excessive bending of the conductive fiber  505 AY may be prevented. As described above, when the blocking member  514 Y is softer than the conductive fiber  505 AY so as to prevent damage to the conductive fiber  505 AY, the blocking member  514 Y may lack flexural rigidity. To address this problem, the blocking member  514 Y may be folded into a complicated shape such as an emboss-like shape or a rib-like shape, thereby increasing its flexural rigidity. 
     The blocking member  514 Y may include an ozone-resistant base material such as chromium-nickel stainless steel having increased oxidation resistance and nonoxidation resistance, stainless steel SUS316L including nickel, stainless steel SUS316 including copper, alumite-treated aluminum, fluorocarbon polymer (e.g., ethylene resin tetrafluoride), and the like. Therefore, degradation of the blocking member  514 Y due to ozone caused by discharge from the conductive fiber  505 AY may be prevented. When a conductive material is used as the base material of the blocking member  514 Y, it may preferably include an insulating surface. 
     Further, the base material of the blocking member  514 Y may preferably have increased heat conductivity, for example, from about 80 W/(m·K) to about 420 W/(m·K). Therefore, heat generated by discharge may be quickly absorbed, and quickly transmitted to the metal holder  506 Y, thereby preventing a temperature increase around the top of the brush  505 Y. 
       FIG. 24  is a schematic view of the charging device  5 YC. The charging device  5 YC includes a metal holder  506 YC and a blocking member  513 BY. The other elements of the charging device  5 YC are common to the charging device  5 YA depicted in  FIG. 22 . 
     The blocking member  513 BY protrudes from a circumferential edge of a fixing surface of the metal holder  506 YC, to which the brush  505 Y is fixed, toward the top of the brush  505 Y. The blocking member  513 BY is integrated with the metal holder  506 YC. Namely, the blocking member  514 Y (depicted in  FIG. 23 ) is integrated into the metal holder  506 YC, so that the number of components and manufacturing processes may be reduced. 
     As in the charging device  5 YA (depicted in  FIGS. 17 ,  19 , and  22 ), provision of the large distance between the conductive fibers  505 AY and the casing  513 Y may prevent generation of abnormal discharge, however, may cause enlargement of the charging device  5 YA instead. Also, as in the charging device  5 YB (depicted in  FIG. 23 ) or the charging device  5 YC (depicted in  FIG. 24 ), provision of the blocking member  514 Y or the blocking member  513 BY may prevent generation of abnormal discharge, however, provision of an installation space in the casing  513 Y may cause enlargement of the charging device  5 YB or the charging device  5 YC. 
     A method (e.g., a brush-grid method) in which the grid electrode  503 Y and the charging brush  507 Y are provided provides an increased charging effect of from about 80% to about 90% depending on conditions, represented by a ratio between an electrical current flowing out from the brush  505 Y and an electrical current flowing into the photoconductor  3 Y. 
       FIG. 25  is a graph illustrating a result of an experiment for examining a relationship between a charging effect and a grid bias (e.g., a grid voltage). By applying a grid voltage above −2.5 kV, a charging effect of 80% or larger may be obtained. Even when the grid electrode  503 Y and the charging brush  507 Y are provided the image forming apparatus  200 , a charging effect of about 50% may be expected. Compared to a conventional wire method including a corotron or a scorotron providing a charging effect of about 10%, the brush-grid method may efficiently perform a charging process. For example, in a case of flowing an electrical current of 100 μA from the brush  505 Y to the photoconductor  3 Y, the wire method needs to supply an electrical current of about 1 mA to the brush  505 Y, but the brush-grid method needs merely about 200 μA. That is, reduction of about 80% of electrical power may be achieved. However, once abnormal discharge generates, the generation of abnormal discharge may decrease the power reduction effect substantially. 
       FIG. 26  is a schematic view of a charging device  5 YD of the image forming apparatus  200  according to yet another exemplary embodiment. The charging device  5 YD includes insulating films  515 Y. The other elements of the charging device  5 YD are common to the charging device  5 YA depicted in  FIG. 17 . 
     The insulating films  515 Y, serving as a directionality improvement member, is provided inside the casing  513 Y and improves discharging directivity from the top of the conductive fiber  505 AY to the grid electrode  503 Y. Improvement of discharging directivity may prevent generation of abnormal discharge between the conductive fibers  505 AY and the inner wall of the casing  513 Y. Therefore, while preventing enlargement of the charging device  5 YD due to provision of the large distance between the conductive fibers  505 AY and the inner wall of the casing  513 Y, or provision of the blocking member  514 Y (depicted in  FIG. 23 ) or  513 BY (depicted in  FIG. 24 ), a waste of power consumption due to abnormal discharge may be prevented. 
     The insulating films  515 Y, serving as a directionality improvement member, includes an electrical charge holder for providing the inner wall of the casing  513 Y of the charging device  5 YD with an electrical charge with a polarity equal to that of a charging bias. When the inner wall of the casing  513 Y of the charging device  5 YD is supplied with an electrical charge with a polarity equal to that of a charging bias to reduce a potential difference between the conductive fibers  505 AY and the inner wall of the casing  513 Y, electrical discharge may not easily generate between the conductive fibers  505 AY and the inner wall of the casing  513 Y, and thereby the directivity of discharging from the top of the conductive fiber  505 AY to the grid electrode  503 Y may be improved. 
       FIG. 27  is a schematic view of a charging device  5 YD′ using a brush-grid method, which does not include the insulating films  515 Y depicted in  FIG. 26 . The other elements of the charging device  5 YD′ are common to the charging device  5 YD depicted in  FIG. 26 . Like the charging devices  5 YA (depicted in  FIG. 17 ),  5 YB (depicted in  FIG. 23 ), and  5 YC (depicted in  FIG. 24 ) according to the above exemplary embodiments, the charging device  5 YD′ includes the metal casing  513 Y, serving as a cover. However, in order to downsize the charging device  5 YD′, the charging device  5 YD′ does not include a large distance between the conductive fibers  505 AY and the casing  513 Y, nor include the blocking member  514 Y (depicted in FIG.  23 ) and  513 BY (depicted in  FIG. 24 ). When the brush  505 Y is supplied with a charging bias, and the gird electrode  503 Y is applied with a grid bias, such that a potential difference of about 2.5 kV is applied between the brush  505 Y and the grid electrode  503 Y, electrical discharge occurs between the top of the conductive fiber  505 AY and the gird electrode  503 Y to discharge electrons from the conductive fiber  505 AY toward the gird electrode  503 Y. Some of the discharged electrons move to the surface of the grid electrode  503 Y, while most of the electrons are attracted to an electrical field formed between the grid electrode  503 Y and the photoconductor  3 Y, pass through the openings  504 Y, and transfer to the surface of the photoconductor  3 Y. 
     Unlike this type of discharge, abnormal discharge irregularly occurs between the conductive fiber  505 AY and the casing  513 Y connected to a ground. The abnormal discharge causes an electron to move from the conductive fiber  505 AY to the inner wall of the casing  513 Y and flow to the ground via a ground wire, not shown, thereby causing a waste of power consumption. 
     As illustrated in  FIG. 26 , the charging device  5 YD also includes the metal casing  513 Y, serving as a cover. However, in order to downsize the charging device  5 YD, the charging device  5 YD does not include a large distance between the conductive fibers  505 AY and the casing  513 Y nor include the blocking member  514 Y (depicted in  FIG. 23 ) and the blocking member  513 BY (depicted in  FIG. 24 ). 
     The insulating film  515 Y is formed in the inner wall of the metal casing  513 Y, and includes an insulating tape (e.g., Teflon (trademark) tape). An electrical field is formed between the conductive fiber  505 AY and the metal casing  513 Y via the insulating film  515 Y. When electrical discharge occurs between the conductive fiber  505 AY and the casing  513 Y, electrons discharged from the conductive fiber  505 AY transfer to a surface of the insulating film  515 Y in a direction of the electrical field and remain thereon for an extended period of time of time without flowing into the casing  513 Y. As an amount of electrons on the surface of the insulating film  515 Y gradually increases according to abnormal discharge, an electric potential of the surface of the insulating film  515 Y gradually becomes negative, so that a electric potential difference between the insulating film  515 Y and the conductive fiber  505 AY gradually becomes small, thereby improving discharging directivity from the top of the conductive fiber  505 AY to the grid electrode  503 Y. 
     According to the present exemplary embodiment, improvement of discharging directivity from the top of the conductive fiber  505 AY to the grid electrode  503 Y may decrease an amount of abnormal discharge. In addition, since the electrons generated by the abnormal discharge remain on the surface of the insulating film  515 Y to improve the discharging directivity, a waste of power consumption may be prevented. 
       FIG. 28  is another schematic view of the charging device  5 YD. When the charging device  5 YD is often activated, a large amount of electrons may be kept on the surface of the insulating film  515 Y, thereby almost eliminating the electric potential difference between the insulating film  515 Y and the conductive fiber  505 AY. In this case, since no electrical field moves from the conductive fiber  505 AY to the insulating film  515 Y, most of the electrons discharged from the conductive fiber  505 AY may transfer to the photoconductor  3 Y. 
     Although the casing  513 Y, serving as a cover, includes a metal material according to the present exemplary embodiment, the casing  513 Y including an insulating material also may include the insulating film  515 Y, serving as directionality improvement member. In this case, a metal layer including a metal plate and a metal sheet may be provided on an outer wall of the insulating casing  513 Y, and connected to a ground. Accordingly, an electric filed is formed between the metal layer on the outer wall of the insulating casing  513 Y and the conductive fiber  505 AY. Thus, electrons and ions generated by abnormal discharge in a direction of the electrical field may be kept on the inner wall of the insulating casing  513 Y. 
       FIG. 29  is a schematic view of the charging device  5 YD and the photoconductor  3 Y. The charging device  5 YD is provided in a manner that the top of the conductive fiber  505 AY opposes a rotational center  3 AY of the photoconductor  3 Y. Therefore, electrical discharge occurs between a circumferential surface of the photoconductor  3 Y and the conductive fiber  505 AY in a direction of a normal line of the circumferential surface of the photoconductor  3 Y. 
       FIG. 30A  is a sectional view of a charging device  5 YE according to yet another exemplary embodiment.  FIG. 30B  is a perspective view of the charging device  5 YE. The casing  513 Y includes a plurality of small openings  513 AY. The other elements of the charging device  5 YE are common to the charging device  5 YD depicted in  FIG. 26 . 
     The plurality of small openings  513 AY is provided in both sides of the casing  513 Y. Since the small opening  513 AY has small capacitance, the insulating film  515 Y may have a potential equal to that of the conductive fiber  505 AY with a decreased amount of electrons. 
       FIG. 31  is a sectional view of a charging device  5 YF according to yet another exemplary embodiment. The charging device  5 YF includes insulating members  516 . The other elements of the charging device  5 YF are common to the charging device  5 YD depicted in  FIG. 26 . 
     The grid electrode  503 Y is fixed to the casing  513 Y via the insulating members  516  to insulate the grid electrode  503 Y from the casing  513 Y. Therefore, electrical charges of the grid electrode  503  may be prevented from moving from the casing  513 Y to the ground, thereby preventing a waste of power consumption. 
       FIG. 32  is a schematic view of a charging device  5 YG according to yet another exemplary embodiment. The charging device  5 YG includes a ventilation opening  502 Y and a fan  517 Y. The other elements of the charging device  5 YG are common to the charging device  5 YD depicted in  FIG. 26 . 
     The ventilation opening  502 Y is provided in the casing  513 Y and opposes the grid electrode  503 Y. The fan  517 Y is provided in an outside of the casing  513 Y, and sends the air toward the ventilation opening  502 Y. The fan  517 Y moves the air from the ventilation opening  502 Y to the surface of the photoconductor  3 Y via the charging brush  507 Y and the openings  504 Y of the grid electrode  503 Y, thereby generating electrical discharge from the top of the conductive fiber  505 AY to the photoconductor  3 Y. Also, the fan  517 Y prevents invasion of toner particles into the casing  513 Y, so that adhesion of the toner particles to an inside of the casing  513 Y may be prevented. 
     The fan  517 Y moves the air with a rotating propeller. The propeller has a circular rotational trajectory. The fan  517 Y may have a diameter of rotation of the propeller almost equal to a width W of the casing  513 Y, so that the air may be efficiently sent to the ventilation opening  502 Y. However, the fan  517 Y may not send the air to the whole area of the ventilation opening  502 Y in a longitudinal direction of the ventilation opening  502 Y (e.g., the longitudinal direction of the photoconductor  3 Y). Therefore, in order to flow the air all over the casing  513 Y in the longitudinal direction, a plurality of fans  517 Y needs to be provided in the longitudinal direction, resulting in cost increase. 
       FIG. 33  is a perspective view of one modification example of the charging device  5 YG.  FIG. 34  is a schematic view of the charging device  5 YG. The charging device  5 YG further includes a paddle  520 Y. The paddle  520 Y includes a rotation axis  518 Y and a plurality of blades  519 Y. 
     As illustrated in  FIG. 33 , the rotation axis  518 Y extends in a longitudinal direction of the casing  513 Y. The plurality of blades  519 Y stands on a circumferential surface of the rotation axis  518 Y. 
     As illustrated in  FIG. 34 , the paddle  520 Y may send the air to the whole area of the ventilation opening  502 Y of the casing  513 Y in the longitudinal direction of the ventilation opening  502 Y with the plurality of blades  519 Y revolving around the rotation axis  518 . Therefore, compared to the plurality of fans  517 Y depicted in  FIG. 32 , the paddle  520 Y may send more air to the whole area of the ventilation opening  502 Y in the longitudinal direction at a low cost. 
       FIG. 35  is a schematic view of a tandem device of the image forming apparatus  200 . The tandem device includes charging devices  5 YG,  5 CG,  5 MG, and  5 KG instead of the charging device  5 Y (depicted in  FIG. 2 ) and development units  7 YG,  7 CG,  7 MG, and  7 KG instead of the development unit  7 Y (depicted in  FIG. 2 ). The development units  7 YG,  7 CG,  7 MG, and  7 KG include development rollers  17 Y,  17 C,  17 M, and  17 K and toner supply rollers  18 Y,  18 C,  18 M, and  18 K, respectively. The toner supply rollers  18 Y,  18 C,  18 M, and  18 K include blades  18 AY,  18 AC,  18 AM, and  18 AK, respectively. 
     Each of the development units  7 YG,  7 CG,  7 MG, and  7 KG uses a one-component development method for developing an electrostatic latent image with toner as one-component developer not including a magnetic carrier. 
     Toner containers, not shown, are provided in the development units  7 YG,  7 CG,  7 MG, and  7 KG, and store yellow, cyan, magenta, and black toner, respectively. Agitators, not shown, are provided in the toner containers, and may rotate to agitate and convey the yellow, cyan, magenta, and black toner. That is, when the agitators rotate in the development units  7 YG,  7 CG,  7 MG, and  7 KG, the yellow, cyan, magenta, and black toner are sent toward the toner supply rollers  18 Y,  18 C,  18 M, and  18 K, respectively. The toner supply rollers  18 Y,  18 C,  18 M, and  18 K include resin foam, and supply the yellow, cyan, magenta, and black toner agitated by the agitators to the development rollers  17 Y,  17 C,  17 M, and  17 K, respectively. Upon contact with the development rollers  17 Y,  17 C,  17 M, and  17 K, the toner supply rollers  18 Y,  18 C,  18 M, and  18 K supply the yellow, cyan, magenta, and black toner to the development rollers  17 Y,  17 C,  17 M, and  17 K, respectively. Therefore, at a development area at which the development rollers  17 Y,  17 C,  17 M, and  17 K carrying the yellow, cyan, magenta, and black toner oppose the photoconductors  3 Y,  3 C,  3 M, and  3 K, respectively, the development rollers  17 Y,  17 C,  17 M, and  17 K cause the yellow, cyan, magenta, and black toner to adhere to electrostatic latent images formed on the photoconductors  3 Y,  3 C,  3 M, and  3 K, respectively. 
       FIG. 36  is a perspective view of the charging device  5 YG, the development roller  17 C, the toner supply roller  18 C, and the photoconductor  3 Y. The toner supply roller  18 C further includes an axis  18 BC.  FIG. 37  is a perspective view of the charging device  5 YG, the development unit  7 CG, and the photoconductor  3 Y. The development unit  7 CG includes a casing  22 C. The casing  22 C includes a ventilation opening  19 C. 
     As illustrated in  FIG. 36 , the axis  18 BC extends from both ends of the toner supply roller  18 C in an axial direction (e.g., a longitudinal direction) of the toner supply roller  18 C, and is rotatably supported by a receiver, not shown. The blades  18 AC protrude from a circumferential surface of the axis  18 BC. When the toner supply roller  18 C rotates, the blades  18 AC revolve around the axis  18 BC to generate airflows F (depicted in  FIG. 37 ) at both ends of the toner supply roller  18 C in the longitudinal direction of the toner supply roller  18 C. As illustrated in  FIG. 37 , the ventilation opening  19 C is provided in the casing  22 C and opposes the charging device  5 YG. The airflows F generated inside the casing  22 C of the development unit  7 CG for the cyan toner enter the ventilation opening  502 Y of the charging device  5 YG via the ventilation opening  19 C. 
     Accordingly, the blades  18 AC and the ventilation opening  19 C provided in the development unit  7 CG for the cyan toner function as a ventilation device for supplying the air to the ventilation opening  502 Y of the charging device  5 YG for the yellow toner. As illustrated in  FIG. 35 , the blades  18 AK and a ventilation opening, not shown, provided in the development unit  7 KG for the black toner function as a ventilation device for supplying the air to the ventilation opening  502 M of the charging device  5 MG for the magenta toner. Also, the blades  18 AM and a ventilation opening, not shown, provided in the development unit  7 MG for the magenta toner function as a ventilation device for supplying the air to the ventilation opening  502 C of the charging device  5 CG for the cyan toner. 
     Therefore, air supply may be performed by using the components provided in the tandem device without adding any component to the tandem device. A dotted line indicated by “LA” represents a laser beam for exposing and scanning the photoconductor  3 Y. 
     Referring to  FIG. 38 , a description is now given of a charging device  5 YH according to yet another exemplary embodiment.  FIG. 38  is a schematic view of the charging device  5 YH and the photoconductor  3 Y. The casing  501 Y includes an air hole  521 Y. The other elements of the charging device  5 YH are common to the charging device  5 YG depicted in  FIG. 32 , except that the casing  501 Y replaces the casing  513 Y. 
     As in the charging device  5 Y (depicted in  FIG. 7 ), the casing  501 Y of the charging device  5 YH includes an insulating material. Four sidewalls of the casing  501 Y extend from the grid electrode  503 Y to the metal holder  506 Y to cover the charging brush  507 . One of the sidewalls is positioned downstream from the photoconductor  3 Y in a direction of movement of the photoconductor  3 Y, and receives an airflow F generated according to rotation of the photoconductor  3 Y. The air hole  521 Y is provided in the above sidewall. 
     Accordingly, since the airflow F generated according to rotation of the photoconductor  3 Y passes through the air hole  521 Y into the casing  501 Y, the airflow F may move from the brush  505 Y to the openings  504 Y of the grid electrode  503 Y in the casing  501 Y. Therefore, the airflow F generated according to rotation of the photoconductor  3 Y may help electrical discharge from the top of the conductive fiber  505 AY to the photoconductor  3 Y, or may prevent toner particles from adhering to the inside of the casing  501 Y. 
     Referring to  FIG. 39 , a description is now given of a charging device  5 YI according to yet another exemplary embodiment.  FIG. 39  is a perspective view of the charging device  5 YI. The charging device  5 YI includes a plurality of brushes  505 Y. The other elements of the charging device  5 YI are common to the charging device  5 Y depicted in  FIG. 7 . 
     In order to uniformly charge the photoconductor  3 Y, the brush  505 Y of the charging brush  507 Y needs to have a large area of a brush surface formed by gathering all the tops of the plurality of conductive fibers  505 AY. However, when the area of the brush surface is too large, electrical charges hardly gather at each top of the conductive fibers  505 AY, thereby increasing discharge starting voltage. 
     Therefore, according to the present exemplary embodiment, the plurality of brushes  505 Y is provided in the charging device  5 YI in a direction of movement of the photoconductor  3 Y. Thus, each of the brush surfaces of the plurality of brushes  505 Y separately opposes the photoconductor  3 Y. Therefore, a proper size of the brush surface area necessary for uniformly charging the photoconductor  3 Y may be provided without excessively enlarging the brush surface area of one brush  505 Y. Accordingly, an increase of the discharge starting voltage due to excessive enlargement of the brush surface area may be prevented, thereby uniformly charging the photoconductor  3 Y. 
       FIG. 40  is a perspective view of one modification example of the charging brush  507 Y. The metal holder  506 Y for holding the brush  505 Y is curved or wound like a snake, for example. Therefore, the single charging brush  507 Y may include a plurality of brushes  505 Y arranged in the direction of movement of the photoconductor  3 Y. When the single metal holder  506 Y is fixed to the casing  501 Y (depicted in  FIG. 39 ), the plurality of brushes  505 Y may be arranged in the direction of movement of the photoconductor  3 Y. Accordingly, compared to a structure in which the plurality of charging brushes  507 Y is separately fixed to the casing  501 Y, the charging brush  507 Y depicted in  FIG. 40  may be fixed to the casing  501 Y with reduced assembly processes. 
     Referring to  FIG. 41 , a description is now given of a charging device  5 YJ according to yet another exemplary embodiment.  FIG. 41  is a schematic view of the charging device  5 YJ. The charging device  5 YJ includes elements common to the charging device  5 YI depicted in  FIG. 39 . 
     Since the photoconductor  3 Y has a drum-like shape, the photoconductor  3 Y has a curved surface opposing the charging device  5 YJ. When the brush  505 Y including a plane brush surface opposes the curved surface of the photoconductor  3 Y, a distance between both ends of the brush surface in the direction of movement of the photoconductor  3 Y and the photoconductor  3 Y is larger than a distance between a center of the brush surface and the photoconductor  3 Y. In order to generate electrical discharge from the top of all the conductive fibers  505 AY to the photoconductor  3 Y, a charging bias needs to be set according to the distance between both ends of the brush surface in the direction of movement of the photoconductor  3 Y and the photoconductor  3 Y. Thus, the charging bias applied at the both ends may become larger than a charging bias set according to the distance between the center of the brush surface and the photoconductor  3 Y. 
     Thus, as illustrated in  FIG. 41 , each top of the plurality of conductive fibers  505 AY of the brush  505 Y is arranged along the curved surface of the photoconductor  3 Y. Specifically, as in the above-described exemplary embodiment depicted in  FIG. 39 , three brushes  505 Y are arranged in the charging device  5 YJ in the direction of movement of the photoconductor  3 Y. However, a length of the conductive fibers  505 AY of the brush  505 Y provided in the center is shorter than that of the conductive fibers  505 AY of each of the brushes  505 Y provided at both ends. Therefore, three brushes  505 Y are provided in a manner that each top of the plurality of conductive fibers  505 AY of the three brushes  505 Y is arranged along the surface of the photoconductor  3 Y. 
     Since the distances between each top of the conductive fibers  505 AY and the photoconductor  3 Y are almost equal, compared to a case in which the distances are different, electrical discharge from each conductive fiber  505 AY may occur at an almost common frequency, so that the photoconductor  3 Y may be uniformly charged. 
     When there is provided one brush  505 Y including a brush surface with a long length in the direction of movement of the photoconductor  3 Y, a length of the conductive fiber  505 AY in the center of the brush  505 Y in the direction of movement of the photoconductor  3 Y may be set to be shorter than that of the conductive fibers  505 AY at both ends. 
       FIG. 42  is a schematic view of one modification example of the charging device  5 YJ. Bases of the plurality of conductive fibers  505 AY of the brushes  505 Y are arranged along the curved surface of the photoconductor  3 Y. Accordingly, the plurality of conductive fibers  505 AY having a length equal to each other may be provided, so that the top of the conductive fibers  505 AY may be arranged along the curved surface of the photoconductor  3 Y. Therefore, the top of the conductive fibers  505 AY may be arranged along the curved surface of the photoconductor  3 Y without any trouble of disposing the brush  505 Y including the conductive fibers  505 AY of different length in a predetermined position, or planting the conductive fibers  505 AY of different length in a predetermined position in the metal holder  506 Y. 
       FIG. 43  is a sectional view of another modification example of the charging device  5 YJ. In addition to the above modification, the grid electrode  503 Y is curved along the curved surface of the photoconductor  3 Y. Therefore, a constant distance is provided between the photoconductor  3 Y and the grid electrode  503 Y in the direction of movement of the photoconductor  3 Y, thereby preventing a decrease of discharge effect due to varied distance between the photoconductor  3 Y and the grid electrode  503 Y. 
     Referring to  FIG. 44 , a description is now given of a charging device  5 YK according to yet another exemplary embodiment.  FIG. 44  is a sectional view of the charging device  5 YK. 
     As described above, the enlargement of the brush surface area of one brush  505 Y may increase discharge starting voltage. The discharge starting voltage may increase not only when a length of the brush surface area of one brush  505 Y is excessively elongated in the direction of movement of the photoconductor  3 Y, but also when a length of the brush surface area is excessively elongated in a longitudinal direction of the brush  505 Y, that is, a direction perpendicular to the direction of movement of the photoconductor  3 Y. Therefore, the brush  505 Y of the charging brush  507 Y includes a portion (e.g., a brush portion), in which the conductive fibers  505 AY are provided, and a portion (e.g., a non-brush portion), in which no conductive fibers  505 AY is provided, alternately disposed in a longitudinal direction of the charging brush  507 Y. Therefore, an increase of discharge starting voltage due to excessive enlargement of the brush surface area of one brush  505 Y may be prevented, so that the photoconductor  3 Y may be uniformly charged. 
     As illustrated in  FIG. 44 , the above-described brush portions are disposed at an equal pitch P in the longitudinal direction of the discharging brush  507 Y. The grid electrode  503 Y includes a plurality of openings  504 Y arranged in a grid pattern. Like the brush portions, the openings  504 Y are also arranged at an equal pitch P in the longitudinal direction of the charging brush  507 Y. Each brush portion is positioned above one of the plurality of openings  504 Y of the grid electrode  503 Y, so as to directly oppose the photoconductor  3 Y through the opening  505 Y. Therefore, electrical discharge from the top of the conductive fibers  505 AY may occur easily, so that discharge starting voltage may be reduced. Moreover, electrical discharge from the top of each of the conductive fibers  505 AY may occur at a reduced discharge starting voltage, thereby preventing the photoconductor  3 Y from being nonuniformly charged. 
     As in the above-described exemplary embodiments depicted in  FIGS. 41 to 43 , the plurality of brushes  505 Y is arranged in the direction of movement of the photoconductor  3 Y at an arrangement pitch equal to the pitch P of the opening  504 Y of the grid electrode  503 Y in the direction of movement of the photoconductor  3 Y. Each brush  505 Y is disposed above each opening  504 Y of the grid electrode  503 Y. 
     Referring to  FIG. 45 , a description is now given of a charging device  5 YL according to yet another exemplary embodiment.  FIG. 45  is a sectional view of the charging device  5 YL. The charging device  5 YL includes an opening electrode  530 Y. The opening electrode  530 Y includes an opening  531 Y. The opening electrode  530 Y replaces the grid electrode  503 Y depicted in FIG.  17 . The other elements of the charging device  5 YL are common to the charging device  5 YA depicted in  FIG. 17 . 
     Instead of the grid electrode  503 Y, the opening electrode  530 Y is provided in the charging device  5 YL. The opening electrode  530 Y is formed by folding one piece of plate-like member into a U-like shape. The opening  531 Y is slit-shaped. A width WA of the slit-like opening  531 Y is almost equal to a width of the opening  504 Y of the grid electrode  503 Y (depicted in  FIG. 17 ). 
     The charging brush  507 Y is fixed to an inside of the opening electrode  530 Y folded into the U-like shape. Therefore, electrical discharge may occur between the top of the conductive fiber  505 AY of the charging brush  507 Y and the photoconductor  3 Y (depicted in  FIG. 17 ) via the opening  531 Y of the opening electrode  530 Y. 
     Accordingly, compared to the charging device  5 YA (depicted in  FIG. 17 ) including the grid electrode  503 Y, a size of the charging device  5 YL in the direction of movement of the photoconductor  3 Y may be decreased. 
     Referring to  FIG. 46 , a description is now given of a charging device  5 YM of the image forming apparatus  200  according to yet another exemplary embodiment.  FIG. 46  is a schematic view of the charging device  5 YM, the development unit  7 Y, and the photoconductor  3 Y. 
     As illustrated in  FIG. 46 , a casing of the charging device  5 YM is integrated into a casing of the development unit  7 Y, thereby the charging device  5 YM may become compact. 
     A ventilation opening, not shown, is provided in the casing of the charging device  5 YM. The fan  517 Y opposes the ventilation opening. 
     As illustrated in  FIG. 7 , according to the above-described exemplary embodiments, electrical discharge occurs from each top of a plurality of conductive fibers (e.g., the plurality of conductive fibers  505 AY depicted in  FIG. 8 ) of a charging brush (e.g., the charging brush  507 Y). Since an arrangement pitch of the plurality of conductive fibers is smaller than an arrangement pitch of teeth of a charging device including a sawtooth discharging electrode, a latent image carrier (e.g., the photoconductor  3 Y) may be uniformly charged. Even when the plurality of conductive fibers is provided in very high density such that the conductive fibers contact each other, the plurality of flexible conductive fibers bends due to a repulsion force of electrical charges concentrating at a top of the plurality of conductive fibers, and separates from each other, thereby the electrical charges are separately concentrated at each top of the plurality of conductive fibers. As a result, electrical discharge may occur at a low electric potential at each top of the plurality of conductive fibers arranged in high density, so that the image carrier may be charged at a lower electric potential than a conventional charging device. 
     The image forming apparatus  200  (depicted in  FIG. 1 ) may be a copier, a facsimile machine, a printer, a multifunction printer having two or more of copying, printing, scanning, and facsimile functions, or the like. According to the above-described non-limiting example embodiments, the image forming apparatus  200  functions as a tandem type color printer for forming a color image on a recording medium (e.g., a sheet) by electrophotography. However, the image forming apparatus  200  is not limited to the color printer and may form a color and/or monochrome image with other structure. 
     As can be appreciated by those skilled in the art, although the present invention has been described above with reference to specific exemplary embodiments the present invention is not limited to the specific embodiments described above, and various modifications and enhancements are possible without departing from the spirit and scope of the invention. It is therefore to be understood that the present invention may be practiced otherwise than as specifically described herein. For example, elements and/or features of different illustrative exemplary embodiments may be combined with each other and/or substituted for each other within the scope of the present invention.