Patent Publication Number: US-7725061-B2

Title: Developing apparatus

Description:
This application is a divisional of U.S. patent application Ser. No. 11/510,637, filed Aug. 28, 2006. 

   BACKGROUND OF THE INVENTION 
   1. Field of the Invention 
   The present invention relates to a developing apparatus for developing an electrostatic image that has been formed through an electrophotographic printing method or an electrostatic recording method on an image bearing member, in particular, having a developer carrying member. 
   2. Description of the Related Art 
   Up to now, in an image forming apparatus such as an electrophotographic copying machine, a powder cloud method, a cascade method, and a magnetic brush method have been known as methods employed for developing apparatuses that are applied to the image forming apparatuses. Among those, in a case of the magnetic brush method of a two-component developing system, a two-component developer mixedly containing magnetic carriers and toner therein is used as the developer. Then, the developer is attracted by magnetic field generating means and stands like the ears of rice in a shape of brush on a magnetic pole portion, and an electrostatic latent image on a drum-shaped electrophotographic photosensitive member (hereinafter referred to as “photosensitive drum”) which serves as the image bearing member is rubbed by the developer to thereby develop and form an image. In this event, because the magnetic carrier per se in the developer serves as a soft developing electrode, it is possible to make the toner adhere to the electrostatic latent image in proportion to charge density of the electrostatic latent image. In other words, the magnetic brush method of the two-component developing system is suitable for a reproduction of a gradation image. Also, the magnetic brush method of the two-component developing system has a feature that the developing apparatus per se can be downsized. 
   The magnetic brush developing method using a development sleeve which is the developer carrying member is generalized as a magnetic brush developing apparatus of the two-component developing system. 
   In the magnetic brush developing method, in order to efficiently develop the electrostatic latent image on the photosensitive drum, the two-component developer containing magnetic powders, for example, magnetic carriers which are ferrite or the like, and toner in which pigment is dispersed in a resin are agitated and mixed together. The agitation and mixture of the developer allow the toner to carry electric charges through frictional charge attributable to the friction between the developers. On the other hand, the developer is held by the development sleeve serving as a hollow cylindrical developer carrying member which has the magnetic pole therein and is made of a nonmagnetic material. The developer that is held by the development sleeve is transported to a development area that faces the photosensitive drum from a developer container by using the development sleeve. The developer that has been transported to the development area is stood like the ears of rice through an action of the magnetic field in the development area, and rubs the surface of the photosensitive drum. As a result, the electrostatic latent image that has been formed on the photosensitive drum is developed by the developer. 
   The two-component magnetic brush developing method using the development sleeve is mainly employed in various products, typical examples of which include a monochrome digital copying machine and a full color copying machine requiring a high image quality. 
   Up to now, in a case where a rotation movement speed of the photosensitive drum is relatively low, that is, in a case of a copying machine having relatively low operating speed, a sufficient and excellent developed image is obtained with a short development period. For that reason, an excellent image is obtained even when a number of the development sleeves is one. 
   However, in a case where the rotation movement rate of the photosensitive drum becomes higher in a course of a demand for increasing the operating speed of the copying machine in recent years, it is not always possible that proper image formation can be conducted by one development sleeve. 
   As a countermeasure against the problem described above, there is a method in which peripheral speed of the development sleeve is increased to enhance development efficiency. However, a centrifugal force that is exerted on the developer constituting the magnetic brush becomes larger as the peripheral speed of the development sleeve increases. This increases a scattering rate of the developer, induces contamination of an interior of the copying machine, and deteriorates functionalities of the apparatus. 
   Under the above-mentioned circumstances, as another countermeasure, there has been proposed a so-called multi-stage magnetic brush developing method using two or more developer carrying members such as the development sleeves. That is, in the multi-stage magnetic brush developing method, a plurality of development sleeves are disposed in such a manner that peripheral surfaces thereof are brought in vicinity of each other so as to be adjacent to each other. Then, the developer is continuously transported through the respective peripheral surfaces, and the development period is extended to enhance development performance. 
   Now, an example of the developing apparatus of the multi-stage magnetic brush developing system having two conventional development sleeves is shown in  FIG. 13 . 
   A developing apparatus  104  includes a developer container  122  that is disposed in parallel to a photosensitive drum  101 , and an interior of the developer container  122  is compartmented into a development chamber R 1  and an agitation chamber R 2  by a partition  123  that is in parallel to the photosensitive drum  101 . A developer  120  into which the toner particles and the magnetic carriers are mixed together is housed in the development chamber R 1  and the agitation chamber R 2 . 
   A transporting screw  124  is housed in the development chamber R 1 , and the transporting screw  124  transports the developer  120  along a longitudinal direction of the developer container  122  which is in parallel to the photosensitive drum  101  through rotational driving. A transporting screw  125  is housed in the agitation chamber R 2 , and transports the developer  120  along a longitudinal direction of the developer container  122  which is in parallel to the photosensitive drum  101  through rotation driving. A developer transporting direction of the transporting screw  125  is opposite to that of the transporting screw  124 . 
   Openings  123   a  and  123   b  are defined in the partition  123  at a back side and a front side of  FIG. 13  as can be understood with reference to  FIG. 14 . The developer  120  that has been transported by the transporting screw  124  is transferred to the transporting screw  125  from the opening  123   a , and the developer  120  that has been transported by the transporting screw  125  is transferred to the transporting screw  124  from the opening  123   b.    
   An opening portion is defined at a portion of the developer container  122  in vicinity of the photosensitive drum  101 , and two developer carrying members consisting of a first development sleeve  126  and a second development sleeve  128  which are made of a non-magnetic material are disposed in the opening portion. The first development sleeve  126  is disposed opposite to the photosensitive drum  101  to define a development area A 1 , and the second development sleeve  128  is disposed opposite to the photosensitive drum  101  to define a development area A 2 . 
   Of the two developer carrying members, the first development sleeve  126  that is disposed opposite to the photosensitive drum  101  at an upstream side in a rotating direction “a” of the photosensitive drum  101  rotates in a direction indicated by an arrow “b” (in a direction opposite to the rotating direction “a” of the photosensitive drum  101 ). 
   Also, in this example, a blade-shaped developer regulating member (layer thickness regulating blade)  121  is disposed at a top end of the opening portion of the developer container  122 , that is, upstream of the development area A 1  in the rotating direction of the development sleeve  126  in general. The development sleeve  126  carries and transports the developer  120  to the first development area A 1  after the retained developer is regulated to an appropriate developer layer thickness by the layer thickness regulating blade  121 . 
   A roller-shaped first magnetic field generating means (hereinafter referred to as “magnet roller”)  127  is fixed and disposed within the development sleeve  126 . The first magnet roller  127  has a development magnetic pole S 1  that faces the first development area A 1 . The magnetic brush of the developer is formed through a development magnetic field that is developed in the first development area A 1  by the development magnetic pole S 1 , and the magnetic brush comes into contact with the photosensitive drum  101  rotating in the direction indicated by the arrow “a” in the first development area A 1  to develop the electrostatic latent image in the first development area A 1 . 
   The first magnet roller  127  has N 1 , S 2 , N 2 , and N 3  poles in addition to the above-mentioned development magnetic pole S 1 , and the N 2  pole and the N 3  pole are identical in polarity with each other and adjacent to each other within the developer container  122  to develop a repulsive magnetic field, thus producing a barrier with respect to the developer  120 . 
   In addition, the second development sleeve  128  that is a second developer carrying member is disposed below the first development sleeve  126  and at a downstream side in the rotating direction “a” of the photosensitive drum  101 . Moreover, the second development sleeve  128  is disposed in an area substantially facing both the first development sleeve  126  and the photosensitive drum  101 , and is also located rotatably in a direction indicated by an arrow “c” which is the same direction as that of the first development sleeve  126 . 
   The second development sleeve  128  is made of a non-magnetic material, as with the first development sleeve  126 , and a roller-shaped second magnet roller  129  that is a second magnetic field generating means is located in a non-rotating state in the interior of the second development sleeve  128 . Also, the second magnet roller  129  has five poles consisting of magnetic poles S 3 , N 4 , S 4 , N 5 , and S 5 . 
   The developer  120  is transported in the stated order of N 2 →S 2 →N 1 →S 1 →N 3  on the first development sleeve  126 . Thereafter, the developer on the first development sleeve  126  is moved to the second development sleeve  128 , and is transported in the stated order of S 3 →N 4 →S 4 →N 5 →S 5  on the second development sleeve  128 . In this example, the developer is transferred by the poles substantially facing each other and having the polarities different from each other (i.e., N 3  pole and S 3  pole). This is because in a case where the poles are identical in polarity with each other, the magnetic force lines are not produced and stable transfer cannot be conducted. 
   In the above-mentioned structure, the magnetic brush produced in the N 4  pole comes into contact with the photosensitive drum  101  at an opposing portion of the second development sleeve  128  and the photosensitive drum  101 , that is, the second development area A 2 . Then, the electrostatic latent image on the photosensitive drum  101  which has passed through the first development area A 1  is further subjected to a second development process. In this way, the two development processes are conducted to achieve high development efficiency. 
   As described above, with the structure in which two development sleeves  126  and  128  are disposed, for example, even if the development period becomes shorter as the peripheral speed of the photosensitive drum  101  increases, high development efficiency can be achieved, thereby making it possible to excellently form an image without deterioration of the development density and occurrence of density unevenness. 
   Incidentally, the developer  120  within the developer container  122  is transferred to a portion of a bearing  140  of the development sleeves  126  and  128  shown in  FIG. 15A  along the surfaces of the development sleeves  126  and  128  by circulation within the developer container  122 . For that reason, the developer enters the portion of the bearing  140  and stays within the bearing  140  to inhibit the function thereof. As a result, there is a case in which a smooth rotation of the development sleeves  126  and  128  is disabled, or the developer passes through the portion of the bearing  140 , causing the developer to leak out from the developer container  122  or scatter. 
   To cope with the scattering of the developer from the end of the development sleeve, there has been proposed a method in which elastic seal members are fitted onto both ends of the development sleeves, and the ends of the seal members are sealed to prevent the toner from leaking. 
   However, in the above-mentioned seal structure, because the elastic sealing members are fitted onto the outer peripheral surfaces of the development sleeves under pressure, there arise such problems that a load on the development sleeves becomes large, and the sealing property is deteriorated due to the deterioration of the elastic seal members. 
   Under such the circumstances, there has been proposed a developing apparatus using a magnetically attractive toner or carrier, in which magnetic sealing is conducted by magnetic force generating means (for example, refer to JP 11-133750 A). 
   Shown in  FIG. 15B  is a structure in which a magnetized magnetic seal member MP is disposed on an opposing surface with respect to a surface of a development sleeve SL at a given gap to magnetically attract and hold the developer. 
   The above-mentioned magnetic seal structure is advantageous in that a rotation load of the development sleeve SL is reduced because the development sleeve SL and the magnetic seal member MP are out of contact, and that the lifetime is prolonged because the development sleeve SL and the magnetic seal member MP are not deteriorated due to the friction. 
   When a plate-shaped magnet is disposed as the magnetic seal member MP to surround the development sleeve SL in a non-contacting fashion, the magnetic brush is produced between a magnetic roller MR and a magnet MP within the development sleeve SL by using the developer, thereby making it possible to prevent the leakage of the developer. In the developing apparatus shown in  FIG. 13 , in a case of using a magnet plate having one surface of N pole and the other surface of S pole as the magnet MP, it is desirable that a surface having a pole different in polarity from a pole (N 2  and N 3 , and S 3  and S 5 ) that produces the repulsive magnetic field of the magnet roller is a surface at the development sleeve side. In a case where the above-mentioned structure is not applied, the leakage of the developer in the longitudinal direction of the sleeve is liable to occur. The reasons will be described below. 
   A description will be given with reference to  FIGS. 16A to 16C  and  FIGS. 17A to 17C . In a case where the repulsive magnetic field and the magnetic seal member are identical in polarity with each other while facing each other, the repulsive magnetic field is also produced between the repulsive magnetic field and the magnetic seal member. For that reason, as shown in  FIG. 16A , the line of magnetic force of the magnetic seal is unintentionally bent toward the outside of the longitudinal direction of the development sleeve SL and extended. In this case, because the developer is arranged along the line of magnetic force as shown in  FIG. 16B , the developer is extended toward the direction of the end of the development sleeve SL, and the developer is liable to leak in the direction of the end thereof. 
     FIG. 16C  schematically shows a force that is applied to the magnetic carrier in an area surrounded by the magnetic seal member MP and the development sleeve SL. The arrows indicate a direction of force at that position, and a length of the arrow indicates a magnitude of the force. 
   In a case where the magnets have the same polarity facing each other, an area (where the direction of the force exerted on the magnet is inverted), in which there is substantially no magnetic force exerted on the magnetic carrier between the magnets, continuously exists in the longitudinal direction between the magnets. In  FIG. 16C , an area in which there is substantially no magnetic force exerted on the magnetic carrier is designated by a mark O. 
   However, as shown in  FIG. 16C , in a case where the area in which there is substantially no force exerted on the magnetic carrier continuously exists between the magnetic seal member MP and the development sleeve SL in the longitudinal direction, no magnetic carrier is attracted to the magnetic seal member MP or the magnet roller MR. For that reason, it is possible to leak the developer according to a flow indicated by a dotted arrow shown in  FIG. 16C . As a result, the developer is liable to flow out of the development area, thereby making it impossible to exercise the excellent sealing property. 
   Under the circumstances, there has been proposed a structure in which the repulsive magnetic field and the magnetic seal member are different in polarity from each other while facing each other. 
   In other words, when the repulsive magnetic field and the magnetic seal member are different in polarity from each other while facing each other, since the line of magnetic force of the magnetic seal is extended toward the direction of the development sleeve as shown in  FIG. 17A , it becomes difficult to extend the line of magnetic force of the magnetic seal toward the outside of the longitudinal direction. As a result, it becomes difficult for the developer to leak toward the direction of the end of the development sleeve. In this situation, the developer is extended toward the direction of the development sleeve as shown in  FIG. 17B , and the magnetic brush is produced between the development sleeve SL and the magnetic seal member MP by the developer. The magnetic brush functions to seal the developer that is to leak toward the end direction, and it is further suppresses leakage of the developer. 
   On the other hand, in a case where the repulsive magnetic field and the magnetic seal member are identical in polarity with each other while facing each other as described above, as shown in  FIG. 16B , the magnetic brush is not extended toward the development sleeve direction from the magnetic seal member MP. Therefore, there is an area in which no developer exists between the magnetic seal member MP and the development sleeve SL, and the developer is liable to leak. 
     FIG. 17C  schematically shows a force that is exerted on the magnetic carrier in an area surrounded by the magnetic seal member MP and the development sleeve SL as in  FIG. 16C . 
   In a case where the magnets are different in polarity from each other while facing each other, an area (indicated by a symbol “O”) in which there is substantially no magnetic force exerted on the magnetic carrier between the magnets exists, but does not continuously exist between the magnets. For that reason, the magnetic carrier in the area surrounded by the magnetic seal member MP and the development sleeve SL is always attracted to the magnetic seal member MP and the magnet roller MR during a process in which the magnetic carrier is moved in the direction of the end of the development sleeve. As a result, it is difficult to that the developer flows out of the development area, thereby making it possible to exercise the excellent seal property. 
   Also, there has been proposed a structure using magnets in which NS poles are magnetized to multiple magnetic poles on an inner peripheral surface as the magnetic seal member MP. In the above-mentioned structure, because the line of magnetic force is extended between the multiple magnetic poles of the magnetic seal member, it is difficult that the line of magnetic force is extended to the outside of the longitudinal direction of the development sleeve, whereby excellent property can be exercised. 
   In a case where the above-mentioned magnetic seal structure is applied to the magnetic brush developing apparatus having the two development sleeves shown in  FIG. 13 , there arise the following problems. 
   As shown in  FIG. 18 , in the case of using magnet plates  130  and  131  each having one surface of N pole and a back surface of S pole as the magnetic seal member, the upstream development sleeve  126  has an S pole surface that is different in polarity from the repulsive magnetic poles (N 2  and N 3 ) as the inner peripheral surface. Also, the downstream development sleeve  128  has an N pole surface that is different in polarity from the repulsive magnetic pole (S 3  and S 5 ) as the inner peripheral surface. From the viewpoint of the above-mentioned conventional art, the above-mentioned structure appears to suppress the leakage of the developer to the outside of the development area, and exert the excellent sealing property. 
   However, according to the inventors&#39; study, it has been found that in the above-mentioned structure, the developer is leaked in the downstream sleeve rotating direction from a space between the upstream and downstream development sleeves  126  and  128  in the end of the development sleeve. The reasons will be described below. 
   Of the S 3  pole and the S 5  pole which produce the repulsive magnetic field of the downstream development sleeve  128 , the S 3  pole also serves as a delivery pole that receives the developer from the upstream development sleeve  126 . As a result, because the upstream development sleeve  126  exists at an opposing portion of the S 3  pole, the magnetic seal member  131  is capable of extending only up to the middle of the repulsive magnetic field as shown in  FIG. 18 . For that reason, the magnetic seal member  131  does not face the S 3  pole of the delivery pole. 
   However, because the line of magnetic force is developed between the magnetic seal member  131  and the S 3  pole of the delivery pole, a part of a developer that has been caught by the magnetic seal member  131  is attracted to the S 3  pole of the delivery pole and moved. The developer that has been moved to the S 3  pole from the magnetic seal member  131  at the end of the development sleeve has no magnetic seal member at a side opposite to the S 3  pole, and leaks in the development sleeve end direction. The developer that has leaked out in the end direction is transported with the rotation of the development sleeve, and leaked from the space between the upstream and downstream development sleeves  126  and  128 . 
   Similarly, in the case of a structure using a magnet, whose NS poles are magnetized to the multiple magnetic poles on the inner peripheral surface, as the magnetic seal member, because the line of magnetic force is formed between the magnetic seal member and the S 3  pole of the delivery pole, the developer is leaked from a space between the upstream and downstream development sleeves as in the above-mentioned case. 
   SUMMARY OF THE INVENTION 
   Under the above-mentioned circumstances, an object of the present invention is to provide a developing apparatus having a plurality of developer carrying members for preventing a developer from leaking from a space between two developer carrying members in a developer carrying member rotating direction when an end of a developer carrying member is sealed by using a magnet member. 
   In order to achieve the above-mentioned object, a developing apparatus for developing an electrostatic image on an image bearing member by using a developer containing magnetic particles therein, the developing apparatus includes: a developer container, which contains the developer; a first developer carrying member that is rotatably disposed within the developer container, for carrying and transporting the developer toward a first development area; a second developer carrying member that is rotatably disposed in vicinity of the first developer carrying member within the developer container, for carrying and transporting the developer delivered from the first developer carrying member toward a second development area in a developer delivery region; a first magnetic field generating means having a plurality of magnetic poles which is disposed within the first developer carrying member, the first magnetic field generating means having a first magnetic pole at a position facing the developer delivery region; a second magnetic field generating means having a plurality of magnetic poles which is disposed within the second developer carrying member, the second magnetic field generating means having a second magnetic pole that is different in polarity from the first magnetic pole at a position facing the developer delivery region; a first magnet member disposed in the vicinity of an end of the first developer carrying member in an axial direction thereof along a peripheral surface of the first developer carrying member to avoid facing the first magnetic pole; and a second magnet member disposed in the vicinity of an end of the second developer carrying member in an axial direction thereof along a peripheral surface of the second developer carrying member to avoid facing the second magnetic pole, a surface facing the second developer carrying member in an end area downstream of the second magnet member in a rotating direction of the second developer carrying member being identical in polarity with the second magnetic pole. 
   Further features of the present invention will become apparent from the following description of exemplary embodiments with reference to the attached drawings. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
       FIG. 1  is a cross-sectional view schematically showing a structure of an image forming apparatus using a developing apparatus according to an embodiment of the present invention. 
       FIG. 2  is a cross-sectional view schematically showing a structure of a developing apparatus according to an embodiment of the present invention. 
       FIG. 3  is a lateral cross-sectional view schematically showing a structure of the developing apparatus. 
       FIG. 4  is a diagram for explaining a flow of a developer that is transported on development sleeves of the developing apparatus. 
       FIG. 5  is a diagram for explaining a developing apparatus in a comparative example. 
       FIG. 6  is a diagram for explaining the developing apparatus according to the embodiment of the present invention. 
       FIG. 7  is a lateral cross-sectional view showing a portion in the vicinity of an end of the development sleeve for explaining the developing apparatus according to the embodiment of the present invention. 
       FIGS. 8A ,  8 B, and  8 C are diagrams for explaining lines of magnetic force in the vicinity of a magnetic seal member and a magnetizable plate, respectively. 
       FIG. 9  is a diagram for explaining magnetic flux density in a repulsive magnetic field. 
       FIG. 10  is a diagram for explaining a developing apparatus according to another embodiment of the present invention. 
       FIG. 11  is a diagram for explaining a developing apparatus according to still another embodiment of the present invention. 
       FIGS. 12A ,  12 B, and  12 C are diagrams for explaining lines of magnetic force in the vicinity of a magnetic seal member in a developing apparatus according to another embodiment of the present invention. 
       FIG. 13  is a diagram for explaining a conventional developing apparatus. 
       FIG. 14  is a lateral cross-sectional view schematically showing a structure of the conventional developing apparatus. 
       FIGS. 15A and 15B  are lateral cross-sectional views for explaining a schematic structure in the vicinity of the end of the development sleeve in the conventional developing apparatus, respectively. 
       FIGS. 16A ,  16 B, and  16 C are diagrams for explaining lines of magnetic force in the vicinity of a magnetic seal member of the conventional developing apparatus, respectively. 
       FIGS. 17A ,  17 B, and  17 C are diagrams for explaining lines of magnetic force in the vicinity of the magnetic seal member of the conventional developing apparatus, respectively. 
       FIG. 18  is a diagram for explaining a flow of the developer that is transported on the development sleeve of the conventional developing apparatus. 
   

   DESCRIPTION OF THE EMBODIMENTS 
   Now, a description will be given in more detail of a developing apparatus and an image forming apparatus according to the present invention with reference to the accompanying drawings. 
   First Embodiment 
   First, a description will be provided of a schematic structure of an image forming apparatus according to an embodiment of the present invention, and thereafter a description will be provided of a developing apparatus that constitutes a characteristic portion of the present invention. In this embodiment, the image forming apparatus is directed to a multicolor image forming apparatus of a tandem type using an electrophotographic printing method. However, the present invention is not limited to the above-mentioned structure. 
   According to this embodiment, the multicolor image forming apparatus has a plurality of image formation sections (i.e., image formation stations) disposed from an upstream side to a downstream side along a rotating direction (i.e., a direction indicated by an arrow R 7 ) of an intermediate transfer belt  7  as an intermediate transfer member. In this embodiment, the image formation sections are constituted of four image formation sections P (i.e., PY, PM, PC, PK) consisting of yellow Y, magenta M, cyan C, and black K. 
   The respective image formation sections P (i.e., PY, PM, PC, PK) are substantially identical in structure with each other, and include drum-shaped electrophotographic photosensitive members, that is, photosensitive drums  1  (i.e.,  1   a ,  1   b ,  1   c , and  1   d ) as image bearing members, respectively. In a full-color image, images of yellow (Y), magenta (M), cyan (C), and black (K) are formed on the photosensitive drums  1  (i.e.,  1   a ,  1   b ,  1   c , and  1   d ), respectively. 
   In more detail, the photosensitive drums  1  (i.e.,  1   a ,  1   b ,  1   c , and  1   d ) are rotationally driven in a direction indicated by an arrow R 1  (i.e., clockwise in  FIG. 1 ), respectively. Chargers (i.e., charging means)  2  (i.e.,  2   a ,  2   b ,  2   c , and  2   d ), exposing devices (i.e., latent image forming means)  3  (i.e.,  3   a ,  3   b ,  3   c , and  3   d ), and developing apparatuses (i.e., developing means)  4  (i.e.,  4   a ,  4   b ,  4   c , and  4   d ) are disposed approximately in the stated order along the rotating direction in the periphery of the respective photosensitive drums  1  (i.e.,  1   a ,  1   b ,  1   c , and  1   d ). Also, primary transfer rollers (i.e., primary transfer means)  5  (i.e.,  5   a ,  5   b ,  5   c , and  5   d ) and drum cleaners (i.e., cleaning devices)  6  (i.e.,  6   a ,  6   b ,  6   c , and  6   d ) are disposed along the rotating direction thereof in the periphery of the respective photosensitive drums (i.e.,  1   a ,  1   b ,  1   c , and  1   d ). 
   The intermediate transfer belt  7  is extended around support rollers  81  and  82 , and a secondary transfer opposed roller  8  that also functions as a driving roller. The intermediate transfer belt  7  rotates in a direction indicated by an arrow R 7  with the rotation of the secondary transfer opposed roller  8  in a direction indicated by an arrow R 8 . The rotating speed of the intermediate transfer belt  7  is set to be substantially identical with the rotating speed (i.e., process speed) of the above-mentioned respective photosensitive drums  1  (i.e.,  1   a ,  1   b ,  1   c , and  1   d ). 
   Also, the intermediate transfer belt  7  is pressed by the primary transfer rollers  5  (i.e.,  5   a ,  5   b ,  5   c , and  5   d ) from the back surface side, and the front surface of the intermediate transfer belt  7  is abutted against the photosensitive drums  1  (i.e.,  1   a ,  1   b ,  1   c , and  1   d ). Primary transfer nips (i.e., primary transfer sections) T 1  (i.e., T 1   a , T 1   b , T 1   c , and T 1   d ) are formed between the intermediate transfer belt  7  and the respective photosensitive drums  1 . 
   A secondary transfer roller (i.e., secondary transfer means)  9  is disposed at a position corresponding to the secondary transfer opposed roller  8 . The secondary transfer roller  9  nips the intermediate transfer belt  7  in association with the secondary transfer opposed roller  8 , and a secondary transfer nip (i.e., secondary transfer section) T 2  is formed between the secondary transfer roller  9  and the intermediate transfer belt  7 . 
   Transferring materials P that are subjected to image formation are housed in a state where the transferring materials P are stacked on a sheet feed cassette  10 . The transferring materials P are supplied to the above-mentioned secondary transfer nip section T 2  by a sheet feeding device (not shown) having a sheet feed roller, a transport roller, and a registration roller. 
   A fixing device  11  having a fixing roller  12  and a pressure roller  13  that is pressed on the fixing roller  12  is disposed downstream of the secondary transfer nip section T 2  along the feeding direction of the transferring material P. A sheet discharge tray (not shown) is disposed downstream of the fixing device  11 . 
   In the image forming apparatus structured as described above, a toner image of four full colors is formed on the transferring material P as follows. 
   First, the photosensitive drums  1  (i.e.,  1   a ,  1   b ,  1   c , and  1   d ) are rotationally driven by a photosensitive drum driving motor (not shown) at a given process speed in a direction indicated by an arrow R 1 . Then, the photosensitive drums  1  are uniformly charged to a given polarity and potential by the chargers  2  (i.e.,  2   a ,  2   b ,  2   c , and  2   d ). The photosensitive drums  1  (i.e.,  1   a ,  1   b ,  1   c ,  1   d ) that have been charged are exposed on the basis of image information by the exposing devices  3  (i.e.,  3   a ,  3   b ,  3   c ,  3   d ), and the electric charges are removed from the exposed portions to form the electrostatic latent images for the respective colors. 
   The electrostatic latent images on those photosensitive drums  1  (i.e.,  1   a ,  1   b ,  1   c , and  1   d ) are developed as the toner images of the respective colors consisting of yellow (Y), magenta (M), cyan (C), and black (K) by the developing apparatuses  4  (i.e.,  4   a ,  4   b ,  4   c , and  4   d ). 
   Those toner images of four colors are sequentially primarily transferred onto the intermediate transfer belt  7  by the primary transfer rollers  5  (i.e.,  5   a ,  5   b ,  5   c , and  5   d ) in the primary transfer nips T 1  (i.e., T 1   a , T 1   b , T 1   c , and T 1   d ). Thus, the toner images of four colors are superimposed on the intermediate transfer belt  7 . 
   The toner (i.e., residual toner) that has remained on the photosensitive drums  1  (i.e.,  1   a ,  1   b ,  1   c , and  1   d ) without being transferred onto the intermediate transfer belt  7  are removed by drum cleaners  6  (i.e.,  6   a ,  6   b ,  6   c , and  6   d ). The photosensitive drums  1  (i.e.,  1   a ,  1   b ,  1   c , and  1   d ) from which the residual toners has been removed are subjected to subsequent image formation. 
   The toner image of four colors which have been superimposed on the intermediate transfer belt  7  in the manner described above is secondarily transferred onto the transferring material P. The transferring material P that has been fed from the sheet feed cassette  10  by the feeding device is supplied to the secondary transfer nip T 2  by the registration roller (not shown) at a timing in accordance with the toner image on the intermediate transfer belt  7 . The toner images of four colors on the intermediate transfer belt  7  are collectively and secondarily transferred onto the supplied transferring material P in the secondary transfer nip T 2  by using the secondary transfer roller  9 . 
   The transferring material P on which the toner image of four colors has been secondarily transferred is fed to the fixing device  11 . The transferring material P is then heated and pressurized to fix the toner image on the surface. The transferring material P on which the toner image has been fixed is discharged to the sheet discharge tray. 
   With the above-mentioned operation, the image formation of four full colors has been completed with respect to one surface (i.e., front surface) of a single transferring material P. 
   Now, a description will be provided in more detail of the developing apparatus  4  according to this embodiment with reference to  FIGS. 2 and 3 . Since the respective developing apparatuses  4   a ,  4   b ,  4   c , and  4   d  that are used in the image forming apparatus main body according to this embodiment are identical in structure with each other, only one developing apparatus  4  will be described. In the following description, the developing apparatus  4  is a generic term used to refer to any one of the developing apparatuses  4   a ,  4   b ,  4   c , and  4   d.    
   In this embodiment, the developing apparatus  4  is identical in structure with the developing apparatus of the multistage magnetic brush developing system having two developing sleeves as the developer carrying member which is described with reference to  FIG. 13  in advance. 
   In other words, in this embodiment, the developing apparatus  4  has a developer container  22 , and the interior of the developer container  22  is compartmented into a development chamber R 1  and an agitation chamber R 2  by a partition  23 . On the other hand, a developer  20  is reserved in the development chamber R 1  and the agitation chamber R 2 . In this embodiment, the developer is made of a two-component developer in which the toner particles and the magnetic carriers (that is, magnetic particles) are mixed together. The magnetic carriers used in this embodiment may be made of ferrite carriers or resin magnetic carriers made of a binder resin, magnetic metal oxide, and nonmagnetic metal oxide. 
   A transporting screw  24  is received in the development chamber R 1 , and transports the developer  20  to the developer container  22  along a longitudinal direction parallel to the photosensitive drum  1  by rotary driving. A transporting screw  25  is received within the agitation chamber R 2 , and transports the developer  20  to the developer container  22  along a longitudinal direction parallel to the photosensitive drum  1  by rotary driving. The developer transporting direction of the transporting screw  25  is opposite to that of the transporting screw  24 . 
   Openings  23   a  and  23   b  are defined in the partition  23  at a backside and a front side of  FIG. 2  as shown in  FIG. 3 . The developer  20  that has been transported by the transporting screw  24  is delivered to the transporting screw  25  from the opening  23   a , and the developer  20  that has been transported by the transporting screw  25  is delivered to the transporting screw  24  from the opening  23   b.    
   An opening portion is defined at a portion of the developer container  22  in vicinity of the photosensitive drum  1 , and two developer carrying members consisting of a first development sleeve (i.e., first developer carrying member)  26  and a second development sleeve (i.e., second developer carrying member)  28  which are made of a non-magnetic material are disposed in the opening portion. In this embodiment, the first and second development sleeves  26  and  28  are made of aluminum, nonmagnetic stainless steel, or other materials and appropriate irregularities are formed on the surfaces of the first and second development sleeves  26  and  28 . 
   Also, the upstream development sleeve  26  that is the first developer carrying member is disposed opposite to the photosensitive drum  1  to define a development area A 1 , and the downstream development sleeve  28  that is the second developer carrying member is disposed opposite to the photosensitive drum  1  to define a development area A 2 . 
   Of the two first and second developer carrying members, the upstream development sleeve  26 , which is disposed opposite to the photosensitive drum  1  at an upstream side in a rotating direction “a” of the photosensitive drum  1 , rotates in a direction indicated by an arrow “b” (i.e., in a direction opposite to the rotating direction “a” of the photosensitive drum  1 ). 
   Also, in this embodiment, a blade-shaped developer regulating member (i.e., layer thickness regulating blade)  21  is disposed at a top end of the opening portion of the developer container  22  upstream of the development area A 1  in the rotating direction of the development sleeve  26 . The development sleeve  26  carries and transports the developer  20  to the upstream development area A 1  after the retained developer is regulated to an appropriate developer layer thickness by the layer thickness regulating blade  21 . 
   A roller-shaped first magnetic field generating means  27  is fixed within the upstream development sleeve  26 . The first magnet roller  27  has a development magnetic pole S 1  that faces the first development area A 1 . The magnetic brush of the developer is produced due to the development magnetic field that is produced in the first development area A 1  by the development magnetic pole S 1 , and the magnetic brush is brought into contact with the photosensitive drum  1  in the direction indicated by the arrow “a” in the first development area A 1  to develop the electrostatic latent image in the first development area A 1 . In this situation, the toner that adheres to the magnetic brush and the toner that adheres to the surface of the development sleeve are also transferred to an image area of the electrostatic latent image and then developed. In this embodiment, the first magnet roller  27  has N 1 , S 2 , N 2  (i.e., fourth magnetic pole), and N 3  (i.e., first magnetic pole) poles in addition to the above-mentioned development magnetic pole S 1 . Among those magnetic poles, the N 2  pole and the N 3  pole are identical in polarity with each other and adjacent to each other to develop a repulsive magnetic field, to thereby produce a barrier with respect to the developer. 
   As described above, the downstream development sleeve  28  that is the second developer carrying member is disposed below the upstream development sleeve  26  and in an area that substantially faces both of the upstream development sleeve  26  and the photosensitive drum  1 . The downstream development sleeve  28  is located rotatably in a direction indicated by an arrow “c” (i.e., the same direction as that of the first development sleeve  26 ). As described above, the downstream development sleeve  28  is also made of a nonmagnetic material, as with the upstream development sleeve  26 , and a roller-shaped second magnet roller  29  that is a second magnetic field generating means is located in a non-rotating state in the interior of the second development sleeve  28 . The second magnet roller  29  has five poles consisting of magnetic poles S 3  (i.e., second magnetic pole), N 4 , S 4 , N 5 , and S 5  (i.e., third magnetic pole). Among those poles, the magnetic brush on the N 4  pole comes in contact with the photosensitive drum  1  in the second development area A 2 , and further conducts a second development on the photosensitive drum  1  that has passed through the first development area A 1 . 
   Also, the S 3  pole and the S 5  pole are identical in polarity with each other, a repulsive magnetic field is developed between the S 3  pole and the S 5  pole to create a barrier with respect to the developer. Among those poles, the S 3  pole faces the N 3  pole of the first magnet roller  27  that is included in the upstream development sleeve  26  in the vicinity of a position closest to both of those sleeves. 
   Hereinafter, a flow of the developer will be described with reference to an enlarged diagram (i.e.,  FIG. 4 ) showing the vicinity of the upstream development sleeve  26  and the downstream development sleeve  28 . 
   A repulsive magnetic field is provided between the N 3  pole and the N 2  pole of the upstream development sleeve  26 , and a repulsive magnetic field is also provided between the S 3  pole and the S 5  pole of the upstream development sleeve  28 . Therefore, the developer that has been transported on the first development sleeve  26  and has passed the development area reaches the N 3  pole, is capable of passing through the closest position of both the sleeves due to the repulsive magnetic field. The developer is moved to the downstream development sleeve  28  side in the developer delivering area according to a line of magnetic force that extends from the N 3  pole to the S 3  pole as indicated by an arrow “d”. The developer is then transported on the downstream development sleeve  28  up to the transporting screw  25  within the agitation chamber R 2 . 
   According to this embodiment, the downstream development sleeve  28  is disposed below the upstream development sleeve  26  with the result that the developer is transported in the stated order of N 2 →S 2 →N 1 →S 1 →N 3  on the upstream development sleeve  26 . Thereafter, the developer on the upstream development sleeve  26  is blocked by the repulsive magnetic field of both the sleeves  26  and  28 , and then moved to the downstream development sleeve  28 . Then, the developer is transported on the downstream development sleeve  28  in the stated order of S 3 →N 4 →S 4 →N 5 →S 5 , blocked by the repulsive magnetic field at the S 5  pole, and peeled off to the agitation chamber R 2 . 
   It is unnecessary that N 3  and S 3  that are delivery poles perfectly face each other. It is possible that the developer is smoothly delivered when the delivery poles substantially face each other within a range which is shifted by 45° from a state in which those delivery poles perfectly face each other. 
   Now, the magnetic seal portion in this embodiment will be described in more detail with reference to  FIGS. 5 and 6 . 
   A magnet member, that is, plate-shaped magnets (i.e., magnet plates)  30  and  31  in this embodiment are disposed as the magnetic seal members in vicinity of the development sleeves  26  and  28  along the development sleeves  26  and  28  in a non-contact state. With this structure, a magnetic brush that is formed by the developer is produced between the magnet rollers  27  and  29  within the development sleeves  26  and  28  and the magnets  30  and  31  that are the magnetic seal members, thereby making it possible to prevent the developer from leaking. 
   In this example, the magnets, that is, the magnet plates each having one surface of an N pole and its back surface of an S pole are used as the magnetic seal members  30  and  31 . Also, as shown in  FIG. 5  as a comparative example, the magnetic seal members  30  and  31  have surfaces that are different in polarity from the poles (i.e., N 2  and N 3 , and S 3  and S 5 ) that produce the repulsive magnetic fields of the magnet rollers  27  and  29  within the upstream and downstream development sleeves  26  and  28  as the surfaces of the development sleeve sides. In this case, the lines of magnetic force is extended between the magnet rollers  27  and  29  within the development sleeves  26  and  28  and the magnets that are magnetic seal members  30  and  31  to form the magnetic brush formed by the developer, thereby making it possible to prevent the developer from leaking. 
   As described in Description of The Related Art, the above is excellent in preventing leakage of the developer to the end direction of the development sleeves  26  and  28 . However, the developer is liable to leak from a space between the upstream development sleeve  26  and the downstream development sleeve  28  on the ends of the development. The reasons will be described below. 
   Of the S 3  pole and the S 5  pole that produce the repulsive magnetic field of the downstream development sleeve  28 , the S 3  pole also serves as a delivery pole that receives the developer from the upstream development sleeve  26 . As a result, because the upstream development sleeve  26  exists at an opposing portion of the S 3  pole of the delivery pole, the magnetic seal member  31  is capable of extending the magnetic seal member only up to the middle of the repulsive magnetic field as shown in  FIG. 5 . For that reason, the S 3  pole of the delivery pole does not face the magnetic seal member  31 . However, the magnetic seal member  31  and the S 3  pole of the delivery pole are different in polarity from each other, and the line of magnetic force is produced between the magnetic seal member  31  and the S 3  pole of the delivery pole. For that reason, a part of the developer that has been caught by the magnetic seal member  31  is attracted to the S 3  pole of the delivery pole, and then moved. The developer that has been moved to the S 3  pole by the magnetic seal member  31  at the end of the development sleeve is leaked in the direction toward the end of the development sleeve  28  because there is no magnetic seal member  31  at a side opposite to the S 3  pole. The leaked developer is transported with the rotation of the development sleeve  28 , and then leaked from the space between the upstream and downstream development sleeves  26  and  28 . 
   In order to prevent the developer from leaking from the space between the upstream and downstream development sleeves  26  and  28  as described above, it is necessary that the line of magnetic force be prevented from being created between the magnetic seal member  31  of the downstream development sleeve  28  and the delivery pole S 3 . In order to achieve this, in the case of using a magnet plate having one surface of N pole and its back surface of S pole as the magnetic seal member  31 , as shown in  FIG. 6 , it is necessary that an S pole that is identical in polarity with the delivery pole S 3  be placed at an opposing surface of the development sleeve. 
   However, in the structure shown in  FIG. 6 , because the S pole surface that is identical in polarity with the S 3  pole and the S 5  pole which produce the repulsive magnetic field of the downstream development sleeve  28  faces the downstream development sleeve  28  as the magnetic seal member  31 , the repulsive magnetic field is also provided between the S 3  pole and the magnetic seal member  31 , or the S 5  pole and the magnetic seal member  31 . For that reason, there arises a problem in sealing property of the magnetic seal member  31  in the longitudinal direction of the development sleeve, depending on relationships among the respective magnetic poles. 
   In order to solve the above-mentioned problem, in this embodiment, a magnetizable plate  32  that surrounds the peripheral surface of the end of the development sleeve  28  in a non-contact fashion is fitted to the inner surface of the side wall  22   a  at each side of the developer container  22 . The same structure can be applied to the upstream development sleeve  26 . 
   First, the downstream development sleeve  28  will be described. The upstream development sleeve  26  will be described later. 
   The magnetizable plate  32  that is disposed at the end of the downstream development sleeve  28  is magnetized by the magnetic force of the magnet roller  29  within the development sleeve  28  and the magnetic force of the magnet  31  that is a magnetic seal member. As a result, a magnetic circuit is formed between the magnetizable plate  32  and the magnet roller  29 , and the magnetic field is concentrated on the fore-end of the magnetizable plate  32  at the downstream development sleeve  28  side as indicated by the lines of magnetic force in  FIG. 8A . 
   The magnetic field allows a dense magnetic brush to be produced in gaps between the magnetizable plate  32  and the downstream development sleeve  28 , and the magnetizable plate  32  and the magnetic seal member  31  by the developer. The magnetic brush functions as an end seal. The magnetic brush blocks the developer that is transported from the interior of the developer container  22  along the surface of the development sleeve  28  due to reciprocating circulation within the developer container  22  between the magnetizable plate  32  and the development sleeve  29 . 
   Further,  FIG. 8C  schematically shows a force that is exerted on the magnetic carrier in an area that is surrounded by the magnetizable plate  31  that is a magnetic seal member and the development sleeve  28 . 
   As in  FIGS. 16C and 17C  in the conventional art, arrows in  FIG. 8C  indicate directions of forces at those positions, and the lengths of the arrows express the magnitudes of the forces. In the case where the magnets are identical in polarity with each other while facing each other, areas in which there is substantially no force that is exerted on the magnetic carrier (i.e., areas where the direction of the magnetic force that is exerted on the magnet is inverted) continuously exist between the magnets (refer to  FIG. 16C  in the conventional art). 
   However, when the magnetizable plate  32  is effectively arranged as in this embodiment, it is possible that areas (indicated by marks O in  FIG. 8C ) in which there is substantially no force that is exerted on the magnetic carrier are not allowed to continuously exist between the magnets. 
   As described above, in this embodiment, since the magnetizable plate  32  is arranged, the areas having substantially no force that is exerted on the magnetic carrier, that is, the areas in which the magnetic force that is exerted on the magnetic carrier is small are not allowed to continuously exist between the magnetic seal member  31 , and the development sleeve  28  and the magnetizable plate  32  in the longitudinal direction, respectively. With the above-mentioned structure, in this embodiment, the magnetic carrier in the areas that are surrounded by the magnetic seal member  31 , and the development sleeve  28  and the magnetizable plate  32  is always attracted to the magnetic seal member  31  and the magnet roller  29  within the development sleeve  28  during a process in which the magnetic carrier move in the direction of the end of the development sleeve  28 . As a result, it is difficult that the developer flows out of the development area, thereby making it possible to exercise the excellent sealing property. 
   In this embodiment, as shown in  FIG. 7 , the magnetizable plate  32  is disposed apart from the magnetic seal member  31  toward the developer container  22  side at a gap (g 1 ) of 0.3 mm. Also, the magnetizable plate  32  is so disposed as to surround the development sleeve  28  with a constant gap (g 2 ) of 0.5 mm in a non-contact fashion, as with the magnetic seal member  31 . A gap (g 3 ) between the surface of the development sleeve  28  and the magnetic seal member  31  is 1 mm. The magnetic seal member  31  as used is 60 mT (milli Tesla). 
   The arrangement of the magnetizable plate  32  is not limited to the above-mentioned conditions. In the area that is surrounded by the magnetic seal member  31  and the development sleeve  28 , it is possible to prevent the leakage so far as the magnetizable plate  32  is disposed such that areas in which there is substantially no force that is exerted on the magnetic carrier are not allowed to continuously exist along the longitudinal direction of the development sleeve. 
   A force (i.e., magnetic force) F that is exerted on the magnetic carrier (i.e., magnetic particles) that is a magnetic material can be measured as follows. 
   The magnetic force F is represented by the following expression with an external magnetic field (i.e., magnetic flux density) as B. The above description is provided on the basis of two dimensions formed of the longitudinal direction of the development sleeve and the direction perpendicular to the surface of the development sleeve. However, in fact, because it is necessary to take the peripheral direction of the development sleeve into consideration, it is necessary to measure the magnetic force three-dimensionally. F=(m·∇)B where F=(Fx, Fy, Fz) 
   In this situation, the magnitude of the magnetic force is represented as follows.
 
| F |=( Fx   2   +Fy   2   +Fz   2 ) 1/2  
 
In this example, since the magnetic bipolar moment “m” in the magnetic carrier in the above-mentioned expression generally has the magnetization that is in proportion to the external magnetic field, the magnetic bipolar moment “m” is represented as follows.
 
           m   =          A        ⁢   B                 F   =            A        ⁢     (     B   ·   ∇     )     ⁢   B     ⁢     
     ⁢           =       -        A          ⁢     ∇     B   2                         Fx   ⁡     (     x   ,   y   ,   z     )       =       -        A          ⁢       {         B   2     ⁡     (     x   ,   y   ,   z     )       -       B   2     ⁡     (       x   +     Δ   ⁢           ⁢   x       ,   y   ,   z     )         }     /   Δ     ⁢           ⁢   x                   Fy   ⁡     (     x   ,   y   ,   z     )       =       -        A          ⁢       {         B   2     ⁡     (     x   ,   y   ,   z     )       -       B   2     ⁡     (     x   ,     y   +     Δ   ⁢           ⁢   y       ,   z     )         }     /   Δy                     Fz   ⁡     (     x   ,   y   ,   z     )       =       -        A          ⁢       {         B   2     ⁡     (     x   ,   y   ,   z     )       -       B   2     ⁡     (     x   ,   y   ,     z   +     Δ   ⁢           ⁢   z         )         }     /   Δ     ⁢           ⁢   z           
where |A| is a function including the magnetic permeability, and represented as follows in the case the carrier is spherical.
 | A |=(4π/μ 0 )×(μ−1)/(μ−2)× r   3    
where r is the radius of carrier, μ is the relative magnetic permeability of carrier, and μ 0  is a space permeability.
 
It is understood from the above-mentioned fact that in the case where the intensity of magnetic field, |B|(={Bx 2 +By 2 +BZ 2 } 1/2 , changes, the magnetic force is developed from a point at which the magnetic flux density is smaller toward a direction along which the magnetic flux density is larger. Conversely, no magnetic force is exerted in a direction along which no intensity of magnetic field |B| changes.
 
   Therefore, when the intensity of the magnetic field (i.e., magnetic flux density) is continuously measured in an area surrounded by the magnetic seal member  31  and the development sleeve  28 , it is possible to obtain the magnitude and the direction of the magnetic force F from its difference on the basis of the above-mentioned expressions. 
   It is possible to measure the intensity of external magnetic field (i.e., magnetic flux density) |B| by a gaussmeter (i.e., teslameter) on the market. The present inventors have employed the gaussmeter model 640 manufactured by FW Bell. Because it is possible to measure the magnetic flux density at a probe fore-end in one direction by the gaussmeter, the magnetic flux densities (herein, Bx, By, Bz) in the three directions are measured by using probes of the three kinds of x-axis, y-axis, and z-axis, and the intensity of magnetic field is derived from the measured results. In this way, the measurement of the magnetic flux density is repeated to derive the distribution of the intensity of the magnetic field, and the magnitude and direction of the magnetic force F are obtained on the basis of the measured results. The measurement is conducted under the conditions where Δx, Δy, and Δz in measurement are set to 250 μm. The distribution of magnetic field can be grasped more precisely as Δx, Δy, and Δz are made smaller. However, there arises a problem that it takes time to conduct measurement. On the other hand, the precise distribution of a magnetic field cannot be grasped as Δx, Δy, and Δz are made larger. It is proper that Δx, Δy, and Δz are about 100 to 300 μm. 
   Since the members other than the magnetic member do not affect the measurement, the measurement is conducted by only the magnet roller, the magnet plate, and the magnetizable plate. In other words, the measurement is conducted by reproducing the actual arrangement in a state where there is no development sleeve. As a result, not only the magnetic flux density can be measured in a narrower area, but also the magnetic force in the interior of the development sleeve can be also grasped. In this situation, the probe is fixed to an xyz stage, and the measurement is continuously conducted while the xyz stage is being moved. 
   The magnetic force that is exerted on the carrier is obtained on the basis of the above-mentioned measurement results and the above-mentioned expression. 
   For example, in the case of the carrier that approximates a sphere that is 17.5 μm in radius, 12 in relative magnetic permeability μ, and 4.8 g/cm 3  in true specific gravity ρ, because the space permeability is 4π×10 −7 , |A|=2.46×10 −6  m 3  is satisfied, and the magnetic force is found on the basis of the measured value of the square B 2  of the intensity of the magnetic field. 
                 Fx   =       ⁢          A        ⁢   Δ   ⁢           ⁢       Bx   2     /   Δ     ⁢           ⁢   x                 =       ⁢         (     2.5   ×     10     -   6         )     /     (     2.5   ×     10     -   4         )       ×   Δ   ⁢           ⁢     Bx   2                   =       ⁢       10     -   2       ×   Δ   ⁢           ⁢       Bx   2     ⁡     (   N   )                     =       ⁢       10     -   2       ×     (       B   x   2     -       B     x   +       ⁢     Δx   2         )     ⁢     (   N   )                   
Since the intensity of magnetic field is a difference in the square of the intensity of magnetic field, the magnetic force becomes larger as the intensity of magnetic field is larger, or as the difference is larger. In the case where the intensity of the magnetic field is smaller, the magnetic force is small even if the difference is relatively large. This coincides with the actual phenomenon.
 
   When the magnetic force that is exerted on the carrier which has been obtained by the above-mentioned expression is small, the carrier is low in the magnetic binding force in the magnetic seal portion, and the possibility that the leakage occurs is high in the development sleeve end in the case where areas in which the magnetic force is small continuously exist in the thrust direction of the development sleeve. 
   According to a study made by inventors of the present invention, there is a case in which the binding force is small, and the leakage starts as Fx becomes smaller than 1×10 −6  (N). 
   For example, in the case where the intensity of magnetic field changes from 100 mT to 90 mT, the following expression is satisfied.
 
 F= 10 −2 ×{(100×10 −3 ) 2 −(90×10 −3 ) 2 }=1.9×10 −5  
 
In this case, the magnetic force of some degree remains.
 
On the other hand, in the case where the intensity of the magnetic field changes from 12 mT to 2 mT, a difference in the intensity of magnetic field is 10 mT which is identical with that in the above-mentioned example. However, the following expression is satisfied.
 
 F= 10 −2 ×{(12×10 −3 ) 2 −(2×10 −3 ) 2 }=1.4×10 −6  
 
Thus, the magnetic force becomes smaller. This is because the intensity per se of the magnetic field becomes smaller.
 
Now, let us consider a case in which the difference in the intensity of the magnetic field is further reduced to 5 mT, that is, a case in which the intensity of the magnetic field changes from 12 mT to 7 mT. As a result, the following expression is satisfied.
 
 F= 10 −2 ×{(12×10 −3 ) 2 −(7×10 −3 ) 2 }=9.5×10 −7  
 
That is, the magnetic force that is exerted on the carrier is further reduced and becomes smaller than 1×10 −6  (N), as a result of which the leakage is liable to occur.
 
   The above-mentioned calculation is conducted one-dimensionally, but in fact, it is necessary to conduct calculation three-dimensionally. Also, in order to conduct accurate measurement, it is necessary that Δx, Δy, and Δz are reduced as much as possible. In fact, there is a limit of a measuring device per se. 
   Under the above-mentioned circumstances, in the case where it is assumed from the distribution of the measured magnetic force that the areas in which the magnetic force is lower than 1×10 −6  (N) continuously exist in the thrust (i.e., longitudinal) direction of the development sleeve, there is the possibility that the leakage occurs. Therefore, in order to prevent the leakage in the thrust direction, it is necessary that the areas in which the magnetic force is equal to or higher than 1×10 6  (N) exist in contact with both of the surface of the magnetic seal member  31  and the surface of the development sleeve  28 . With the above-mentioned structure, the areas in which the magnetic force is lower than 1×10 −6  (N) cannot continuously exist in the thrust (i.e., longitudinal) direction of the development sleeve. In other words, the areas having a capability of trapping the magnetic particles continuously exist between the surface of the magnetic seal member  31  and the surface of the development sleeve  28 , thereby making it possible to ensure the sealing property in the longitudinal direction. 
   With the above-mentioned structure, it is possible to prevent the developer from leaking from a space between the upstream and downstream development sleeves while ensuring the magnetic sealing property at the ends of the developing sleeves. 
   In this embodiment, as shown in  FIG. 6 , the magnet  31  that is the magnetic seal member is extended up to the opposing portion of the N 5  pole which is positioned further upstream of the S 5  pole that produces the repulsive magnetic field in the sleeve rotating direction. As a result, since the magnetic seal member  31  is different in polarity from the N 5  pole and faces the N 5  pole, the magnetic seal property can be more positively ensured. Also, when there exist the portions that are different in polarity from each other while facing each other, the lines of magnetic force are extended directly between the magnet plate  31  and the magnet roller  29  within the development sleeve at that position. As a result, the developer that has been magnetically sealed is smoothly returned to the development sleeve side, that is, the interior of the developer container. From the above-mentioned viewpoint, it is important to provide one or more areas in which members different in polarity face each other for the purpose of preventing the developer from being reserved in the magnetic seal portion. 
   The above description is mainly provided of the downstream development sleeve  28 . The following description will be provided of the upstream development sleeve  26 . 
   In this embodiment, in the case of the downstream development sleeve  28 , a magnet plate having one surface of N pole as the magnetic seal member  31  and its back surface of S pole is used as the magnetic seal member  31 . In this case, as described above, when a surface that is different in polarity from the poles (S 3  and S 5 ) that produce the repulsive magnetic field of the magnet roller  29  within the downstream development sleeve  28  is a surface at the development sleeve side, the developer is leaked from a space between the upstream and downstream development sleeves  26  and  28  at the ends of the development sleeves. 
   However, in the upstream development sleeve  26 , a surface that is different in polarity from the poles (N 2  and N 3 ) that produce the repulsive magnetic field of the magnet roller  27  within the upstream development sleeve  26  is a surface at the development sleeve side as the magnetic seal member  30 . Even with the above-mentioned structure, the developer is not leaked from the space between the upstream and downstream development sleeves  26  and  28  at the ends of the development sleeves. 
   This is because as in the case of the downstream development sleeve  28 , the lines of magnetic force are produced between the magnetic seal member  30  and the delivery pole N 3  pole of the upstream development sleeve  26 , and the developer is attracted to the delivery pole N 3  pole. However, the rotating direction of the development sleeve at the position of the delivery pole N 3  pole is different from that of the downstream development sleeve  28  at the S 3  pole of the delivery pole, and rotates in a direction of transporting the developer within the developer container. Hence, the developer that has been leaked in the direction of the development sleeve end is also suitably collected within the developer container  22 . 
   For that reason, in the upstream development sleeve  26 , there arises no problem even when the surface that is different in polarity from the poles (N 2  and N 3 ) that produce the repulsive magnetic field of the magnet roller  27  within the upstream development sleeve  26  is a surface at the development sleeve side as the magnetic seal member  30 . 
   Under the circumstances, in this embodiment, in the magnet plate  30  serving as the magnetic seal member of the development sleeve  26 , a surface that is different in polarity from the repulsive magnetic field (N 2  and N 3 ) of the magnet roller  27 , that is, the S pole is the surface at the development sleeve side. This is because the lines of magnetic force are positively developed between the magnet roller  27  and the magnetic seal member  30 , thereby making it possible to more positively prevent the leakage. 
   In this embodiment, the magnetizable plate  32  that is used for the end seal of the downstream development sleeve  28  is extended up to the periphery of the upstream development sleeve  26 . This is also to more positively prevent the leakage. 
   In this example, it is preferable that the magnetizable plate  32  is made of a ferromagnetic material such as iron, nickel, or cobalt, or an alloy of those materials. The thickness of the magnetizable plate  32  is set to about 0.2 to 1 mm. Those ferromagnetic materials are equal to or lower than 0.7 J/m 2  in (½)·(BH)max. B indicates a residual magnetic flux density, H indicates a coercive force, and (BH)max indicates the maximum energy product that is the maximum of the energy product (B×H). 
   As described above, the gap g 2  between the magnetizable plates  32  and the development sleeves  26  and  28  is set to 0.5 mm in this embodiment. However, the present invention is not limited to this embodiment, and is appropriately set in a range of 0.3 to 2 mm. 
   The magnetizable plates  32  are formed of annular plates that are concentric with the development sleeves  26  and  28 . However, the magnetizable plates  32  may not be always formed of the annular plates as long as a uniform gap g 2  can be defined between the magnetizable plates  32  and the development sleeves  26  and  28 , and diverse configurations can be applied. It is preferable that angles θ (refer to  FIG. 7 ) defined between the plate surfaces of the magnetizable plates  26  and  28  and the surfaces orthogonal to the axes of the development sleeves  26  and  28  are equal to or lower than 20° from the viewpoint of more positively preventing the leakage of the developer. 
   The magnet plates  30  and  31  can be made of, for example, a rubber magnet (a magnet produced by mixing magnetic particles and rubber together) or a Nd-based (Neo-Dymium-based) magnet which has been magnetized with the magnetic flux density of 40 to 100 mT (milli Tesla). When the magnetic flux density is equal to or lower than 40 mT, it is difficult to produce the magnetic brush, leading to a problem on the magnetic sealing property. As described above, the rubber magnet having an appropriate elasticity is suitable for adhesion along the development sleeves. As described above, the distances g 3  between the development sleeves  26  and  28  and the magnet plates  30  and  31  are set to 1 mm in this embodiment. However, the present invention is not limited to the above-mentioned structure. It is preferable that the distances g 3  be in a range that enables the magnetic brush erected on at least the magnet plates to come into light contact with the outer peripheral surfaces of the development sleeves to lightly seal the outer peripheral surfaces of the development sleeves, and that the distances g 3  be set to be in a range of 0.3 to 2.0 mm. 
   It is important that all of the magnetizable plates  32  and the magnet plates  30  and  31  are selected and disposed so that the distribution of the magnetic force becomes desirable. 
   In this embodiment, the case where the developer is the two-component developer containing the nonmagnetic toner and the magnetic carrier therein has been described. The two-component developer is applicable to all of cases in which the magnetic particles are contained in the developer such as a case using the magnetic toner or a case using the magnetic toner and the magnetic carrier. 
   As described above, with the above-mentioned structure, the sealing of the ends of the development sleeves using the magnetic brush can be extremely effectively achieved in the developing apparatus having the plurality of development sleeves. 
   Second Embodiment 
   This embodiment is substantially identical in structure with the above-mentioned first embodiment. Hereinafter, structures different from those in the first embodiment will be mainly described. 
   In the first embodiment, in order to prevent the developer from leaking from a space between the upstream and downstream development sleeves  26  and  28 , the magnet  31  is disposed such that the same pole surface as that of the S 3  pole of the delivery pole of the downstream development sleeve  28  is the development sleeve surface. However, in fact, it is unnecessary to make all surfaces identical in polarity with each other. 
   The repulsive magnetic field area that is low in the magnetic flux density due to the S 3  pole and the S 5  pole of the repulsive magnetic poles appears between the S 3  pole and the S 5  pole of the downstream development sleeve  28 . 
   According to a study made by the inventors of the present invention, it is found that the developer can be prevented from leaking from a space between the upstream and downstream development sleeves when the magnetic flux density of the repulsive magnetic field area is substantially 0, that is, when the S 3  pole side (downstream side in the sleeve rotating direction) with respect to the opposing portion of the center position R of the minimum magnetic force has the same polarity. 
   In other words, when the magnetic seal member whose polarity is different exists at the S 3  pole side with respect to the opposing portion of the center position R at which the magnetic flux density of the repulsive magnetic field area is substantially 0 as shown in  FIG. 9 , the developer is transported from the magnetic seal member to the S 3  pole of the delivery pole. Therefore, the leakage occurs from the space between the upstream and downstream development sleeves. However, even when the magnetic seal member having the different polarity exists at the S 5  polarity side (upstream side in the sleeve rotating direction) with respect to the opposing portion of the center position R at which the magnetic flux density of the repulsive magnetic field area is substantially 0, the developer is not leaked from the space between the upstream and downstream development sleeves because the developer of the magnetic seal member is attracted toward the S 5  pole. 
   Under the circumstances, in this embodiment, as shown in  FIG. 10 , the magnet serving as the magnetic seal member has the same polarity as that of the magnet roller and faces the magnet roller at the downstream side of the opposing portion of the center position R of the minimum magnetic force in the sleeve rotating direction, at which the magnetic flux density of the repulsive magnetic field area of the downstream development sleeve  28  is substantially 0. On the other hand, the magnets having different polarities face each other at the upstream side. 
   As described above, the magnet has the same polarity as that of the magnet roller and faces the magnet roller at the downstream side of the opposing portion of the center position R in the sleeve rotating direction, at which the magnetic flux density of the repulsive magnetic field area is substantially 0. Therefore, the developer is not leaked from the space between the upstream and downstream development sleeves. On the other hand, the magnet whose polarity is different from that of the magnet roller faces the magnet roller at the upstream side of the opposing portion of the center position R in the sleeve rotating direction, at which the magnetic flux density of the repulsive magnetic field area is substantially 0. The reason is stated below. In the case where the magnet is different in polarity from the magnet roller and faces the magnet roller, because the lines of magnetic force are developed between the magnet serving as the magnetic seal member and the magnet roller within the development sleeve, the magnetic sealing property can be further strongly ensured. 
   In the case where the magnet has the same polarity as that of the magnet roller and faces the magnet roller at the downstream side of the opposing portion of the center position R in the sleeve rotating direction, at which the magnetic flux density of the repulsive magnetic field area is substantially 0, a magnet plate that is alternately magnetized with N and S magnetic poles may be used as the magnetic seal member in the upstream portion as shown in  FIG. 11 . 
   Even with the above-mentioned structure, in the developing apparatus having the plurality of development sleeves, the development sleeve end seal using the magnetic brush can be perfectly effected. 
   Third Embodiment 
   This embodiment is substantially identical in structure with the above-mentioned first embodiment. Hereinafter, structures different from those in the first embodiment will be mainly described. 
   In the first embodiment, in the case where the magnet plate  31  serving as the magnetic seal member and the magnet roller  29  are identical in polarity with each other while facing each other, the magnetizable plates  32  are fitted onto the inner surfaces of the side walls at both sides of the developer container  22  so as to surround the peripheral surfaces of the ends of the development sleeves  26  and  28  in the non-contact fashion. With the above-mentioned structure, a problem of the leakage of the developer in the development sleeve end direction is solved. 
   In other words, because the magnetic field is concentrated on the fore-end of the magnetizable plate  32  and the magnetic brush is formed with a high density, which produces an effect of blocking the movement of the developer in the developer container external direction. In addition, when the magnetizable plate  32  is disposed, the developer that is to leak is suitably held by the magnet plate or the magnet roller to avoid leakage because the areas in which there is substantially no force that is exerted on the magnetic carrier can be prevented from continuously existing along the longitudinal direction of the development sleeve. 
   With regard to the leakage of the developer in the longitudinal direction of the development sleeves, it is important that the areas in which there is substantially no force that is exerted on the magnetic carrier is prevented from continuously existing along the longitudinal direction of the development sleeve. Under the circumstances, in the first embodiment, the above-mentioned structure is achieved by using the magnetizable plate  32 . However, even if the magnetizable plate is not used, the above-mentioned structure can be achieved. 
   For example, even in the case where the magnet plate  31  and the magnet roller  29 , which are identical in polarity with each other and formed of the magnetic seal members, face each other, the magnetic force of the magnet plate  31  can be relatively increased with respect to the magnetic force of the magnet roller  29 . As a result, as shown in  FIG. 12A , the lines of magnetic force from the magnet plate  31  is extended up to the vicinity of the magnet roller  29  within the development sleeve. Therefore, as shown in  FIG. 12C , the areas having substantially no magnetic force which are continuously developed between the magnet plate  31  and the magnet roller  29  is formed within the development sleeve  28 . As a result, because a force is always exerted between the magnet plate  31  and the development sleeve  28  in the direction of the magnet plate, the developer is suitably held by the magnet plate  31  to avoid leakage. 
   In addition, because the magnetic brush from the magnet plate  31  is produced along the lines of magnetic force, the magnetic brush is produced from the magnet plate  31  in the direction of the development sleeve  28  as shown in  FIG. 12B , and the magnetic brush comes into light contact with the development sleeve  28 , thereby making it possible to block the leakage of the developer. 
   In this embodiment, there is used the magnet plate  31  whose surface is 100 mT in the magnetic force which is higher than that of the first embodiment. Also, as shown in  FIG. 7 , the gap g 3  between the magnet plate  31  and the development sleeve  28  is narrowly set to 0.5 mm, to thereby achieve the above-mentioned structure. 
   Even with the above-mentioned structure, in the developing apparatus having the plurality of development sleeves, the development sleeve end seal using the magnetic brush can be perfectly effected. 
   The developing apparatus according to the present invention is properly applied to the image forming apparatus of the above-mentioned embodiments which have been described as a color image forming apparatus of the intermediate transfer system. However, the present invention is not limited to those structures. The developing apparatus according to the present invention can be applied to diverse image forming apparatuses such as an image forming apparatus having a feed belt that feeds a transferring material instead of the intermediate transfer belt, or an image forming apparatus having one image bearing member which are known to those skilled in the art. 
   While the present invention has been described with reference to exemplary embodiments, it is to be understood that the invention is not limited to the disclosed exemplary embodiments. The scope of the following claims is to be accorded the broadest interpretation so as to encompass all such modifications and equivalent structures and functions. 
   This application claims the benefit of Japanese Patent Application No. 2005-259969, filed Sep. 7, 2005, which is hereby incorporated by reference herein in its entirety.