Abstract:
An image heating apparatus includes: a rotatable heating member; an coil provided outside the heating member and configured to generate heat by electromagnetic induction in the heating member; a coil holder configured to hold the coil; a plurality of magnetic cores arranged opposed to the heating member along a longitudinal direction of the heating member with the coil interposed therebetween; a core holder configured to hold at least one of the magnetic cores which is movable; and a moving mechanism configured to move the core holder between a first position and a second position which is farther away from the heating member than the first position. The core holder is provided with a stopper portion configured to stop movement of the core holder from the second position to the first position by abutment to the coil holder.

Description:
FIELD OF THE INVENTION AND RELATED ART 
     The present invention relates to an image heating apparatus for heating an image on a sheet of recording medium. An image heating apparatus is employed by an image forming apparatus such a copying machine, a printer, a facsimile machine, and so on, which records an electrophotographic, electrostatic, magnetic, or the like image forming method. It relates to also a multifunction image forming apparatus capable of functioning as two or more of the preceding examples of an image forming apparatus. 
     A fixing apparatus (or image heating device) for an electrophotographic image forming apparatus, for example, fixes an unfixed toner image formed on a sheet of recording medium, by the application of heat and pressure to the unfixed toner image. 
     As the heating method used by the fixing device for an electrophotographic image forming apparatus, a heating method based on electromagnetic induction has been known, which heats a fixing member (circularly movable heating member) by electromagnetic induction. This heating method makes it possible to place a heat source closer to toner than other heating methods, such as a heating method which uses a halogen lamp, being therefore advantageous in that it can reduce the length of time necessary to increase the surface temperature of the fixing member to a target level when the fixing device is started up. Further, its heat transmission route from the heat source to the toner is short and simple. Therefore, it is higher in thermal efficiency. 
     One of fixing devices employing a heating method based on electromagnetic induction is disclosed in Japanese Laid-open Patent Application 2010-160388. This fixing device which has multiple magnetic cores aligned in parallel in the lengthwise direction of its fixing member (widthwise direction of recording medium), and is structured so that one or more of the magnetic cores can be moved away from its excitation coil according to the widthwise direction of the recording medium. With the employment of this structural arrangement, as a magnetic core is moved away from the excitation coil, the portion of the fixing member, which corresponds in position to the moved magnetic core, reduces in the amount of heat it generates. Thus, this structural arrangement can prevent the lengthwise end portions of the fixing member from excessively increasing in temperature. 
     However, in a case where a fixing device is structured so that its magnetic cores can be moved away from its excitation coil, the effect which the positional relationship among the three members of the fixing device, more specifically, the magnetic core, excitation coil, and fixing member, has upon the heat generation efficiency of the fixing member. Thus, if the positional relation among the abovementioned three components becomes deviant at one or more magnetic cores, the fixing member is likely to become nonuniform in temperature (heat generation) in terms of its lengthwise direction. In particular, the deviation in the position (distance) of the protrusion of each magnetic core relative to the fixing member has a serious effect upon the nonuniformity of the temperature distribution of the fixing member. 
     In the case of the apparatus disclosed in Japanese Laid-open Patent Application 2010-160388, however, regarding the shape of each magnetic core, the portion (arch portion) of each magnetic core, which is positioned so that it becomes roughly concentric with the outward surface of the wound portion of the excitation coil, and the portion (protrusion) of the magnetic core, which protrudes toward the center of the wound portion of the excitation coil, that is, toward the fixing member, are formed as integral parts of the magnetic core. 
     In the case of this kind of structural arrangement, the heat generation efficiency of the fixing member is significantly affected by the positional relationship among the magnetic core, excitation coil, and fixing member. Therefore, if the above described positional relationship among each magnetic core, excitation coil, and fixing member becomes incorrect, the fixing device is likely to becomes nonuniform in the temperature of its fixing member in terms of its lengthwise direction. In particular, the deviation in position (distance) of the protrusion of the magnetic core relative to the fixing member has a serious effect upon the nonuniformity of the temperature of the fixing member. 
     In this case, the arched portions of each core, and the protrusion of each core, are formed as integral parts of each magnetic core. Therefore, the magnetic core of this type is more likely to be formed wrong in shape (and/or measurement) than a magnetic core, the arched portion and protrusion of which are physically independent from each other. That is, in the case of the magnetic core of this type, the position of the protrusion of each magnetic core, which has a substantial effect upon the heat generation efficiency of the fixing member, is affected by the accuracy in measurement of the arched portion of the magnetic core. Therefore, the fixing device is likely to become nonuniform in the distance between the protrusion of each magnetic core and the fixing member, in terms of the lengthwise direction of the fixing device. 
     SUMMARY OF THE INVENTION 
     According to an aspect of the present invention, there is provided an image heating apparatus comprising a rotatable heating member configured to heat an image on a recording sheet; an excitation coil provided outside said rotatable heating member and configured to generate heat by electromagnetic induction in said rotatable heating member; a coil holder configured and positioned to hold said excitation coil; a plurality of magnetic cores arranged opposed to said rotatable heating member along a longitudinal direction of the rotatable heating member with said excitation coil interposed therebetween; a core holder configured and positioned to hold at least one of magnetic cores which is movable; and a moving mechanism configured to move said core holder between a first position and a second position which is more away from said rotatable heating member than the first position, wherein said core holder is provided with a stopper portion configured and positioned to stop movement of said core holder from the second position to the first position by abutment to said coil holder. 
     These and other objects, features, and advantages of the present invention will become more apparent upon consideration of the following description of the preferred embodiments of the present invention, taken in conjunction with the accompanying drawings. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a schematic sectional view of an example of an image forming apparatus equipped with the fixing device (electromagnetic induction heating device) in the first embodiment of the present invention. 
         FIG. 2  is a schematic sectional view of the fixing device (apparatus) in the first embodiment of the present invention. 
         FIG. 3  is a schematic front view of the fixing device in the first embodiment. 
         FIG. 4  is a schematic sectional view of the fixation belt of the fixing device in the first embodiment. 
         FIG. 5  is an exploded perspective view of the fixing device, in this embodiment, minus the portion which are not directly related to the present invention. 
         FIG. 6  is a schematic sectional view of the fixing device in the first embodiment. 
         FIG. 7  is an exploded perspective view of the movable magnetic core and core holder in the first embodiment. 
         FIG. 8  is an exploded sectional view of the movable magnetic core, core holder, and coil holder in the first embodiment. 
         FIG. 9  is a sectional view of the core holder and coil holder in the first embodiment, and shows the positional relationship between the core holder and coil holder, when the core holder is in the first and second positions. 
         FIG. 10  is a schematic side view of an example of a core moving mechanism. 
         FIG. 11  is an exploded perspective view of a modified version of the fixing device in the first embodiment, minus the portions which are not directly related to the present invention. 
         FIG. 12  is a perspective view of the electrically conductive member employed by the modified version of the fixing device in the first embodiment of the present invention. 
         FIG. 13  is a sectional view of the core holder and coil holder of the modified version of the fixing device in the first embodiment, and shows the positional relationship between the core holder and coil holder when the core holder is in the first and second position. 
         FIG. 14  is a schematic sectional view of the modified version of the fixing device in the first embodiment of the present invention. 
         FIG. 15  is a schematic sectional view of the modified version of the fixing device in the first embodiment of the present invention. 
         FIG. 16  is a drawing for describing the position of the movable magnetic core, and the temperature distribution of the fixation belt, in terms of the lengthwise direction of the fixing device. 
         FIG. 17  is a drawing for describing the position of the movable magnetic core, and the temperature distribution of the fixation belt, in terms of the lengthwise direction of the fixing device. 
         FIG. 18  is a block diagram of the control system of the image forming apparatus in the first embodiment, which is for controlling the portion of the apparatus, to which the present invention is related. 
         FIG. 19  is a flowchart for roughly describing the image forming operation of the image forming apparatus in this embodiment. 
         FIG. 20  is a perspective view of the fixing device in another embodiment of the present invention, minus the portions of the fixing device which are not directly related to the present invention. 
         FIG. 21  is a sectional view of the core holder and coil holder in another (second) embodiment of the present invention, and shows positional relation between the core holder and coil holder when the core holder is in the first and second positions. 
         FIG. 22  is a drawing for describing the relationship between the positioning of the magnetic core, and the temperature distribution of the fixation belt, in another (third) embodiment of the present invention. 
         FIG. 23  is a schematic sectional view of a referential fixing device. 
         FIG. 24  is an exploded perspective view of the referential fixing device, minus the portions of the device, which are not directly related to the present invention. 
         FIG. 25  is a drawing for describing the relationship between the positioning of the magnetic core, and the temperature distribution of the fixation belt, in terms of the lengthwise direction of a fixing device. 
     
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     Hereinafter, the image heating apparatuses in accordance with the present invention are described in detail with reference to the appended drawings. 
     [Embodiment 1] 
     1. Image Forming Apparatus 
     First, the general structure and operation of the image forming apparatus equipped with the image heating device in the first embodiment of the present invention is described.  FIG. 1  is a schematic sectional view of the image forming apparatus  1  equipped with a fixing device  100  as the image heating device in this embodiment. 
     The image forming apparatus  1  is a color image forming apparatus which uses an electrophotographic image forming method. The image forming apparatus  1  has multiple image forming stations, more specifically, the first, second, third, and fourth image formation stations  10 Y,  10 M,  10 C and  10 K, which form yellow (Y), magenta (M), cyan (C) and black (K) toner images, respectively. The four image formation stations  10 Y,  10 M,  10 C, and  10 K are vertically aligned in parallel, listing from the bottom side of the apparatus  1 . 
     The four image formation stations  10 Y,  10 M,  10 C and  10 K are practically the same in structure and operation, although they are different in the color of the toners they use. Therefore, in order to describe them together, the suffixes Y, C, M and K, which indicate the color of the toner they use, are omitted unless the four stations  10  need to be differentiated. 
     The image formation station  10  has a photosensitive drum  11 , which is an electrophotographic image bearing member (photosensitive member). The photosensitive drum  11  is rotationally driven in the direction (counterclockwise direction) indicated by an arrow mark in  FIG. 1 . There are provided the following means, more specifically, a charge roller  12  as a charging means which is the form of a roller, an exposing device  13  as an exposing means, a developing device  14  as a developing means, an intermediary transfer unit  20  as a transferring device, and a drum cleaning device  15 , which are positioned in the listed order in the adjacencies of the peripheral surface of the photosensitive drum  11 . 
     There are stored yellow, cyan, magenta and black toners, as developers, in the developing devices  14 Y,  14 M,  14 C and  14 K of the first, second, third, and fourth image formation stations  10 Y,  10 M,  10 C and  10 K, respectively. 
     The intermediary transfer unit  20  has an intermediary transfer belt  21 , as an intermediary transferring member, which is an endless belt made of film. The intermediary transfer belt  21  is suspended and kept stretched by three rollers, more specifically, a driving roller  22 , a belt backing roller  23 , and a tension roller  24 . The intermediary transfer belt  21  is circularly driven by the driving roller  22  in the direction (clockwise direction) indicated by an arrow mark in  FIG. 1 . There are positioned primary transfer rollers  25 , as primary transferring means (members), which are in the form of a roller, on the inward side of the loop which the intermediary transfer belt  21  forms, opposing the photosensitive drums  11 , one for one. Each primary transfer roller  21  is kept pressed against the corresponding photosensitive drum  11  with the presence of the intermediary transfer belt  21  between itself and photosensitive drum  11 , forming the primary transfer station N 1 , that is, the area of contact between the intermediary transfer belt  21  and photosensitive drum (in which intermediary transfer belt  21  is kept pressed upon photosensitive drum  11 ). There is also the secondary transfer roller  26  as the secondary transferring means (secondary transferring member), which is on the outward side of the abovementioned belt loop, opposing the aforementioned belt backing roller  23 . The secondary transfer roller  26  is kept pressed upon the portion of the intermediary transfer belt  21 , which is in contact with the peripheral surface of the belt backing roller  23  as if it partially wraps around the belt backing roller  23 . The area of contact between the secondary transfer roller  26  and intermediary transfer belt  21  is the secondary transfer station N 2 . There is also a belt cleaning device  27  as a means for cleaning the intermediary transfer belt  21 , which is positioned on the outward side of the loop which the intermediary transfer belt  21  forms, in such a manner that it opposes the belt backing roller  23 . 
     The exposing device  13  is structured as an optical system for projecting a beam of light upon the photosensitive drum  11  of each image formation station  10 , while modulating the beam of light according to the information of the image to be formed. The optical system in this embodiment is an optical system which scans the peripheral surface of the photosensitive drum  11  with a beam of laser light it outputs. 
     There is provided a recording medium feeding/conveying device  30  on the upstream side of the secondary transfer station N 2 , in terms of the direction in which a sheet P of recording medium is conveyed. 
     Next, the image forming operation of this image forming apparatus will be described with reference to its operation for forming a full-color image. First, the peripheral surface of the photosensitive drum  11  is roughly uniformly charged by the charge roller  12 , in each image formation station  10 . The charged photosensitive drum  11  is scanned (exposed) by the exposing device  13  based on the image data. Consequently, an electrostatic latent image (electrostatic image), which corresponds to the pattern of exposure of the peripheral surface of the photosensitive drum  11  is effected on the photosensitive drum  11 . The electrostatic latent image formed on the photosensitive drum  11  is developed into a toner image by the developing apparatus  14 , with the use of toner as developer. 
     On the photosensitive drums  11 Y,  11 M,  11 C and  11 K of the first, second, third, and fourth image formation stations  10 Y,  10 M,  10 C and  10 K, yellow, magenta, cyan and black toner images are formed, respectively. 
     The toner images, different in color, formed on photosensitive drums  11  in the image formation stations  10 , one for one, are sequentially transferred (primary transfer) onto the intermediary transfer belt  21 , in the primary image formations N 1 , one for one, in such a manner that they are aligned in layers in terms of the direction perpendicular to the surface of the intermediary transfer belt  21 . During this primary transfer, the intermediary transfer belt  21  is circularly moved in synchronism with the rotation of each photosensitive drum  11  at roughly the same speed as the photosensitive drum  11 . Also during this primary transfer, the primary transfer bias, the polarity of which is opposite from the normal polarity to which the toner for developing an electrostatic latent image is charged, is applied to each primary roller  25  from a primary bias power source (unshown). As a result, an unfixed full-color image is synthetically formed of unfixed toner images, different in color, on the intermediary transfer belt  21 . 
     In each image formation station  10 , the toner (residual toner from primary transfer) remaining on the photosensitive drum  11  after the primary transfer is removed from the peripheral surface of the photosensitive drum  11  by the drum cleaning device  15 , and then, is recovered by the cleaning device  15 . 
     As described above, during the formation of a full-color image, yellow, magenta, cyan and black toner images, are sequentially layered on the intermediary transfer belt  21  in synchronism with the circular movement of the intermediary transfer belt  21 , in such a manner that they are alignment in terms of the direction perpendicular to the circular movement of the intermediary transfer belt  21 . By the way, during the formation of a monochromatic image (monochromatic mode) of a specific color, a toner image is formed in only the image formation station  10 , which uses the toner of the specific color. Then, only the monochromatic toner image of the specific color is transferred (primary transfer) onto the intermediary transfer belt  21 . 
     Meanwhile, a sheets P of recording medium in a recording medium cassette  31  is separated, one by one, from the rest in the cassette  31 , by the sheet feeding/conveying roller  32  of the recording medium conveying device  30 , and then, is conveyed to the secondary transfer station N 2  by a pair of registration rollers  33  with a reset timing. 
     The toner images transferred onto the intermediary transfer belt  21  are transferred together (secondary transfer) onto the sheet P of recording medium, in the secondary transfer station N 2 . During the secondary transfer, the secondary transfer bias, the polarity of which is opposite to the normal polarity to which the toner is charged to develop an electrostatic latent image, is applied to the secondary transfer roller  26 . 
     The toner (residual toner from secondary transfer) remaining on the intermediary transfer belt  21  after the secondary transfer is removed from the intermediary transfer belt  21  by the belt cleaning device  27 , and is recovered by the device  27 . 
     The toner images, different in color, transferred (secondary transfer) onto the sheet P of recording medium are melted by the fixing device  100  while being mixed, and then, are fixed to the sheet P. The structure and operation of the fixing device  100  are described later in detail. 
     After the fixation of the toner images to the sheet P of recording medium, the sheet P is conveyed as a full-color print through the sheet discharge passage  41 , and then, is discharged into the delivery tray  42 . 
     2. General Structure and Operation of Fixing Device 
     Next, the general structure of the fixing device  100  as an image heating device is described. 
     By the way, in the following description of the fixing device  100 , the “lengthwise direction, widthwise direction, front surface, rear surface, left, right, upstream and downstream” means the following: The lengthwise direction (widthwise direction), is the direction which is roughly perpendicular to the direction in which the sheet P of recording medium is conveyed through the recording medium conveyance passage. The widthwise direction is the direction perpendicular to the above described lengthwise direction, that is, the direction which is roughly parallel to the direction in which the sheet P is conveyed through the recording medium conveyance passage. The front surface is the surface of the fixing device, which is on the side of the fixing device  100 , from which the sheet P is entered into the fixing device  100 . The rear surface is the opposite surface of the fixing device  100  from the front surface (surface which is on the side from which sheet P is outputted from the fixing device  100 ). The left side means the left side of the fixing device as seen from the front side of the fixing device. The right side means the right side of the fixing device as seen from the front side of the fixing device  100 . “Upstream” means the upstream in terms of the direction in which a sheet of recording medium is conveyed. “Downstream” means the downstream in terms of the recording medium conveyance direction. 
       FIG. 2  is a schematic sectional view of the essential portions of the fixing device  100  in this embodiment. 
     The fixing device  100  has: a fixation belt  101 , as a circularly movable heating member (fixing member), which is an endless belt; a pressure roller  102  as a rotatable member (pressure applying member); and an induction heating section  200  as an induction heating means (heat source). The fixation belt  101  has a metallic layer as an induction heat generating member, as will be described later. The pressure roller  102  is kept in contact with the outward surface of the fixation belt  101 . On the inward side of the fixation belt  101 , with reference to the loop which the fixation belt  101  forms, there is positioned a pressure applying member  104  which forms the fixation nip N by pressing the fixation belt  101  upon the pressure roller  102 . The pressure applying member  104  is held by a metallic stay  105 . Also on the inward side of the loop which the fixation belt  101  forms, there is a magnetism shielding core  106 , as a magnetism blocking member, for preventing the stay  105  from being increased in temperature by the heat generated therein by electromagnetic induction. The magnetism blocking core  106  is on the induction heating section  200  side of the stay  105 . 
     Also on the inward side of the loop which the fixation belt  101 , a temperature sensor  107  (temperature detection element) as a temperature detecting means is provided. As the temperature sensor  107 , a thermistor or the like is employed. In terms of the lengthwise direction of the fixing device  100 , the temperature sensor  107  is positioned at roughly the center of the fixation belt  101 , being kept in contact with the inward surface of the fixation belt  101 . The temperature sensor  107  is indirectly attached to the pressure applying member  104  with the placement of an elastic supporting member  107   a  between itself and the pressure applying member  104 . Thus, even if the surface of the fixation belt  101 , with which the temperature sensor  107  is kept in contact, is changed in position by the undulation, or the like movement of the fixation belt  101 , the temperature sensor  107  is made to follow the undulation or the like of the fixation belt  101 , by the elastic supporting member  107   a , being thereby enabled to remain satisfactorily in contact with the fixation belt  101 . 
       FIG. 3  is a schematic sectional view of the essential portions of the fixing device  100  in this embodiment. 
     The fixing device  100  is provided with left and right fixation flanges  108   a  and  108   b , which are positioned at the ends of the fixation belt  101  in terms of the lengthwise direction, one for one. The fixation flanges  108   a  and  108   b  are for regulating the fixation belt  101  in the movement of the belt  101  in the lengthwise direction and also, in the shape in terms of the circumferential direction. The stay  105  is positioned on the inward side of the loop which the fixation belt  101  forms, and is put through the left and right fixation flanges  108   a  and  108   b . There are stay pressing springs  110   a  and  110   b , which are compression springs as pressure applying means, being kept compressed between lengthwise ends portion  105   a  and  105   b  of the stay  105 , and the spring supporting members  109   a  and  109   b  of the chassis of the fixing device  100 , respectively. The stay pressing springs  110   a  and  110   b  generate the force which is for causing the stay  105  to press (press downward) the fixation belt  101  upon the pressure roller  102 . Therefore, the bottom surface of the pressure applying member  104  held by the stay  105 , and the top surface of the pressure roller  102  are pressed against each other, with the presence of the fixation belt  101  between the two surfaces, forming thereby the fixation nip N, which has a preset width in terms of the direction in which a sheet P of recording medium is conveyed. In this embodiment, the fixing device  100  is structured so that the left and right flanges  108   a  and  108   b  contact the metallic core  102   a  of the pressure roller  102  at the lengthwise ends of the metallic core  102   a,  one for one (unshown). Therefore, it is possible to prevent the elastic layer  102   b  of the pressure roller  102  and the fixation belt  101  from being permanently deformed. Further, the fixing device  100  has lateral supporting plates  111   a  and  111   b  for rotatably supporting the fixation belt  101 , with the presence of the fixation flanges  110   a  and  110   b  between themselves and fixation belt  101 . The fixation belt  101  is regulated in its position in terms of the lengthwise direction of the fixing device  100  by the lateral supporting plates  111   a  and  111   b , with the presence of the fixation flanges  110   a  and  110   b  between the lateral supporting plates  111   a  and  111   b  and the fixation belt  101 . The pressure roller  102  also is rotatably supported by the lateral supporting plates  111   a  and  111   b , by this metallic core  102   a , at the lengthwise ends. 
       FIG. 4  is a schematic sectional view of a part of the fixation belt  101 , and shows the laminar structure of the belt  101 . The fixation belt  101  has a metallic layer (electrically conductive layer)  101   a,  which is the base layer of the fixation belt  101 . As the metallic substances usable as the material for the metallic base layer  101   a , iron alloy, copper, silver, and the like can be preferably used. From the standpoint of reducing the fixation belt  101  in diameter (reducing fixation belt  101  in thermal capacity) or the like reason, the internal diameter of the metallic layer  101   a  is desired to be in a range of 20 mm-60 mm. In this embodiment, it is 60 mm. From the standpoint of thermal capacity, and the heat generation efficiency by magnetic flux, the thickness of the metallic layer  101   a  is desired to be set to a value in a range of 10 μm˜70 μm. In this embodiment, the thickness of the metallic layer  101   a  is 60 μm. The fixation belt  101  has an elastic layer  101   b , which is on the outward surface of the metallic layer  101   a . The elastic layer  101   b  is a rubber layer formed of heat resistant rubber. In consideration of the need for reducing the fixation belt  101  in thermal capacity to reduce the length of time (warm-up time) necessary for temperature increase, and also, for satisfactorily fixing a color image, the thickness of the elastic layer  101   b  is desired to be set to a value in a range of 100 μm-800 μm. In this embodiment, the thickness of the elastic layer  101   b  is 100 μm. There is a layer formed of fluorinated resin, as a separating layer  101   c  on the outward surface of the elastic layer  101   b.  As the fluorinated resin, PFA and TTFE, for example, is used. In this embodiment, from the standpoint of thermal conductivity and durability, the thickness of the separation layer  101   c  is desired to be set to a value in a range of 20 μm-200 μm. In this embodiment, the thickness of the separation layer  101   c  is 150 μm. In order to reduce the coefficient of friction between the inward surface of the fixation belt  101  and the temperature sensor  107 , there may be provided a highly lubricous layer  101   d , on the inward side of the metallic layer  101   a . The thickness of the lubricous layer  101   d  is desired to be set to a value in a range of 10 μm-50 μm. 
     The pressure roller  102  has: a metallic core  102   a;  a rubber layer as an elastic layer  102   b  which is on the outward surface of the metallic core  102   a ; a parting layer  102   c  as the surface layer which covers the outward surface of the elastic layer  102   b . In this embodiment, the external diameter of the metallic core  102   a  is 40 mm. The thickness of the elastic layer  102   b  is 20 mm. The thickness of the parting layer  102   c  is 150 μm. 
     The pressure applying member  104  is formed of heat resistant resin. The stay  105  is required to be rigid to apply pressure to the fixation nip N. In this embodiment, therefore, it is formed of iron. Further, the pressure applying member  104  is very close to the excitation coil  202  of the induction heating section  200 , which will be described later, in particular, at the ends in terms of the widthwise direction. Therefore, in order to shield the pressure applying member  104  from the magnetic field generated by the excitation coil  104  to prevent heat from being generated in the pressure applying member  104 , there is provided a magnetism blocking core  106  which extends across virtually the entire lengthwise range of the fixing device  100 , on the induction heating section  200  side of the pressure applying member  104 . 
     The base layer  101   a  of the fixation belt  101  is formed of a metallic substance. Therefore, what is necessary as the means for regulating the deviation of the fixation belt  101  in the lengthwise direction even while the fixation belt  101  is circularly moved, are nothing but the fixation flange  108   a  and  108   b  which simply catch the end portions of the fixation belt  101  in terms of the lengthwise direction. Therefore, the fixing device  100  can be simplified in structure. 
     The induction heating section  200  is positioned on the opposite side of the fixation belt  101  from the pressure roller  102 , in such a manner that it opposes the pressure roller  102 . The induction heating section  200  is positioned roughly in parallel to the lengthwise direction of the fixation belt  101 , with the presence of a preset amount of gap between itself and fixation belt  101 . The induction heating section  200  heats the fixation belt  101  by heating the metallic layer  101   a  of the fixation belt  101  as an induction heat generating member, by induction heating, from the outward side of the fixation belt  101 . The structure and operation of the induction heating section  200  are described later in detail. 
     Next, the fixation process of the fixing device  100  is described in general terms. 
     As electric power is supplied to the excitation coil  202  of the induction heating section  200 , from an electric power source  103  ( FIG. 18 ), which is under the control of the control section  50  ( FIG. 18 ), the temperature of the fixation belt  101  is increased to a preset level (fixation level), and is kept at the fixation level. The electric power source  103  has an excitation circuit (electromagnetic induction heating driving circuit: high frequency converter), an AC power source, and so on. With the temperature of the fixation belt  101  being kept at the preset fixation level, the sheet P of recording medium, on which an unfixed toner image T is borne, is introduced between the fixation belt  101  and pressure roller  102 , in the fixation nip N, while being guided by the guiding member (unshown), in such an attitude that the surface of the sheet P, on which the unfixed toner image T is present, faces the fixation belt  101 . Then, the sheet P is moved along with the fixation belt  101  through the fixation nip N, while remaining pinched between the fixation belt  101  and pressure roller  102 , being therefore airtightly pressed on the outward surface of the fixation belt  101 . Thus, heat is applied to the sheet P and the unfixed toner image T thereon, primarily from the fixation belt  101 . Further, the sheet P and the unfixed toner image T thereon are subjected to the pressure applied by the pressure roller  102 . Consequently, the unfixed toner image T is fixed to the surface of the sheet P. After being conveyed through the fixation nip N, the sheet P separates itself from the outward surface of the fixation belt  101 , because the fixation belt  101  is deformed at the exit portion of the fixation nip N. Then, the sheet P is conveyed out of the fixing device  100 . 
     Here, the measurement of the sheet P of recording medium in terms of the direction roughly perpendicular to the direction in which the sheet P is conveyed is referred to as the width of the sheet P. Regarding where a sheet P of recording medium is positioned relative to the fixing device  100  in terms of the lengthwise direction of the fixing device  100 , a sheet P of recording medium is positioned so that the center of the sheet P coincides with the center of the fixing device in terms of the lengthwise direction, regardless of the size of the sheet P. That is, the sheet P is conveyed in the so-called central alignment. A referential code “O” in  FIG. 3  stands for the central alignment line (theoretical line) of the fixing device  100 . A referential code “A” in  FIG. 3  stands for the width of the path of the largest sheet P of recording medium (which may be referred simply as largest sheet P of recording medium), in terms of width, which can be dealt with by the fixing device  100 . A referential code “B” in  FIG. 3  stands for the width of the path of any sheet P of recording medium (which may be referred to simply as small sheet P of recording medium), in terms of width, which is smaller than the largest sheet P of recording medium. A referential code “C” in  FIG. 3  stands for the areas which fall between the edge of the path of the largest sheet P of recording medium and the edge of the small sheet P of recording medium, that is, the areas which are not used for fixation when the small sheet P is used for image formation, or the out-of-sheet-path area. 
     The temperature sensor  107  detects the temperature of roughly the center (which corresponds to abovementioned central referential line O) of the inward surface of the fixation belt  101 , in terms of the lengthwise direction, and inputs the information of the detected temperature level into the control section  50  ( FIG. 18 ). That is, regardless of the width of a sheet P of recording medium used for image formation, the temperature sensor  107  contacts the inward surface of the portion of the fixation belt  101 , which corresponds in position to the path of the sheet P in use, and detect the temperature of the portion of the fixation belt  101 , which corresponds in position to the sheet P in use. The control section  50  controls the electric power to be inputted into the excitation coil  202  of the induction heating section  200  from the electric power source  103  ( FIG. 18 ), in such a manner that the temperature level detected by the temperature sensor  107  remains at the preset target level (fixation level). That is, as the temperature level detected by the temperature sensor  107  increases to the preset fixation level, the electric power which is being supplied to the excitation coil  202  is shut off. In this embodiment, the control section  50  changes, in frequency, the high frequency electric current applied to the excitation coil from the electric power source  103 , so that the temperature level detected by the temperature sensor  107  remains roughly stable at 180° C., which is the preset target level (fixation level). In other words, the control section  50  controls the temperature of the fixation belt  101  by controlling the electric power to be inputted into the excitation coil  202 . 
     At least while the image forming apparatus  1  is being used for image formation, the fixation belt  101  is rotated in the direction (clockwise direction) indicated by an arrow mark in  FIG. 2 , by the friction between the outward surface of the fixation belt  101  and the peripheral surface of the pressure roller  102 , which occurs as the pressure roller  102  is rotationally driven in the direction (counterclockwise direction) indicated by an arrow mark in  FIG. 2 . The pressure roller  102  is rotationally driven by a motor M 1  ( FIG. 18 ), as a driving means, which is controlled by the control section  50  ( FIG. 18 ). The fixation belt  101  is rotated at the peripheral velocity which is roughly the same as the speed at which the sheet P of recording medium, which is conveyed from the secondary transfer nip N 2  while bearing the unfixed toner image T. In this embodiment, the peripheral velocity of the fixation belt  101  is 200 mm/sec. Thus, it can process 50 sheets P of recording medium if the sheets P are of the size A4. Further, it can process 32 sheets P of recording medium, if the sheets P are of the size A4R. 
     3. General Structure of Induction Heating Section 
     Next, the general structure of the induction heating section  200  is described. 
       FIG. 5  is an exploded perspective view of the fixing device  100 , minus its portions which are not directly related to the present invention. The induction heating section  200  of the fixing device  100  is an induction heating means (heat source) which inductively heats the fixation belt  101  having the metallic layer  101   a  as an induction heat generating member. 
     The induction heating section  200  has a magnetic flux generating means  201  having the excitation coil  202  and magnetic core  203  ( 204 ,  205  and  206 ). It has also a coil holder (coil holding member)  207  which holds the excitation coil  202 . 
     The excitation coil  202  is formed by winding electric wire roughly in the form of an elliptic (shaped like bottom of boat), the long axis of which is parallel to the lengthwise direction of the fixing device  100 . Referring to  FIG. 5 , a referential code  202   b  stands for the center/top portion of the excitation coil  202 , and a referential code  202   a  stands for the wound portion of the excitation coil  202 . The overall shape of the excitation coil  202  is such that it is bent in curvature so that the wound portion  202   a  matches in contour a part of the outward surface of the fixation belt  101 . The excitation coil  202  is positioned so that it opposes a part of the outward surface of the fixation belt  101 . Further, the excitation coil  202  is positioned so that its lengthwise ends oppose the ends of the fixation belt  101  in terms of the lengthwise direction. As the electric wire for the excitation coil  202 , Litz wire, for example, is used. The excitation coil  202  is held to the coil holder  207  by being solidly attached to the coil holder  207 . 
     The magnetic core  203  has multiple external magnetic cores  204 , which are aligned in parallel in the lengthwise direction of the fixing device  100  with the presence (provision) of preset intervals. Each of the external magnetic cores  204  is in such a shape that it envelops the center portion of the wound portion of the excitation coil  202  (it is roughly in the shape of an arch). That is, the external magnetic core  204  has a portion (portion R), which coincides in position to the outward surface of the wound portion  202   a  of the excitation coil  202 . The external magnetic core  204  has a portion (protrusion), which protrudes toward the center portion  202   b  of the wound portion  202   a  of the excitation coil  202 , so that it will be in the adjacencies of the metallic layer (induction heat generating member)  101   a  of the fixation belt  101  after the assembly of the fixing device  100 . In this embodiment, the magnetic core  203  has  16  external magnetic cores  204 , which are aligned in parallel in the lengthwise direction of the fixing device  100 , with the placement of roughly the same intervals which are less across the center portion of fixing device  100 ), from one end of the fixing device  100  to the other (interval between central two cores  204  is less than the others). As will be described later in detail, the fixing device  100  is structured so that at least one of the multiple external magnetic cores  204  is changeable in position relative to the excitation coil  202 . Hereafter, the external magnetic core (cores) changeable in position relative to the excitation coil  202  (which will be referred to as movable magnetic core, hereafter) has the first core  241  (end core, arch-shaped core), which opposes the wound portion  202   a  of the excitation coil  202 , and the second core  242  (center core, T-shaped core), which has a protrusion  242   a  which protrudes toward the center portion  202   b  of the wound portion  202   a  of the excitation coil  202 . The method for holding the external magnetic cores  204  by the coil holder  207  is described later. 
     The magnetic core  203  has an upstream magnetic core  205 , which is on the upstream side of the excitation coil  202 , and extends in the lengthwise direction of the fixing device  100 . Further, the magnetic core  203  has a downstream magnetic core  206 , which is on the downstream side of the excitation coil  202  and extends in the lengthwise direction of the fixing device  100 . The upstream and downstream magnetic cores  205  and  206  are held by the coil holder  207  by being solidly attached to the coil holder  207 . 
     The above-described external magnetic core  204  covers the excitation coil  202  in such a manner that the upstream and downstream magnetic cores  205  and  206  make it practically impossible for the magnetic field generated by the excitation coil  202  to leak, except toward the metallic layer  101   a  (induction heat generating member) of the fixation belt  101 . Further, the magnetic core  203  plays the role of efficiently guide the alternating magnetic flux generated by the excitation coil  202 , to the metallic layer (induction heat generating member)  101   a  of the fixation belt  101 . In other words, the magnetic core  203  plays both the role of increasing the magnetic circuit (magnetic path) in efficiency, and the role of blocking the magnetism. As the material for the magnetic core  203 , ferrite or the like, which is high in permeability, and low is residual magnetic flux density, is desired. 
     In this embodiment, each of the upstream and downstream magnetic cores  205  and  206  is a single (one-piece) member which extends in the lengthwise direction of the fixing device  100 . However, each of the upstream and downstream magnetic cores  205  and  206  may be made up of multiple sub-cores aligned in the lengthwise direction of the fixing device  100 . 
     The coil holder  207  is formed of dielectric resin. It is in the form of a topless rectangular box. It is positioned so that its lengthwise direction is parallel to the lengthwise direction of the fixing device  100 , and also, so that its bottom portion  207  faces the fixation belt  101 . The bottom portion  271  has a curved (arch-like) portion  271   a , which matches in curvature the portion of the fixation belt  101 , which corresponds in position to the magnetic blocking core  106 . Further, the opposite side of the coil holder  207  from the bottom portion  271  is open as an opening  272 . In this embodiment, the coil holder  207  is positioned above the fixation belt  101  in such a manner that it opposes the fixation belt  101 , with the presence of a preset amount of gap between the arch-like portion  271   a  of the bottom portion  271 , and the outward surface of the fixation belt  101 . 
     When the fixation belt  101  is being rotated, high frequency electric current which is 20 kHz-50 kHz in frequency is applied to the excitation coil  202  of the induction heating section  200 , from the electric power source  103  ( FIG. 18 ). Thus, heat is inductively generated in the metallic layer  101   a , as an induction heat generating member, of the fixation belt  101  by the magnetic field generated by the excitation coil  202 , increasing thereby the fixation belt  101  in temperature. That is, the excitation coil  202  is made to generate an alternating magnetic flux by the alternating current supplied by the electric power source  103 . This alternating magnetic flux is guided by the magnetic core  203  of the induction heating section  200 , and acts on the metallic layer (induction heat generating member)  101   a  of he fixation belt  101 , generating eddy current in the metallic layer  101   a . This eddy current generates Joule&#39;s heat in the metallic layer (induction heat generating member)  101   a  of the fixation belt  101 , by the amount proportional to the specific resistance of the material of the metallic layer  101   a . As described above, the metallic layer (induction heat generating member)  101   a  of the fixation belt  101  is made to generate heat in itself by electromagnetic induction; it is made to generate heat by the function of the magnetic flux by supplying the excitation coil  202  with alternating electric current. 
     In this embodiment, the fixation belt  101  and the excitation coil  202  of the induction heating section  200  are kept electrically insulated from each other by the coil holder  207  which is roughly 2 mm in thickness. The excitation coil  202  is held by the coil holder  207  in such a manner that a roughly preset amount of distance is maintained between the fixation belt  101  and excitation coil  202  to ensure that the fixation belt  101  is roughly uniform in the amount of heat generation. In this embodiment, the temperature of the fixation belt  101  is controlled so that the temperature of the fixation belt  101  detected by the temperature sensor  107  remains roughly stable at 180° C., which is the preset target level (fixation level), as described above. In this embodiment, the induction heating section  200 , which includes the excitation coil  202 , is positioned outside the loop which the fixation belt  101  forms, instead of the inside of the loop, which becomes higher in temperature than the outside of the loop. Therefore, the excitation coil  202  is unlikely to become excessively high in temperature, being therefore unlikely to increase in electrical resistance. Therefore, it is likely to be less in the amount of loss in terms of Joule&#39;s heat which occurs as high frequency electric current is flowed. Further, the positioning of the excitation coil  202  outside the loop, which the fixation belt  101  forms, contribute to reduce the fixation belt  101  in diameter (reduction in thermal capacity). Thus, the fixing device  100  is excellent in terms of energy conservation. 
     4. External Magnetic Core 
     Next, the structure of the external magnetic core is described further. As described above, in this embodiment, the multiple external magnetic cores  204  are aligned in parallel in the lengthwise direction (which is roughly perpendicular to conveyance direction of sheet P), with the presence of roughly equal intervals. Each external magnetic core  204  is structured in the form of such an arch that appears as if it surrounds the center of the excitation coil  202  and the adjacencies of the center (roughly in the form of arch). To describe in detail, each external magnetic core  204  has a pair of first cores (end cores, arch-shaped cores)  241 , and a single second core (center core, T-shaped core) (hereafter, group of multiple external magnetic cores may be collectively referred to simply as external magnetic core). 
     In this embodiment, the external magnetic cores, which are within a preset range (movable core range) E (in which external magnetic cores are movable), that is, the lengthwise end ranges of the fixing device  100 , are movable magnetic cores, which can be changed in their position relative to the excitation coil  202 . In particular, in this embodiment, the five external magnetic cores  204 , which are on one of the lengthwise end sides of the fixing device  100 , and the five external magnetic cores  20 , which are on the other lengthwise end side of the fixing device  100 , are the movable magnetic cores. These movable external magnetic cores  204  are movable by a core moving mechanism as a core moving means, as will be described later in detail. 
     Also in this embodiment, the external magnetic cores  204  which are positioned in a preset range (stationary core range) D (in which external magnetic cores are not movable), which corresponds in position to the center portion of the fixing device  100 , are solidly attached to the coil holder  207 , as will be described later in detail. 
     The dimension of the stationary core range D in terms of lengthwise direction equals to the width of a small sheet P of recording medium. The dimension of the combination of the stationary core range D and movable core ranges E equals to the width of the large sheet P of recording medium. That is, in this embodiment, the range D corresponds the range B in  FIG. 3 , and the range E corresponds to the range C in  FIG. 3 . Further, the combination of the range D and ranges E corresponds to the range A in  FIG. 3 . 
     Although the following will be described later in detail, referring to  FIG. 6 , the external magnetic cores  204  positioned in the ranges E are enabled to be moved in the direction to move away from the excitation coil  202  so that in the ranges E, the distance between the excitation coil  202  and external magnetic core  204  can be increased. As the distance between the excitation coil  202  and external magnetic core  204  is increased, the magnetic circuit created in the adjacencies of the excitation coil  202  by the magnetic core  203 , metallic layer (induction heat generating member)  101   a  of the fixation belt  101 , and so on, is reduced in efficiency, and therefore, the fixation belt  101  reduces in the amount of heat it generates. Therefore, the out-of-sheet path portions of the fixation belt  101  are prevented from excessively increasing in temperature. Therefore, the magnetic core  203 , excitation coil  202 , and so on, are prevented from abnormally increasing in temperature.  FIG. 6  is a schematic sectional view of the essential portions of the fixation belt  101  in this embodiment. It shows the state of the essential portions when the external magnetic cores  204  positioned in the ranges E have been moved away from the excitation coil  202 . 
     Here, referring to  FIGS. 23-25 , the problems which a fixing device structured so that its magnetic core is moved to reduce its induction heat generating member in the amount of heat generation it generates are described. 
       FIG. 23  is a schematic sectional view of the essential portions of the referential fixing device.  FIG. 24  is an exploded perspective view of the referential fixing device, minus the portions of the apparatus, which are not directly related to the present invention. In the following description of the referential fixing device, the components of the fixing device, their portions, and so on, which correspond in function and structure to the counterparts of the fixing device  100  in this embodiment are given the same referential codes as those given to the counterparts. 
     Basically, the referential fixing device  100  is similar in structure to the fixing device  100  in this embodiment. In the case of the referential fixing device  100 , the magnetic core  203  (which corresponds to group of external magnetic cores  204  in this embodiment) is a one-piece core, unlike the magnetic core  203  in this embodiment, which is made up of multiple external magnetic cores  204  aligned with the presence of preset intervals. That is, in the case of the referential fixing device  100 , the magnetic core  203  has a portion  203 R which is concentric with the outward surface of the wound portion  202   a  of the excitation coil  202 . Further, this magnetic core  203  has a protrusion  203 T which protrudes toward the center portion  202   b  of the wound portion  202   a  of the excitation coil  202 , and is in the adjacencies of the metallic layer (induction heat generating member)  101   a  of the fixation belt  101 . Further, the R-portion  203 R and protrusion  203 T are integral parts of the magnetic core  203 . Further, the referential fixing device  100  is not provided with such cores that correspond to the upstream and downstream magnetic cores  205  and  206  with which the fixing device  100  in this embodiment are provided. However, the description of the referential fixing device  100 , which will be given next, holds true even if the referential fixing device  100  is provided with the upstream and downstream magnetic cores  205  and  206 . 
     From the standpoint of increasing the fixing device  100  in the efficiency with which heat is generated in the metallic layer (induction heat generating member)  101   a  of the fixation belt  101 , by the magnetic flux generating means  201  of the induction heating section  200 , it is effective to place the magnetic core  203  as close as possible to the excitation coil  202  of the magnetic flux generating means  201 . It is also effective to reduce the distance between the protrusion  203 T of the magnetic core  203  and the metallic layer (induction heat generating member)  101   a  of the fixation belt  101 . 
     However, if the distance between the excitation coil  202  and magnetic core  203  is reduced, the heat generation efficiency of the induction heating section  200  becomes excessively sensitive to the positional relationship among the magnetic core  203 , excitation coil  202 , and metallic layer (induction heat generating member)  101   a  of the fixation belt  101 . 
       FIG. 25  is a drawing for describing the relationship between the position of the magnetic core  203  and the temperature distribution (heat generation distribution) of the fixation belt  101  in terms of the lengthwise direction.  FIG. 25(   a ) shows the relationship between the desirable temperature distribution and the position of the magnetic core  203 , and  FIG. 25(   b ) shows the relationship between the problematic temperature distribution and the position of the magnetic core  203 . 
     If the multiple magnetic cores  203  aligned in parallel in the lengthwise direction of the fixing device  100  become different in their positional relationship relative to the excitation coil  202  and fixation belt  101 , the fixation belt  101  is likely to become nonuniform in temperature. In particular, the positional deviation of the protrusion  203 T of the magnetic core  203  relative to the metallic layer (induction heat generating member)  101   a  of the fixation belt  101  (in height direction of drawing) has notable effect upon the nonuniformity of the fixation belt  101  in terms of the temperature distribution in the lengthwise direction. 
     From the standpoint of making the fixation belt  101  uniform in temperature in the lengthwise direction, it is desired that the following measure is taken. That is, referring to  FIG. 25(   a ), the multiple magnetic cores  203  aligned in parallel in the lengthwise direction of the fixing device  100  are individually controlled in terms of their positional relationship relative to the excitation coil  202  and fixation belt  101 . More specifically, it is desired that the magnetic cores  203  are rendered the same in the position (in height direction in drawing) of their protrusion  203 T relative to the metallic layer (induction heat generating member)  101   a , in terms of the lengthwise direction. It is also desired that the R portion  203 R, and the outward surface of the excitation coil  202 , which the R portion  203 R opposes, are concentric. 
     In the case of the above-described referential fixing device  100 , the magnetic cores  203  positioned in the movable core ranges E are movable to the first positions which are their closest positions to the excitation coil  202 , and the second positions which are their farthest positions from the excitation coil  202 . In this case, in order to keep the fixation belt  101  roughly uniform in temperature in terms of the lengthwise direction, it is possible to place the magnetic cores  203  in contact with the coil holder  207  to accurately position the magnetic cores  203 . 
     However, if an attempt is made to place the magnetic cores  203  in contact with the coil holder  207 , and move the magnetic cores  203  from their second positions to their first positions, it is possible that the magnetic cores  203  will be damaged by the impact which occurs as the magnetic cores  203  are placed in contact with the coil holder  207 . In particular, in the case of the referential fixing device  100 , the R portion  203 R and protrusion  203 T of the magnetic core, which are integral parts of the magnetic core  203 , the impact caused by the movement of the magnetic core  203  is entirely caught by the protrusion  203 T. Therefore, it is very likely for the magnetic core  203  (protrusion  203 T) to be damaged. Generally, the magnetic core  203  is made of ferrite or the like, by sintering ferrite powder or the like,. Thus, it is relatively susceptible to impact. 
     5. Core Holder 
     Next, referring to  FIGS. 7-9 , the core holder (core holding member)  208  for holding the movable magnetic core  204 , that is, the external magnetic cores  204  positioned in the movable core ranges E, is described. 
       FIG. 7  is an exploded perspective view of the external magnetic cores  204  and core holder  208  positioned in the movable core ranges E.  FIG. 8  is an exploded sectional view of the external magnetic cores  204 , core holder  208 , and coil holder  207 , which are positioned in the ranges E.  FIGS. 9(   a ) and  9 ( b ) are schematic sectional views of the core holder  208  and coil holder  207 , when the core holder  208  is in the first and second positions, respectively, which will be described later. 
     In order to deal with the above described problem, in this embodiment, such a structural arrangement that at least the movable magnetic cores  204 , that is, the magnetic cores  204  positioned in the movable core range E, are separated as follows. That is, the first core  241  which opposes the wound portion  202   a  of the excitation coil  202 , and the second core  242  which has the protrusion  242   a  which protrudes toward the center portion  202   b  of the excitation coil  202 , are made physically independent from each other. The movable magnetic cores, that is, the external magnetic cores  204  positioned in the movable core range E, is held by the core holder  208 . The core holder  208  enables the first and second cores  241  and  242  it holds, to be moved to the first position which is relatively close to the excitation coil  202 , and the second position which is farther from the excitation coil  202  than the first position. Further, as the core holder  208  moves from the second position to the first position, the core holder  208  comes into contact with the first area  273  of contact of the coil holder  207 , being enabled to accurately position the first core  241  held by the core holder  208 , relative to the excitation coil  202 . Further, as the core holder  208  is moved from the second position to the first position, the second core  242  of the external magnetic core  204  held by the core holder  208  comes into contact with the second area  274  of contact of the coil holder  207 , being thereby accurately positioned relative to the excitation coil  202 . In this embodiment, in order to ensure that the first area  273  of contact moves through the center (hole) of the excitation coil  202 , it is formed as the tip of the protrusion  275  which protrudes from the bottom portion  271  of the coil holder  207  in the opposite direction from the fixation belt  101 . Further, in this embodiment, the second area  274  of contact is a part of the bottom portion  271  of the coil holder  207 , which opposes the center of the wound portion of the excitation coil  202 . Further, in this embodiment, the tip  242   b  of the protrusion  242   a  of the second core  242  comes into contact with the second area  274  of contact of the coil holder  207 . Next, the abovementioned described structural arrangements are described in detail. 
     In this embodiment, the combination of the external magnetic cores  204  has a preset width in terms of the lengthwise direction of the fixing device  100 , and is shaped so that it appears roughly arched in cross section at a plane which is roughly perpendicular to the lengthwise direction of the fixing device  100 . The first cores  241  and  241  of the external magnetic core  204  extend from their base portion s  241   a  and  241   a  which are the lengthwise ends of the aforementioned roughly arched figures, toward the top portions  241   b  and  241   b , which correspond to the peak of the abovementioned roughly arched figure, in a curvature that is the same as the curvature of the excitation coil  202 . More specifically, preset ranges of the base portions  241   a  and  241   a  of the first cores  241  and  241  are areas  241   e  and  241   e  of engagement, which are roughly flat and extend toward the excitation coil  202 . The portion of the first core  241 , which is between the area  241   e  of engagement and the top portion  241   b  is the arched portion  241   f.  Further, the surface of each first core  241 , which faces the excitation coil  202  is provided with the first step-shaped portion  241   c , which is between the area  241   e  of engagement and arched portion  241   f.  Further, the surface of the each first core  214 , which faces the external magnetic core  204 , is provided with the second step-like portion  241   d , which is adjacent to the top portion  241   b.    
     The second core  242  of the external magnetic core  204  has a base portion  242   c , which is roughly flat and makes up the top portion of the abovementioned roughly arched figure, and a protrusion  242   c  which protrudes toward the center portion of the wound portion  202   a  of the excitation coil  202 , from the base portion  242   c . That is, the second core  242  is roughly T-shaped in cross section which is roughly perpendicular to the lengthwise direction of the fixing device  100 . 
     Referring to  FIGS. 7 and 9 , the external magnetic core  204  shaped as described above is attached to the core holder  208 . By the way,  FIG. 7  shows one of the core holders  208  aligned in parallel in the lengthwise direction of the fixing device  100 . Each core holder  208  holds three of the external magnetic cores  204  aligned in parallel in the lengthwise direction of the fixing device  100 . However, the fixing device  100  may be structured so that each core holder  208  holds only one external magnetic core  204 , and/or two or more external magnetic cores  204  are held by a single holder  208 . Hereafter, the core holder  208  is described, with special attention being paid to the portion of the core holder  208 , which holds one of the three external magnetic cores  204  which each core holder  208  can hold. 
     The core holder  208  is structured like a frame. Each section of the core holder  208 , which holds one external magnetic core, has a pair of long lateral portions  281  and  281 , which oppose each other and extend in the widthwise direction of the fixing device  100 , and short lateral portions  282  and  282 , which oppose each other and extend in the lengthwise direction of the fixing device  100 . In this embodiment, the end portion of the long lateral portions  281 , which is on the excitation coil  202  side, is provided with a void for accommodating the arched portion  272   a  of the bottom portion  271  of the coil holder  207 . Each of the pair of short lateral portions  282  and  282  is roughly in the form of a rectangle. The core holder  208  is also provided with first and second bridge beams  283  and  283 , and second bridge beams  284  and  284 , which bridge between the pair of the long lateral portions  281  and  281 . In terms of the widthwise direction of the fixing device  100 , the first bridge beams  283  and  283  are positioned roughly at the center between the pair of long lateral portions  281  and  281 , with the presence of a preset gap (d 2 ) between the first bridge beams  283  and  283 . The second bridge beams  284  and  284  have the core supporting first portion  284   a  and  284   a , which protrude from the excitation coil  202  side end of the second bridge beams  284  and  284 , toward the short lateral portion  282 . Further, the excitation coil side of the second bridge beams  284  and  284  have the core supporting second portion  284   b    284   b , which protrude in the opposite direction from the direction in which the coil supporting first portion  284   a  and  284   a . Further, the second bridge beams  284  and  284  have portions  284   c  and  284   c  of engagement which extend from the end portions which have the core supporting second portion  284   b  and  284   b . The core holder  208  is formed of a dielectric resin. 
     In this embodiment, referring to  FIG. 8 , the first core  241  and  241  are attached to the core holder  208 , as if it is dropped into the core holder  208  from the top side of the core holder (opposite side from excitation coil  202 ), as indicated by an arrow mark G. That is, the engagement portions  241   e  and  241   e , with which the base portions  241   a  and  241   a  of the first cores  241  and  241  are provided, one for one, are fitted into the gap (groove)  285  and  285  provided between the short lateral portions  281  and  281  and first bridge beams  283  and  283  of the core holder  208 , one for one. The amount d 1  of the gap  285 , and the thickness d 3  of the engaging portion  241   e , are roughly the same. The first step-like portion  241   c  of each first core  241  is placed in contact with the top surface of the bridge beam  283  with which the core holder  208  is provided. In this state, each first step-like portion is solidly attached, as solidly attaching means, to the bridge beam  283  by thermal welding. Further, the second step-like portions  241   d  of each first core  241  is placed in contact with the top surface of the core supporting first portion  284 , with which the second bridge beam  284  of the core holder  208  is provided, In this state, the second step-like portion  241   d  is solidly attached to the second bridge beam  284  by thermal welding as a means for solidly attaching the second step-like portion  241   d . Through above described steps, the pair of second bridge beams  214   d  and  214   d  are solidly attached to the core holder  208 , being thereby held by the core holder  208 . The choice of the solidly attaching means is optional. That is, welding, gluing, binding, snap-fitting, or the like may be used as the solidly attaching means. 
     Referring to  FIG. 8 , in this embodiment, the second core  242  also is attached to the core holder  208  as if it is dropped into the core holder  208 , from above (from opposite side from excitation coil  202 ), as indicated by the arrow mark G. That is, the second core  242  is inserted into the gap  284   d  between the second bridge beams  284  and  284  of the core holder  208 , from the tip  242   b  side of its protrusion  242   a . The gap d 2  between the pair of engagement portions  284   c  and  284   c  is made slightly larger than the distance d 4  between the pair of engagement portions  284   c  and  284   c,  providing thereby a certain amount of play for allowing the base portion  242   c  to easily move. On the other hand, the gap between the second core supporting portions  284   b  and  284   b  of the second bridge beams  284  and  284  is made smaller than the width of the base portion  242   c  of the second core  242 , in the same direction. Therefore, the second core  242  can be suspended by the second bridge beams  284  and  284 , with the bottom surface of the base portion  242   c  being in contact with the top surface of the second core supporting portions  284   b  and  284   b  of the second bridge beams  284  and  284  of the core holder  208 . The second core  242  is not solidly attached to the core holder  208 , being therefore allowed to freely move in the direction indicated by the arrow mark G, and also, in the opposite direction from the direction indicated by the arrow mark G, while remaining in the space (gap)  284   d  between the second bridge beams  284  and  284 . 
     In this embodiment, the core holder  208  which is holding the external magnetic core  204  attached to the core holder  208  as described above is placed in the coil holder  207  from the top side (opposite side from excitation coil  202 ) in the direction indicated by the arrow mark G in  FIG. 8 , as if it is dropped into the coil holder  207 . As will be described later, the core holder  208  can be slid into the coil holder  207  by the core moving mechanism, as core moving means, in the direction indicated by the arrow mark G in  FIG. 8 , or the opposite direction from the direction indicated by the arrow mark G, and can be positioned in the first position ( FIG. 9(   a )), and the second position ( FIG. 9(   b )). As described above, the coil holder  207  is shaped like a rectangular box without a lid, the lengthwise direction of which is parallel to the lengthwise direction of the fixing device  100 . The bottom portion  271  of the coil holder  207  has a semi-cylindrical recess, which arches inward of the coil holder  207 , in such a curvature that matches the curvature of the outward surface of the fixation belt  101 . The opposite side of the coil holder  207  from the bottom portion  271  is open, as an opening  272 . The distance between the pair of long lateral walls  276  and  276  of the coil holder  207 , which extend in the lengthwise direction of the fixing device  100 , is made slightly greater than the distance between the widthwise portions  282  and  282  of the core holder  208 , providing thereby a gap (play), between the core holder  208  and coil holder  207 , which is sufficient to allow the core holder  208  to move within the coil holder  207 . 
     The excitation coil  202  is solidly attached to the coil holder  207 , being thereby held by the coil holder  207 , in such a manner that the contour of the bottom side of the excitation coil  202  matches the contour of the semi-cylindrical portion  271   a  of the bottom portion  271  of the coil holder  207 , with the presence of virtually no gap between the excitation coil  202  and cylindrical portion  271   a . The coil holder  207  has the protrusion  275  which protrudes from the arched portion  271   a  of the bottom portion  271  in the opposite direction from the fixation belt  101  so that it penetrates through the center portion (hole)  202   b  of the wound portion  202   a  of the excitation coil  202 . The opposite end of the protrusion  275  from the fixation belt  101  protrudes beyond the outward surface of the wound portion  202   a  of the excitation coil  202 . This end of the protrusion  275  is the first area  273  of contact, with which the core holder  208  comes into contact. This first area  273  of contact comes into contact with the catching portions  286  and  286 , which are the bottom surface of the second core supporting portions  284   b  and  284   b  of the second bridge beams  284  and  284  of the core holder  208 . Further, the arch portion  271   a  of the bottom portion  271 , which is on the inward side of the protrusion  275  (which corresponds in position to the center portion  202   b  of excitation coil  202 ) is the second area  274  of contact, which comes into contact with the second core  242  of the external magnetic core  204 . This second area  274  of contact comes into contact with the tip of the protrusion  242   a  of the second core  242  of the external magnetic core  204 . Further, it is on the flat portions  271   b  and  271   b , which are roughly flat portions of the bottom portion  271 , which are between the long lateral walls  276  and  276  and bottom portion  271 , that the upstream and downstream magnetic cores  205  and  206  are positioned. The upstream and downstream magnetic cores  205  and  206  are solidly attached to the coil holder  207  by thermal welding as solidly attaching means. 
     Referring to  FIG. 5 , in this embodiment, the stationary external magnetic core  204 , that is, the magnetic core positioned in the stationary core range D, is also made up of the first and second cores  241  and  242 , which are physically separated from each other. That is, in this embodiment, all the external magnetic cores  204  are practically the same in structure, dimension, and shape. However, the stationary external magnetic core  204  positioned in the stationary core range D is solidly attached to the coil holder  207 . In this embodiment, the external magnetic core  204 , which is a stationary magnetic core positioned in the range D, is solidly attached to the coil holder  207 , with the placement of a stationary core holder between the external magnetic core  204  and coil holder  207 . This stationary core holder is similar in structure to the above described core holder in terms of the method for holding the external magnetic core  204 . However, it is solidly attached to the core holder  208  so that it cannot be moved out of the first position. By the way, the stationary external magnetic core  204  positioned in the range D may be directly and solidly attached to the coil holder  207 . 
     Next, referring to  FIGS. 9(   a ) and  9 ( b ), the positional relationship among the first and second cores  241  and  242  of the external magnetic core  204 , and the excitation coil  202 , is described. 
     Referring to  FIG. 9(   a ), when the core holder  208  is in the first position, the catching portions  286  and  286  with which the second bridge beams  284  and  284  of the core holder  208  are provided are in contact with the first areas  273  and  273  of the protrusions  275  and  275  of the coil holder  207 . Therefore, the first core  241  of the external magnetic core  204  held by the core holder  208  is roughly concentrically positioned with the outward surface of the wound portion  202   a  of the excitation coil  202 , with the presence of the core holder  208  and coil holder  207  between the external magnetic core  204  and excitation coil  202 . 
     Here, referring to  FIG. 5 , the distance between the first area  273  of contact and the metallic layer (induction heat generating member)  101   a  of the fixation belt  101 , is roughly uniform across the entire range of the coil holder  207  which extends in the lengthwise direction of the fixing device  100 . Further, the core holders  208  by which the multiple external magnetic cores  204  positioned in the movable core ranges E are held, one for one, are practically the same in structure. Therefore, all the first cores  241  of the multiple external magnetic cores  204  positioned in the movable core ranges E are roughly concentrically positioned with the outward surface of the wound portion  202   a  of the excitation coil  202 . Further, as described above, in this embodiment, the first core  241  of each of the multiple external magnetic cores  204  positioned in the range D is attached to the coil holder  207  with the placement of a stationary core holder which is similar in structure to the above described core holder  208 , between the first core  241  and coil holder  207 . Therefore, the first core  241  of each of the multiple external magnetic cores  204  aligned in parallel in the lengthwise direction of the fixing device  100 , in the ranges E and D is roughly concentrically positioned with the outward surface of the wound portion  202   a  of the excitation coil  202 . 
     On the other hand, when the core holder  208  is in the first position, there is a gap (space)  287  between the bottom surface of the base portion  242   c  of the second core of the external magnetic core  204 , and the top surface of the second core supporting portions  284   b  and  284   b  of the core holder  208 , in terms of the direction indicated by the arrow mark G in  FIG. 9(   a ). Further, the tip  242   b  of the protrusion  242   a  is in contact with the second area  274  of contact, with which the coil holder  207 , keeping the second core  242  in the state shown in  FIG. 9(   a ). Further, the protrusion  242   a  of the second core  242  is the center portion (hole)  202   b  of the wound portion  202   a  of the excitation coil  202 . Further, the base portion  242   c  of the second core  242  makes up a part of the arch which the curved portion  241   f  of the first core  241  forms with the base portion  242   c , and which is roughly the same in curvature as the excitation coil  202 . 
     Here, referring to  FIG. 5 , there is a certain amount of distance between the second area  274  of contact and the metallic layer (induction heat generating member)  101   a  of the fixation belt  101 , which is roughly uniform across the entire range of the coil holder  207  in terms of the lengthwise direction of the fixing device  100 . Therefore, the second core  242  of each of the multiple external magnetic cores  204  positioned in parallel in the lengthwise direction of the fixing device  100 , along the second area  274  of contact of the coil holder  207 , in the movable core range E is positioned roughly the same distance from the metallic layer (induction heat generating member)  101   a  of the fixing device  100  as the other second cores  242 . Further, as described above, in this embodiment, the multiple external magnetic cores  204  positioned in parallel in the lengthwise direction of the fixing device  100 , in the stationary core range D, are solidly attached to the coil holder  207 , with the placement of the stationary core holder which is similar in structure as the above described core holder  208 , between the external magnetic cores  204  and the coil holder  207 . Therefore, the second core  242  of each of the multiple external magnetic cores  204  positioned in parallel in the lengthwise direction of the fixing device  100 , along the second area  274  of contact of the coil holder  207 , in the ranges E and D is positioned roughly the same distance from the metallic layer (induction heat generating member)  101   a  of the fixing device  100  as the other second cores  242 . 
     As described above, this embodiment of the present invention makes it possible to make roughly uniform the positional relationship among the first and second cores  241  and  242  of the multiple external magnetic cores  204 , excitation coil  202 , and the metallic layer (induction heat generating member)  101   a  of the fixation belt  101 , in terms of the lengthwise direction of the fixing device  100 . Therefore, it can make the fixation belt  101  of the fixing device  100  roughly uniform in temperature, in terms of the lengthwise direction of the fixing device  100 . 
     Further, referring to  FIG. 9(   a ), when the core holder  208  is in the second position, the catching portions  286  and  286 , with which the second bridge beams  284  and  284  are provided are separated from the first area of contact with which the protrusion  275  of the coil holder  207  is provided. Further, the first core  241  of the external magnetic cores  204  held by the core holder  208  are positioned farther from the excitation coil  202  than when the core holder  208  is in the first position. Also when the core holder  208  is in the second position, the second core  242  of the external magnetic core  204  is suspended by the second core supporting portion (holding portions)  284   b  and  284   b , with which the second bridge beams  284  and  284  of the core holder  208 , with the bottom surface of the base portion  242   c  of the second core  242  being in engagement with the top surface of the second core supporting portions (holding portions)  284   b  and  284   b . Thus, the second core  242  held by the core holder  208  as described above is positioned farther away from the excitation coil  202  than when the core holder  208  is in the first position. Also when the core holder  208  is in the second position, the protrusion  242   a  of the second core  242  is outside the center portion (hole)  202   b  of the wound portion  202   a  of the excitation coil  202 . 
     As described above, in this embodiment, the external magnetic cores  204 , which are the movable magnetic cores positioned in the movable core ranges E, are movable to their closest positions to the excitation coil  202 , and their farthest positions from the excitation coil  202 . When the core holder  208  is in the first position, the external magnetic cores  204 , which are movable magnetic cores, are positioned in their closest position to the excitation coil  202 . Further, the theoretical circle, which coincides with the first core  241 , is concentric with the theoretical circle which coincides with the outward surface of the wound portion  202   a  of the excitation coil  202 , and also, the protrusions  242   a  of the second cores  242  are positioned in the center portion (hole) of the excitation coil  202 . Also when the core holder  208  is in the second position, the external magnetic cores  204  which are movable magnetic cores are positioned in their farthest position from the excitation coil  202 . Further, as the core holder  208  is moved into the second position, the first core  241  is moved in the radius direction of the excitation coil  202  so that it is positioned on the outward side of the excitation coil  202 , and not only is the theoretical circle which coincides with the first core  241  displaced from the theoretical circle which coincides with the outward surface of the wound portion of the excitation coil  202 , but also, the protrusion  242  of the second core  242  is placed outside the center portion (hole) of the wound portion of the excitation coil  202 . 
     Also in this embodiment, the core holder  208  has a blocker sheet (disengagement preventing portion)  288 , which is attached to the adjacencies of the tips of the second bridge beams  284  and  284  to bridge between the second bridge beams  284  and  284 . The blocker sheet  288  is positioned on the opposite side of the second core  242  from the fixation belt  101  (inductive heat generating member), and plays a role of preventing the second core  242  from moving in the opposite direction from the fixation belt  101  (that is, inductive heat generating member), more than a preset distance (α). Therefore, when the core holder  208  is moved from the second position to the first position, the second core  242  of the external magnetic core  204  is prevented from moving in the opposite direction from the direction indicated by the arrow mark G in  FIG. 9(   a ), far enough to become disengaged from the core holder  208 . As the material for the blocker sheet  288 , aramid polymer fiber which is heat resistant, heat resistant paper, or the like can be preferably used. Further, in this embodiment, each disengagement prevention sheet  288  is thermally welded to the core holder  208  at two points. 
     As described above, when the core holder  208  is in the first position, the tip  242   b  of the protrusion  242   a  of the second core  242  of the external magnetic core  204  is in contact with the second area  274  of contact of the coil holder  207 . Further, the gap (space)  287  is present between the bottom surface of the base portion  242   c  of the second core  242 , and the second core supporting portion  284   b  and  284   b  of the core holder  208 . In this embodiment, therefore, when the core holder  208  is in the first position, there is the preset amount of clearance between the second core  242  and disengagement prevention sheet  288 . Therefore, it can be prevented that the second core  242  comes into contact with the disengagement prevention sheet  288 , whereby the core holder  208  is lifted in the opposite direction from the direction indicated by the arrow mark G in  FIG. 9(   a ). 
     According to this embodiment, it is possible to reduce the impact to which the external magnetic core  204  positioned in the movable core range E is subjected when it is moved from its farthest position from the excitation coil  202 , to its closet position to the excitation coil  202 . 
     That is, when the core holder  208  is moved from the second position to the first position, the catching portions  286  and  286  of the core holder  208  come first into contact with the first area  273  of contact of the coil holder  207 , whereby the first core  241  of the external magnetic core  204  is accurately positioned relative to the excitation coil  202 . Therefore, it does not occur that the first core  241  comes directly in contact with the coil holder  207 . Therefore, it is possible to reduce the impact to which the first core  241  is subjected. 
     The positional relationship between the core holder  208  and second core  242  is set so that when the core holder  208  is moved from the second position to the first position, the second core  242  of the external magnetic core  204  is placed in contact with the coil holder  207  to improve the fixing device  100  in the accuracy with which the second core  242  is positioned. Regarding the amount of force to which the protrusion  242   a  of the second core  242  is subjected by the coil holder  207 , the second core  242  is loosely fitted in the core holder  208  so that after it comes into contact with the coil holder  207 , it is allowed to move upward (distance α) as shown in  FIG. 9 . Further, the second core  242  is physically independent from the first core  241 . Therefore, it is only the weight of the second core  242  that affects the amount of impact to which the second core  242  is subjected. Therefore, the impact is slight. Further, the second core  242  is prevented by the above described disengagement prevention sheet  288 , from disengaging from the core holder  208  when the core holder  208  is moved from the second position to the first position. 
     That is, although the effect of the positioning of the protrusion  242   a , which is the bottom side of the second core  242 , relative to the excitation coil  202  (fixation belt  101 ), upon the amount (9.4°/mm, in this embodiment) by which heat is generated by electromagnetic induction, is substantial, the fixing device  100  is structured so that the protrusion  242   a  comes into contact with the coil holder  207  while preventing the second core  242  from being damaged. Therefore, it is possible to prevent the fixation belt  101  from becoming nonuniform in temperature in terms of its lengthwise direction. Further, the first and second cores  241  and  242  are made physically independent from each other. Therefore, the second core  242  can be positioned relative to the excitation coil  202  (fixation belt  101 ) at a high level of accuracy, regardless of the errors in the shape of the first core  241 . 
     For example, in a case where the magnetic core  203  (which is equivalent to external magnetic core in this embodiment) is a one-piece core, like the one in the referential fixing device  100 , the projection  203 T catches the entire load which the magnetic core  203  carries while it is moved. In comparison, in this embodiment, the force to which the second core  242  is subjected comes from the weight of the second core  242  alone, being therefore, substantially smaller. 
     As described above, according to this embodiment, it is possible to reduce the impact to which the external magnetic core  204  is subjected when the external magnetic core  204  positioned in the movable core range E is moved from its farthest position from the excitation coil  202 , to its closest position. Therefore, it is possible to reduce the possibility that the external magnetic core  204  will be damaged during the above-described movement of the external magnetic core  204 . 
     6. Core Moving Mechanism 
     In this embodiment, a method for sliding the core holder  208  is used as the method for moving the core holder  208 . However, the structure of the mechanism for sliding the core holder  208  to the first or second position is optional. 
       FIG. 10  is a schematic side view of an example of a core moving mechanism  300 . It shows the general structure of the mechanism  300 . This core moving mechanism  300  has a pivotally movable lever  312 , a solenoid  315  as a driving means for moving the lever  312 , and so on. The fixing device  100  may be provided with multiple core moving mechanisms  300  so that each core holder  208  which holds a single or multiple external magnetic cores  204  positioned in the movable core range E is provided with its own core moving mechanism  300 . 
     To describe in detail, the pivotally movable lever  312  is fitted around the shaft  311  (pivot) with which the frame of the fixing device  100  is provided, so that the lever  312  can be pivotally moved about the shaft  311 . The pin shaft  313  with which the core holder  208  is provided is fitted in the elongated hole  312   a  with which the opposite end portion of the lever  312  from the shaft  311  is provided. Thus, the lever  312  becomes connected to the core holder  208 . Further, the solenoid  315  is solidly attached to the supporting plate  314  with which the frame of the fixing device  100  is provided. Further, the pin shaft  315   b  with which the plunger  315   a  of the solenoid  315  is provided is fitted in the elongated hole  312   b  with which the opposite end portion of the lever  311  from the elongated hole  312   a  is provided; the solenoid  315  and lever  312  are connected to each other. Further, a tension spring  316  is placed between the spring anchor  312   c  with which the arm side of the lever  312  is provided, and the spring anchor  314   a  with which the supporting plate  314  is provided, in such a manner that the spring  316  remains stretched. Thus, the lever  312  remains under the tensile force of the tension spring  316 , which works in the direction to cause the lever  312  to pivot about the shaft  311  in the direction to cause the core holder  208  to move toward the bottom portion  271  of the coil holder  207 . 
     When the solenoid  315  is off, the plunger  315   a  is not under the force which works in the direction to pull the plunger  315   a  into the solenoid  315 . Therefore, the lever  312  is pivotally moved about the shaft  311  by the tensile force of the tension spring  316 , causing thereby the core holder  208  to move toward the bottom portion  271  of the coil holder  207 . Consequently, the core holder  208  is moved into the first position. On the other hand, as the electric power for the solenoid  315  is turned on, the plunger  315   a  is pulled into the solenoid  315 . Therefore, the lever  312  is pivotally moved about the shaft  311 , causing thereby the core holder  208  to move away from the bottom portion  271  of the coil holder  207 , while stretching the spring  316  against the tensile force of the spring  316 . Consequently, the core holder  208  is moved into the second position. 
     In this embodiment, a method for sliding the core holder  208  is used as the method for moving the core holder  208 . However, the method for moving the core holder  208  does not need to be the method for sliding the core holder  208 . That is, it may be a method other than the sliding method, as long as it can ensure that a preset positional relationship is maintained between the excitation coil  202  and the movable external magnetic core  204 . A case in which another method is used as the method for moving the movable external magnetic core  204  is described during the description of the second embodiment of the present invention. 
     Further, in this embodiment, a case in which the external magnetic core  204  is positioned on the inward side of the wound portion  202   a  of the excitation coil  202  in terms of the lengthwise direction of the fixing device  100  is described as an example of positioning of the external magnetic core  204  on the inward side of the wound portion  202   a  of the excitation coil  202  in terms of the lengthwise direction of the external magnetic core  204 . However, an external magnetic core  204  may be positioned on the outward side of the wound portion  202   a  of the excitation coil  202 , in order to increase the fixation belt  101  in terms of its range across which it generates heat. In the case in which the external magnetic core  204  is placed on the outward side of the wound portion  202   a  of the excitation coil  202 , however, the external magnetic core  204  positioned on the outward side of the wound portion  202   a  of the excitation coil  202  also is accurately positioned relative to the coil holder  207  as described above. 
     7. Electrically Conductive Member 
     Next, the structure of one of the modified version of the fixing device in this embodiment of the present invention, which moves the magnetic cores to reduce the amount of the leakage of magnetic flux is described.  FIG. 11  is an exploded perspective view of this modified version of the fixing device in this embodiment, minus its portions which are not directly related to the present invention.  FIG. 12  is a schematic sectional view of the electrically conductive member of this modified version, which will be described later.  FIGS. 13(   a ) and  13 ( b ) are sectional views of the combination of the core holder  208  and coil holder  207  of the fixing device in this modification the first embodiment, when the core holder  208  is in the first and second positions, respectively.  FIGS. 14 and 15  are schematic sectional views of the fixing device  100  in this modification of the first embodiment, when the external magnetic cores  204  positioned in the movable core ranges E are in their closest position to the excitation coil  202 , and in their farthest position from the excitation coil  202 , respectively. The basic structure and operation of the fixing device in this modification of the first embodiment is practically the same as those of the fixing device in this embodiment. Thus, the elements of the fixing device in this modification of the first embodiment, which are the same as, or equivalent to, the counterparts in the fixing device in this embodiment, in function and structure, are given the same referential codes as those given to the counterparts. 
     In this modification of the first embodiment, the fixing device  100  is provided with a pair of electrically conductive members  289 , which are positioned on the outward side of the external magnetic core  204  which is a movable magnetic core and is positioned in the movable core range E, and on the inward side of the lateral long walls  276  and  276  of the coil holder  207 . These conductive members  289  are solidly attached to the inward surfaces of the upstream and downstream long lateral walls  276  and  276 , at the end portions of the coil holder  207 , which correspond in position to the ranges E, one for one, in terms of the lengthwise direction. The conductive members  289  are positioned so that they oppose the space through which the external magnetic core  204  positioned in the movable core range E move to be placed in its farthest position from the excitation coil  202 . The conductive members  289  are magnetic flux adjusting members for reducing this space in magnetic flux density. They are made of thin plate of metallic substance which is low in permeability, for example, and are solidly attached to the aforementioned lateral walls of the coil holder  207 , with the use adhesive as solidly attaching means. 
     First, the effect (effect A) of the conductive member  289  attributable to the movement of the magnetic core is described. 
     Referring to  FIG. 14 , when a large sheet P of paper is used as recording medium, the external magnetic core  204  positioned in the movable core range E is moved into, and kept in, its closest position to the excitation coil  202 . It is when the fixing device  100  is in the state shown in  FIG. 14  that the pressure roller  102  is rotationally driven, and the excitation coil  202  is supplied with electric power, to make the fixing device to perform the fixing operation. In this case, therefore, the fixation belt  101 , the entirety of the portion of the fixation belt  101 , which corresponds in position to the path of the large sheet P of recording medium (range A in  FIG. 3 ) roughly uniformly generates heat. The magnetic circuits formed in the adjacencies of the excitation coil  202  when the external magnetic cores  204  positioned in the left and right ranges E are in their closest position to the excitation coil  202  are indicated by sold bold lines H in  FIG. 14 . These magnetic circuits are formed by the external magnetic cores  204  positioned in the left and right ranges E, upstream and downstream magnetic cores  205  and  206 , and the metallic layer (induction heat generating member)  101   a  of the fixation belt  101 . 
     Referring to  FIG. 15 , in a case where recording medium is a small sheet P of recording medium, the distance (gap) between the external magnetic cores  204  positioned in the left and right movable core ranges E, and the excitation coil  202 , is greater than when a large sheet P recording medium is used. It is when the fixing device  100  is in the state shown in  FIG. 15  that the pressure roller  102  is rotationally driven, and the excitation coil  202  is supplied with electric power to make the fixing device  100  to perform a fixing operation. The magnetic circuits formed in the adjacencies of the excitation coil  202  when the external magnetic cores  204  positioned in the left and right ranges E are in their farthest position from the excitation coil  202  are indicated by solid bold lines H in  FIG. 15 . When the fixing device  100  is in the state shown in  FIG. 15 , the magnetic circuits formed in the adjacencies of the excitation coil  202  are lower in efficiency. In this case, therefore, the portion of the fixation belt  101 , which correspond in position to the areas (areas C in  FIG. 3 ) between the left edge of the path of a small sheet P of recording medium and the left edge of the path of a large sheet P of recording medium, and the portion of the fixation belt  101 , which corresponds in position to the right edge of the path of the small sheet P and the right edge of the large sheet P, are less in the amount of heat generation. 
     Next, the function (function B) of the electrically conductive member  289  is described. 
     When the fixing device  100  is in the state shown in  FIG. 15 , the conductive members  289  are in the spaces formed by the movement of the external magnetic cores  204  positioned in the left and right ranges E, and are held by the coil holder  207 . The conductive members  289  can reduce the amount by which the electromagnetic flux leaks out of the fixing device  100 . In addition, it plays also the following role. 
     That is, since the electrically conductive member  289  intersects with a part of the magnetic flux H generated by the excitation coil  202 , the magnetic flux H ( FIG. 15 ) from the excitation coil  202  is affected by the electromagnetic induction. More specifically, since the electrically conductive member  289  is positioned so that it intersects with the magnetic flux H generated by the excitation coil  202 , electromagnetic force is generated in the conductive member  289  by an amount proportional to the rate of change of the magnetic flux H (principle of electromagnetic induction), creating thereby a closed circuit (intersectional circuit) which induces electric current in the conductive member  289 . The direction of this force, or the direction of the electric current flowed by this electromagnetic force is such that the magnetic flux generated by this current impedes the change in the intersectional magnetic flux. Therefore, the areas in which the electrically conductive members  289  intersect with the magnetic flux H, that is, the ranges E, reduces in magnetic flux density. Therefore, the portions of the fixation belt  101 , which correspond in position to the ranges E reduces in the amount by which they generate heat. 
     As described above, basically, the unwanted increase in temperature which is likely to occur across the out-of-sheet path portions of the fixation belt  101 , when a small sheet P of recording medium is used, can be controlled (prevented) by the above described function A of the electrically conductive member  289 . Further, in a case where the fixing device  100  is provided with the electrically conductive members  289 , the unwanted increase in temperature which is likely to occur across the out-of-sheet path portions of the fixation belt  101 , when a small sheet P of recording medium is used, can be controlled (prevented) at a higher level of effectiveness, because the combined effects of the above described functions A and B. 
     Further, the fixing device  100  is structured so that the electrically conductive members  289  are stationary, and only the external magnetic cores  204  positioned in the left and right ranges E are movable. Therefore, the fixing device  100  is less complicated in overall structure, and is smaller than the fixing device in this embodiment. 
     From the standpoint of reducing the fixing device  100  in electric power consumption while preventing the electrically conductive members  289  from being increased in temperature by the heat generated in themselves, it is desired that the conductive members  289  are formed of a conductive substance which is low in permeability. That is, the conductive members  289  is desired to be no less than 0.9, and no more than 1.1, in permeability. As the desirable material for the electrically conductive member  289 , copper, aluminum, silver, lead, and the like can be listed, which are 0.999991, 1.00002, 0.99998 and 0.999983, respectively, in permeability. Further, from the standpoint of reducing the amount by which the electrically conductive members  289  generate heat, the electrically conductive members  289  are desired to be made of metallic plate which is low in electrical resistance. 
     The principle of electric magnetic induction is that as electric current flows through an object with which a magnetic flux intersects, heat is generated in the object by the electric power, amount of which is proportional to the skin resistance Rs of the object. The skin depth δ of the object can be expressed as follows:
 
δ=(2ρ/μω)1/2
 
wherein ω, μ and ρ stand for angular frequency, permeability, and specific resistance, respectively.
 
     Further, the skin resistance Rs is expressed as follows:
 
 Rs=ρ/δ 
 
     The amount of electric power W generated in the object with which the magnetic flux intersects can be expressed as follows:
 
 W∝Rs ∫|I| 2 dS  
 
     wherein I stands for electric current. 
     Thus, the smaller the electrically conductive members  289  in permeability, the smaller the conductive members  289  in the amount of electric power W generated therein, and therefore, the smaller the conductive members  289  in the amount by which heat is generated therein. Further, the smaller the conductive members  289  in specific resistance, the smaller the conductive members  289  in the amount of electric power W generated therein, and therefore, the smaller in the amount by which heat is generated therein. 
     On the other hand, in this modified version of this embodiment, in order to prevent the magnetic flux from leaking through the electrically conductive members  289 , the electrically conductive members  289  are structured so that their thickness t ( FIG. 12 ) is greater than its skin depth δ. As described above, the skin depth δ is determined by the permeability μ of the electrically conductive member  289 , specific resistance ρ of the electrically conductive member  289 , and angular frequency ω of the magnetic flux. In this connection, in a case where the thickness t of the electrically conductive member  289  is less than the skin depth δ of the conductive member  289 , the skin resistance Rs of the conductive member  289  is expressible as follows based on the principle of electromagnetic induction:
 
 Rs≈ρ/t ( t : thickness)
 
     In this case, therefore, the conductive member  289  is greater in the amount of the heat generated therein. 
     Further, from the standpoint of ensuring that the electrically conductive members  289  sufficiently reduce the magnetic flux, the electrically conductive members  289  should be positioned in the areas in which the magnetic flux has not dispersed. That is, it is desired that the electrically conductive members  289  are positioned in the areas which are near the excitation coil  202 , and in which the external magnetic cores  204  in the left and right ranges E, upstream and downstream magnetic cores  205  and  206 , induction heat generating member  101   a , and conductive members  298  form the magnetic circuit, to prevent as much as possible the magnetic flux from leaking out. 
     In this modification of this embodiment, the electrically conductive members  289  are positioned in the adjacencies of the paths of the external magnetic cores  204  positioned in the left and right ranges E, one for one, and are directly and solidly attached to the coil holder  207 . Also in this modification of the first embodiment, the fixing device  100  is structured so that the length L ( FIG. 12 ) of the external magnetic cores  204  positioned in the left and right ranges E, in terms of the direction in which the external magnetic cores  204  are moved, is longer than the distance d ( FIG. 15 ) by which the external magnetic cores  204  positioned in the left and right ranges E are moved. Therefore, even if the distance between the external magnetic cores  204  and excitation coil  202  is widened by the movement of the external magnetic cores  204 , the presence of the electrically conductive members  289  minimize the magnetic flux leakage, minimizing thereby the effects of the magnetic flux upon the components which are in the adjacencies of the fixing device  100 . Further, for the purpose of maximizing the magnetic flux reducing effect of the electrically conductive members  289 , the length W ( FIG. 12 ) of the conductive member  289  in terms of the lengthwise direction of the fixing device  100  is made longer than the dimension of the movable core range E in the same direction. By the way, the electrically conductive members  289  have only to be positioned in the spaces through which the external magnetic cores  204  positioned in the left and right ranges E move. That is, the fixing device  100  may be structured so that the electrically conductive members  289  cover the outward surface of the external magnetic cores  204  positioned in the left and right ranges E, or the entirety of the magnetic core  203 . 
     8. Prevention of Temperature Increase of Out-of-sheet Path Portion of Fixation Belt 
     Next, referring to  FIGS. 16 and 17 , the effects of this embodiment (and above described modification of this embodiment) in terms of the prevention of the temperature increase of the portions of the fixation belt  101 , which are out of the sheet path, are described more concretely. 
       FIGS. 16 and 17  are schematic drawings for describing the effects which the external magnetic cores  204  have when a small sheet P of recording medium, more specifically, a sheet P of recording medium which is W 1  in width, is used. 
     The graph in  FIG. 16  shows the temperature distribution of the fixation belt  101  in terms of the lengthwise direction, after the conveyance of the first (dotted line) and 500th (solid line) small sheets P of recording medium, which is W 1  in width, when the width W 2  of the range across which the magnetic flux is stronger because of the presence of the external magnetic cores  204 , is greater than W 1 . The preset target temperature level (fixation level) of the temperature of the fixing device  100  is 180° C. According to this graph, if the fixing device  100  was set to make the sheet path portion of the fixation belt  101  uniform in temperature distribution, for the first sheet P of recording medium, the temperature of the portions of the fixation belt  101 , which are in the adjacencies of the lateral edges of the 500th sheet P, became 270° C. That is, the temperature of these portions of the fixation belt  101  had increased to a level which is substantially higher than the target level. This excessive amount of temperature increase is likely to lead to the endurance rupture of the fixation belt  101 . Therefore, it is desired that the fixing device  100  is structured so that these portions of the fixation belt  101  are prevented from excessively increasing in temperature. 
     In this embodiment (also in above described modification of this embodiment), in order to deal with the excessive temperature increase of the above described portion of the fixation belt  101 , the distance between the excitation coil  202  and external magnetic cores  204  is widened across the ranges which are outside the sheet path, so that the fixing device  100  is reduced in the density of the magnetic flux which passes the fixation belt  101 , to reduce the amount by which the fixation belt  101  generates heat. 
       FIG. 17  shows the temperature distribution of the fixation belt  101  in terms of the lengthwise direction, after the conveyance of the first (dotted line) and 500th (solid line) small sheets P of recording medium, which is W 1  in width, when the width W 3  of the range across which the magnetic flux is stronger because of the presence of the external magnetic cores  204 , is the same as W 1 . According to this graph, the portion of the fixation belt  101 , which corresponds in position to the sheet path, was uniform in temperature distribution, being therefore satisfactory in fixation. Further, even after the conveyance of the 500th sheet P of recording medium, the temperature of the portions of the fixation belt  101 , which are outside the path of the sheet P which is W 1  in width, that is, the out-of-sheet path portions of the fixation belt  101 , were kept below the temperature level beyond which the fixation belt is likely to be ruptured (enturance rupture). That is, the graph shows, this embodiment (also modified version of this embodiment) can reduce the possibility of the endurance rupture of the fixation belt  101  due to the excessive temperature increase which occurs to the portions of the fixation belt  101 , which are outside the path of the sheet P of recording medium. 
     As described above, this embodiment (also modified version of this embodiment) widens the distance (gap) between the excitation coil  202  and external magnetic cores  204 , across the areas which are outside the path of recording medium, in terms of the lengthwise direction of the fixing device  100 , when a small sheet P of recording medium is used as recording medium. Therefore, not only can it keep the fixing device satisfactory in fixation, but also, minimize the possibility that the fixation belt  101  will suffer from endurance rupture. 
     9. Control 
       FIG. 18  is a block diagram of the control of the essential portions of the image forming apparatus  1  in this embodiment. It shows the general control of the apparatus  1 . The operation of the image forming apparatus  1  is integrally controlled by the control section  50  with which the apparatus  1  is provided. The control section  50  has a CPU  51  as controlling means, a ROM  52  as storing means, a RAM  53  as storing means, and so on. The control section  50  controls the operation of each of the various portions of the image forming apparatus  1 , based on the programs and/or data stored in the ROM 52  and read out into the RAM  53  as necessary. Regarding the relationship between this embodiment and control section  50 , the control section  50  is in connection to the electric power source  103  which applies high frequency current to the inductive heat generating section  200  of the fixing device  100 , motor M 1  which rotationally drives the pressure roller  102  of the fixing device  100 , and so on. Further, the control section  50  is in connection to the temperature sensor  107  which detects the temperature of the fixation belt  101  of the fixing device  100 , driving means for driving the core moving mechanism  300  which moves the core holder  208  of the fixing device  100 , and so on. 
       FIG. 19  is a flowchart of the operation to be carried out by the image forming apparatus  1  to form an image. It shows the general control of the operation of the apparatus  1 . When the image forming apparatus  1  is kept on standby, the control section  50  keeps the core holders  208  which are holding the external magnetic cores  204  in the ranges E, in the first position by the core moving mechanism  300  (Step 1). Then, as a print start signal is inputted (Step 2), the control section  50  reads the size of the sheet P of recording medium to be used for the image formation, from the information inputted from an external host apparatus, or through the recording medium size inputting means of the control panel (unshown) of the image forming apparatus  1  (Step 3). Then, the control section  50  determines whether the inputted value indicates that it is a small or large sheet of recording medium that is used for the image formation (Step 4). If the control section  50  determines that the recording medium is a small sheet P of recording medium, it moves the core holder  208  which are holding the external magnetic cores  204  positioned in the ranges E, to the second position, with the use of the core moving mechanism  300  (Step 5). Then, it makes the image forming apparatus  1  carry out a printing job set for outputting a preset number of prints (Step 6). As soon as the printing job is completed (Step 7), the control section  50  puts the image forming apparatus  1  on standby, and waits for the print start signal for the next printing job (Step 8). On the other hand, if the control section  50  determines that the recording medium is a large sheet P of recording medium, it keeps the core holder  208  in the first position, and makes the image forming apparatus  1  perform the printing job for outputting a preset number of prints (Step 6). Then, as soon as the printing job is finished (Step 7), the control section  50  puts the image forming apparatus  1  on standby, and waits for the inputting of the print start signal for the next printing job (Step 8). 
     10. Effects 
     As described above, in this embodiment, among the multiple external magnetic cores  204  positioned in parallel in the lengthwise direction of the fixing device  100 , those positioned in the ranges E are movable. Therefore, it is possible to prevent the portions of the fixation belt  101 , which are outside the path of a small sheet P of recording medium, from excessively increasing in temperature. Further, according to this embodiment, it is possible to make the fixing device  100  roughly uniform in the positional relationship between the external magnetic cores  204  positioned in the ranges E, excitation coil  202 , and metallic layer (induction heat generating member)  101   a  of the fixation belt  101 , in terms of the lengthwise direction of the fixing device  100 . Therefore, it is possible to make the fixation belt  101  roughly uniform in temperature in terms of the lengthwise direction. 
     Further, it is possible to minimize the impact to which the external magnetic cores  204  positioned in the ranges E are subjected when the core holder  208  is moved from the second position to the first position to move the external magnetic cores  204  from their farthest position from the excitation coil  202  to their closest position to the excitation coil  202 . 
     As described above, according to this embodiment, it is possible to achieve both the objective of preventing the portions of the fixation belt  101 , which are out of the path of a small sheet P of recording medium, from excessively increasing in temperature, and the objective of keeping the fixation belt uniform in temperature in the lengthwise direction, while minimizing the possibility that the external magnetic cores  204  will be damaged. 
     [Embodiment 2] 
     Next, the another embodiment of the present invention is described. The image forming apparatus and its fixing device in this embodiment are the same in basic structure and operation as those in the first embodiment. Thus, elements of the image forming apparatus and fixing device in this embodiment, which are equivalent in function and structure as the counterparts in the first embodiment are given the same referential codes as those given to the counterparts, and are not described in detail here. 
     This embodiment is different from the first embodiment in the structure of the coil holder  207 , core holder  208 , and the core moving mechanism  300 . 
       FIG. 20  is an external perspective view of the induction heating section  200  in this embodiment, minus its portions which are not directly related to the present invention.  FIGS. 21(   a ) and  21 ( b ) are sectional views of the combination of the core holder  208  and coil holder  207  when the core holder  208  is in its first and second positions, respectively. 
     In the first embodiment, the method for sliding the core holder  208  was used as the method for moving the core holder  208 . In this embodiment, a method for rotating (pivotally moving) the core holder  208 is used as the method for moving the core holder  208 . 
     In this embodiment, the multiple external magnetic cores  204  positioned in the ranges E are held by the core holder  208  which is movable to the first and second positions by the core holder moving mechanism  300  as a core holder moving means. 
     Next, referring to  FIG. 20 , the fixing device  100  is structured so that multiple core holders  208  hold multiple external magnetic cores, one for one. However, it may be structured so that some of the core holders  208  hold two or more external magnetic cores  204  as it was in the first embodiment. Also in this embodiment, the multiple external magnetic cores  204  positioned in the range D, are solidly attached to the coil holder  207 , with the placement of a stationary core holder between themselves and the coil holder  207 , as in the first embodiment. This stationary core holder is the same as the above described core holder  208  in terms of how they hold the external magnetic cores  204 . It however is kept stationary only in its first position. 
     In terms of how each external magnetic core  204  is held by a core holder, the core holder  208  in this embodiment is roughly the same in structure as that in the first embodiment. Next, referring to  FIGS. 21(   a ) and  21 ( b ), the short lateral portions  281  and  281 , and long lateral portions  282  and  282 , in this embodiment are different in shape, and the attributes related to shape, as those in the first embodiment, but, are practically equivalent to those in the first embodiment in functionality. 
     Further, the coil holder  207  in this embodiment has a protrusion  275  which protrudes from the arch portion  271   a  of the bottom portion  271  in the opposite direction from the fixation belt  101 , as the coil holder  207  in the first embodiment does. In this embodiment, however, in practical terms, it is only the tip portion of the protrusion  275 , which extends in the lengthwise direction of the fixing device  100  toward the center (hole)  202   b  of the wound portion  202   a  of the excitation coil  202 , on the downstream side, that protrudes beyond the outward surface of the wound portion  202   a  of the excitation coil  202 . This tip portion is the first area  273  of contact. In this embodiment, therefore, it is the bottom surface of the second core supporting portion  284   a  of only the core supporting downstream bridge beam  284  of the core supporting second bridge beams  284 , that functions as the catching portion  286  which comes into contact with the first area  273  of contact. 
     Further, in this embodiment, the fixing device  100  is provided with a pair of electrically conductive members  289  which are similar to those in the modified version of the first embodiment described above. The pair of electrically conductive members  289  are on the inward surfaces of the portions of the downstream lateral long wall  276 , which correspond in position to the ranges E. Also in this embodiment, the fixing device  100  is structured so that the opposite end portion of the upstream lateral long wall  276  of the coil holder  207  extends lower than the corresponding end portion of the upstream lateral long wall  276 . This opposite end portion of the upstream lateral long wall  276  from the fixation belt  101  is tilted so that it remains parallel to the tangential line of the theoretical circle, the center of which coincides with the pivot (axial line) of the core moving mechanism which will be described later. It is on this tilted end portion of the upstream lateral wall  276 , which corresponds in position to the movable core range E, that the electrically conductive member  289  is positioned. 
     The core moving mechanism  300  in this embodiment is made up of a pivot (axle)  301 , an arm  302 , a base  303 , a coil spring  304 , a cam  305 , a cam shaft  306 , and so on. The axle  301  is on the upstream side of the coil holder  207  and core holder  208 , and extends in the lengthwise direction of the fixing device  100 . It is supported by the base  303  attached to the coil holder  207 , by its lengthwise ends. The arm  302  is pivotally supported by the axle  301  (pivot). It is connected to the upstream lateral short wall  283  of the core holder  208 . Thus, the core holder  208  is supported by the base  303  in such a manner that it can be pivotally moved about the shaft (pivot)  301 . 
     In this embodiment, the arm  302  is an integral part of the core holder  208 . However, the arm  302  may be independently formed from the core holder  208  to be solidly attached to the core holder  208  with the use of a proper means for solidly attaching the arm  302  to the core holder  208 . Also in this embodiment, the base  303  and coil holder  207  are independently formed from each other, and are solidly attached to each other by thermal welding. However, they may be formed together in a singe piece. 
     The core holder  208  is kept under a preset amount of pressure generated in the direction indicated by an arrow mark F in  FIGS. 21(   a ) and  21 ( b ) by the torsional coil spring  304 , as a pressure applying means, the axial line of which coincides with the axial line of the shaft (pivot)  301 . That is, the core holder  208  is kept under the preset amount of pressure generated by the coil spring  304  in the direction to cause the core holder  208  to pivotally move toward the coil holder  207 . More specifically, when the core holder  208  is in the first position shown in  FIG. 21(   a ), the core holder  208  is kept pressured toward the coil holder  207  so that the catching portion  286  of the core holder  208  remains in contact with the first area  273  of contact of the coil holder  207 . Also when the core holder  208  is in the first position, the tip  272   b  of the protrusion  242   a  of the second core  242  of the external magnetic core  204  remains in contact with the second area  274  of contact of the coil holder  207 . 
     That is, the second core  242  is properly positioned by the contact between the tip  242   b  and coil holder  207 . The reason where the fixing device  100  is structured as described above is as follows. The magnetic flux generated by the excitation coil  202  concentrates inward of the wound portion  202   a  of the excitation coil  202 . Therefore, the strength of the magnetic flux which acts on the fixation belt  101  is significantly affected by the distance between the second core  242  which is on the inward side of the wound wire of the excitation coil  202 , and the fixation belt  101 . Thus, if the magnetic flux which acts on the fixation belt  101  becomes nonuniform in strength in terms of the lengthwise direction, it is possible that the fixation belt  101  becomes nonuniform in heating performance in terms of the lengthwise direction. Thus, from the standpoint of preventing the fixation belt  101  from becoming nonuniform in heating performance in terms of the lengthwise direction, it is desired that the fixing device  100  is structured so that the multiple second cores  242  aligned in parallel in the lengthwise direction do not become nonuniform in their distance from the fixation belt  101 . In this embodiment, therefore, in order to prevent the second cores  242  aligned in parallel in the lengthwise direction from becoming nonuniform in their distance from the fixation belt  101 , the fixing device  100  is structured so that each second core  242  is positioned relative to the coil holder  207  by the contact between the tip of the second core  242  and coil holder  207 . 
     Like in the first embodiment, the second core  242  is held in such a manner that it is movable relative to the core holder  208  in the direction indicated by an arrow mark G, and the opposite direction from the direction G, when the core holder  208  is in the first position. Therefore, it does not occur that the second core  242  is pressed upon the coil holder  207  by the force generated by the torsional coil spring  304 . 
     The core holder  208  is moved with the use of the cam  305  and cam shaft  306 . The cam shaft  306  is positioned on the upstream side of the coil holder  207  and core holder  208 , and on the downstream side of the shaft (pivot)  301 , and extends in the lengthwise direction of the fixing device  100 . The cam  305  is between the arm  302  and base  303 , and is solidly attached to the cam shaft  306 . In this embodiment, all the core holders  208  which hold the external magnetic cores  204  positioned in the ranges E are synchronously moved to the first or second position. Thus, all the cams  305  are practically the same in profile in terms of their rotational directions. 
     Referring to  FIG. 21(   a ), when it is necessary to place the core holder  208  in the first position, the cam shaft  306  is to be rotated by a cam shaft driving motor (unshown) as a driving means to move the cam  305  away from the arm  302  so that the external magnetic cores  204  positioned in the ranges E are moved into their closest position to the excitation coil  202 . 
     On the other hand, when it is necessary to place the core holder  208  in the second position, the cam shaft  306  is to be rotated by the abovementioned motor (unshown) as a driving means to cause the cam  305  to push up the arm  302  so that the arm  302  is moved against the resiliency of the torsional coil spring  304 . Thus, the external magnetic cores  204  positioned in the ranges E are moved into their farthest positions from the excitation coil  202 . 
     As described above, according to this embodiment, not only can the same effects as those obtainable by the first embodiment be obtained, but also, it is possible to simplify the core moving mechanism  300  in structure. 
     [Embodiment 3] 
     Next, another embodiment of the present invention is described. The image forming apparatus and its fixing device in this embodiment are the same in basic structure and operation as those in the first embodiment. Thus, elements of the image forming apparatus and fixing device in this embodiment, which are the same as, or equivalent to the counterparts in the first embodiment in function and structure, are given the same referential codes as those given to the counterparts, and are not described in detail here. 
     This embodiment is different from the first embodiment in the position of the external magnetic cores  204  in terms of the lengthwise direction of the fixing device  100 . 
     In this embodiment, the multiple external magnetic cores  204  positioned in the ranges E are held by the core holder  208  which are movable to the first or second position by the core moving mechanism  300  as core moving means, as in the first embodiment. 
     Also in this embodiment, the multiple external magnetic cores  204  positioned in the ranges E are held so that they are movable relative to the coil holder  207 , with the placement of the core holder  208  between the external magnetic cores and coil holder  207 . Further, the multiple external magnetic cores  204  positioned in the range D, are kept stationary relative to the coil holder  207 , with the presence of the stationary core holder between the external magnetic cores  204  and coil holder  207 . In terms of the method for holding the external magnetic cores  204 , this stationary core holder is the same in structure as the above described core holder  208 . However, this stationary core holder is permanently kept in the first position. 
       FIG. 22  is a schematic drawing for showing the relationship between the position of the external magnetic cores  204  and the temperature distribution of the fixation belt  101  in terms of the lengthwise direction. 
     In the first embodiment, the gap between the multiple external magnetic cores  204  aligned in parallel in the lengthwise direction of the fixing device  100 , and the metallic layer (induction heat generating member)  101   a  of the fixation belt  101 , was made roughly uniform in terms of the lengthwise direction of the fixing device  100 . Therefore, it was possible to make the fixation belt  101  uniform in temperature in terms of the lengthwise direction ( FIG. 25(   a )). 
     In comparison, in this embodiment, regarding the temperature distribution of the fixation belt  101  in terms of the lengthwise direction, the fixing device  100  is designed so that the end portions of the fixation belt  101  in terms of the lengthwise direction become higher in temperature than the center portion (referential center line O), as shown in  FIG. 22 , in order to make the end portions of the fixation belt  101 , in terms of the lengthwise direction of the fixing device  100 , greater in the recording medium conveyance speed than the center portion of the fixation belt  101  to deal with the problem that while a sheet of paper or the like recording medium is conveyed, the sheet is made to wrinkle by the difference between the center portion and the end portions of the sheet P, in the conveyance speed. 
     In this embodiment, therefore, the distance between the second area  274  of contact of the coil holder  207 , and the metallic layer (induction heat generating member)  101   a  of the fixation belt  101 , is made larger across the center portion of the fixing device  100  than across the end portions of the fixing device  100  in terms of the lengthwise direction of the fixing device  100 . That is, in terms of the lengthwise direction of the fixing device  100 , the second area  274  of contact of the coil holder  207 , which is the bottom portion  271  of the coil holder  207 , is given such a curvature that the height of the roughly center portion (referential center line O) of the bottom portion  271  relative to the lengthwise end of the bottom portion  271  is Δx. Further, in this embodiment, the distance between the first area  273  of contact of the coil holder  207  and the metallic layer (induction heat generating member)  101   a  of the fixation belt  101  is made greater across roughly the center portion of the fixing device  100  than the end portions of the fixing device  100 , in terms of the lengthwise direction of the fixing device  100 . That is, in terms of the lengthwise direction of the fixing device  100 , the first area  273  of contact of the coil holder  207 , which is the tip of the protrusion  275  of the coil holder  207 , is also given such a curvature that the height of the roughly center portion (referential center line O) of the tip of the protrusion  275  relative to the lengthwise end of the bottom portion  271  is Δx. 
     In this embodiment, the positional relationship between the multiple external magnetic cores  204  aligned in parallel in the lengthwise direction of the fixing device  100 , and the fixation belt  101 , is maintained by the direct contact between the external magnetic cores  204  and coil holder  207 , or the indirect contact between external magnetic core  204  and coil holder  207  through the core holder  208 , as in the first embodiment. Therefore, the position of each external magnetic core  204  relative to the metallic layer (induction heat generating member)  101   a  of the fixation belt  101  is determined by the curvature of the bottom portion  271  of the coil holder  207 . Therefore, the fixation belt  101  can be heated so that its temperature distribution in terms of the lengthwise direction becomes as shown in  FIG. 22 . 
     As described above, according to the present invention, even in a case where the external magnetic cores  204  aligned in parallel in the lengthwise direction of the fixing device  100  are not roughly uniformly changed in their distance relative to the metallic layer (induction heat generating member)  101   a  of the fixation belt  101 , each external magnetic core  204  can be properly positioned. 
     [Miscellanies] 
     The foregoing are the description of the present invention with the reference to the concrete embodiments of the present invention. However, these embodiments are not intended to limit the present invention in scope. 
     The image forming apparatus and fixing device may be structured so that the position of a sheet of recording medium relative to the image forming apparatus (fixing device) in terms of the direction perpendicular to the recoding medium conveyance direction is set by the placement of one of the lateral edges of the sheet P in contact with the corresponding edge of the recording medium conveyance passage. Thus, all that is necessary when a small sheet of recording medium is used as recording medium is to move the movable magnetic cores which are on the opposite side of the recording medium passage from the positional referential edge of the recording medium passage, in the direction which is roughly perpendicular to the recording medium conveyance direction. 
     Further, not only may the heating device (apparatus) be a fixing device (apparatus) for fixing the unfixed image on recording medium, but also, a temporarily fixing device for temporarily fixing the unfixed image on recording medium to the recording medium, a surface property altering device (apparatus) (for example, gloss increasing device for increasing image in gloss) for altering in surface properties the image on recording medium by heating the recording medium. Further, the heating device (apparatus) may be a thermal drying device for quickly drying the ink, that is, liquid which contains dye and/or pigment, deposited on recording medium by an image forming apparatus of the inkjet type to form an image on the recording medium. Further, the heating device (apparatus) may be a thermal pressing device for removing wrinkles from paper money, a thermal laminating device, a thermal drying device for evaporating water from a sheet of paper, a heating device for thermally processing a sheet of paper. 
     Further, the number by which the multiple movable cores are moved may be changed according to the width of a sheet of recording medium. In such a case, the core moving mechanism is desired to be such that it can move each movable magnetic core independently from the other. 
     Further, in the above described embodiments, the fixing device was divided into the areas (sections) in which multiple magnetic cores aligned in parallel in the lengthwise direction of the fixing device are movable, and the area (section) in which the magnetic cores are not movable. However, the fixing device may be structured so that all the multiple magnetic cores aligned in parallel in the lengthwise direction of the fixing device are movable to their closest position to the excitation coil, and their farthest position from the excitation coil, to enable the fixing device to select the magnetic cores to be moved, according to the width of the sheet of recording medium used for image formation. 
     While the invention has been described with reference to the structures disclosed herein, it is not confined to the details set forth, and this application is intended to cover such modifications or changes as may come within the purposes of the improvements or the scope of the following claims. 
     This application claims priority from Japanese Patent Application No. 168932/2012 filed Jul. 30, 2012, which is hereby incorporated by reference.