Patent Publication Number: US-7725052-B2

Title: Ion generating device and image forming apparatus including same

Description:
This Nonprovisional application claims priority under 35 U.S.C. §119(a) on Patent Application Nos. 099908/2007 and 099909/2007 filed in Japan on Apr. 5, 2007, the entire contents of which are hereby incorporated by reference. 
   FIELD OF THE INVENTION 
   The present invention relates to an ion generating device that generates ions along with creeping discharge and an image forming apparatus, such as a copier, a facsimile machine, or a printer, which includes the ion generating device. 
   BACKGROUND OF THE INVENTION 
   Some image forming apparatuses such as copiers, printers, and facsimile machines employ an electrophotographic method as a recording method, and the market for such image forming apparatuses have been rapidly growing. 
   Conventionally, in such an image forming apparatus employing an electrophotographic method, ion generating devices have been used as a charging device for charging a photoreceptor or the like, as an electricity-removing device for removing electricity from a photoreceptor or the like, and as a transfer device for transferring a toner image onto a transfer receiving material. Currently mainstream examples of the ion generating devices include corotron or scorotron corona charging devices and roller-type contact charging devices. 
   However, now that image forming apparatuses have been miniaturized, there is a demand for miniaturization of charging devices. In reality, such ion generating devices as corona charging devices and roller-type contact device described above cannot sufficiently meet the demand for miniaturization. 
   Specifically, a corona charging device is arranged such that a corona wire is so covered with a case as to be approximately 1 cm away from the case. This makes it difficult to miniaturize the charging device. A roller-type contact charging device also requires a space, albeit not as large as a corona charging device does, in which a roller is disposed. 
   Studied in response to the demand as an ion generating device capable of charging even in a small space is an ion generating device, referred to also as “solid ion generating device, which uses a creeping discharge element that generates ions along with creeping discharge. Such an ion generating device can greatly reduce the amount of ozone to be generated. 
     FIG. 16  shows an arrangement of a conventional ion generating device for generating ions along with creeping discharge. As shown in  FIG. 16 , the ion generating device includes: a dielectric material  101 ; and a creeping discharge element  104 , which has a discharge electrode  103  and an inductive electrode  102  disposed so as to face each other via the dielectric material  101 . 
   Each of the discharge electrode  103  and the inductive electrode  102  is formed by forming an electrode film such as a tungsten electrode film on (a surface of) the dielectric material  101  and shaping the electrode film into a ribbon (stripe) with use of a photolithographic technique or the like. 
   When an alternating voltage is applied between the discharge electrode  103  and the inductive electrode  102 , creeping discharge occurs near the discharge electrode  103 , so that ions are generated accordingly. In the case of the ribbon discharge electrode  103 , discharge occurs in two edge portions that extend in a longitudinal direction of the ribbon discharge electrode  103 . 
   Further, commonly-used examples of the dielectric material  101  include mica paper made by joining pieces of mica on top of each other with resin. Other usable examples of the dielectric material include raw mica, ceramic, and a resin sheet. However, it is very difficult to obtain raw mica having a large amount of space, and such raw mica is expensive. Further, ceramic is expensive, albeit not as expensive as raw mica. Moreover, ceramic is prone to breakages and cracks. This makes it difficult to shape ceramic into a thin plate. 
   Conventionally, a creeping discharge element in such an ion generating device or the device has been judged to have reached the end of its life when it becomes unable to ensure uniformity in ion generating ability or when its ion generating ability falls short of an allowable lower limit. 
   In the case of the dielectric material  101  made of mica paper as shown in  FIG. 16 , there is a minute hole  106  formed in a space between mica flakes  105 . As the accumulated amount of discharge time becomes larger, the minute hole  106  grows larger or comes to contain water. That part of the minute hole  106  which has grown larger or contains water deteriorates in insulation resistance, and therefore becomes incapable of discharging. This makes it impossible to ensure uniformity in discharge, thereby causing nonuniformity in ion generating ability. 
   Further, when a high voltage is applied to the discharge electrode  103 , a strong electric field is formed between the discharge electrode  103  and a part of the dielectric material  101  in contact with the discharge electrode  103 . Therefore, as the accumulated amount of discharge time becomes larger, the dielectric material  101  suffers from a change in color (burn-in). When the dielectric material  101  suffers from such a change in color, the dielectric material  101  cannot maintain the required ion generating ability. 
   As described above, conventionally, the creeping discharge element  104  or the ion generating device has been judged to have reached the end of its life at a point of time where the dielectric material  101  deteriorates. 
   Examples of prior art documents of an ion generating device including a creeping discharge element include Patent Documents 1 and 2. In order to prevent deterioration from being caused by discharge, Patent Documents 1 and 2 teach that a surface of a discharge electrode and a surface of a dielectric material on which the discharge electrode is provided are coated with an inorganic coating agent. Patent Documents 1 and 2 also teach that: the coating agent performs a function of preventing the discharge electrode from being wearing due to discharge and prevents the dielectric material from deteriorating due to discharge, thereby greatly improving the life of the ion generating device. 
   [Patent Document 1] 
   Japanese Unexamined Patent Application Publication No. 237368/2002 (Tokukai 2002-237368; published on Aug. 23, 2006) 
   [Patent Document 2] 
   Japanese Unexamined Patent Application Publication No. 47642/2006 (Tokukai 2006-47642; published on Feb. 16, 2006) 
   However, even in the arrangement, described in Patent Documents 1 and 2, in which the discharge electrode and the dielectric material are coated with the inorganic coating agent, it is hard to say that the life of the ion generating device is sufficiently long. A proposal for a technique for further extending the life of a creeping discharge element or an ion generating device has been expected. 
   Furthermore, such a conventional ion generating device as described in Patent Documents 1 and 2 has problems with cost of manufacturing and running cost. 
   That is, in the above arrangement, the discharge electrode  103  and the inductive electrode  102  are formed by forming an electrode film on a surface of the dielectric material  101  and patterning the electrode film with use of a photolithographic technique or the like. This inevitably causes a rise in cost of manufacturing. 
   Further, in the case of the discharge electrode  103  made by patterning the electrode film, creeping discharge occurs mainly in the edge portions as shown in  FIG. 16 , so that the amount of space of discharge is small. Therefore, the amount of discharge is small with respect to the alternating voltage applied between the discharge electrode  103  and the inductive electrode  102 . In order to ensure the amount of discharge, it is necessary to apply a high alternating voltage to some extent. This causes an increase in power consumption, and shortens the life of an ion generating device or a creeping discharge element by accelerating deterioration of the dielectric material  101 , thereby causing a rise in running cost. 
   SUMMARY OF THE INVENTION 
   It is a first object of the present invention to provide: a long-life ion generating device whose life has been extended by making effective use of a dielectric material; and an image forming apparatus including the ion generating device. 
   As a result of diligent studies to attain the foregoing object, the applicant of the present application finally completed the present invention by finding that since ceramic or mica material has low mechanical strength and is hard to be processed, the material is formed so as to have an amount of space larger than necessary for actual charging, and that a large number of undeteriorated areas that can still be used are left at a point of time where a creeping discharge element or an ion generating device is judged to have reached the end of its life. 
   In order to attain the first object, an ion generating device of the present invention is an ion generating device for generating ions along with creeping discharge, the ion generating device including: a dielectric material; an inductive electrode disposed on a first surface of the dielectric material; a discharge electrode disposed on a second surface of the dielectric material; an alternating voltage applying section for applying an alternating voltage between the inductive electrode and the discharge electrode; and a DC bias voltage applying section for applying a DC bias voltage to the discharge electrode, the discharge electrode and the dielectric material being provided so as to be movable with respect to each other. 
   According to the foregoing arrangement, the discharge electrode and the dielectric material are provided so as to be movable with respect to each other. Therefore, in cases where an increase in accumulated amount of discharge time causes the aforementioned deterioration in a portion of the dielectric material in contact with the discharge electrode and the deterioration causes a defect in discharge, the portion of the dielectric material in contact with the discharge electrode can be switched to a new area free of deterioration by changing the position of the discharge electrode with respect to the dielectric material. 
   This makes it possible to efficiently use a dielectric material that has conventionally been wastefully disposed with an undeteriorated area remaining thereon. This makes it possible to extend the life of a creeping discharge element or an ion generating device while using a dielectric material of the same size. Moreover, this makes it possible to reduce the total cost including maintenance cost for replacing the creeping discharge element and the ion generating device. 
   In order to attain the first object, an image forming apparatus of the present invention is arranged so as to include an ion generating device of the present invention as a pretransfer charging device for giving electric charge to toner carried on a carrier. 
   By using an ion generating device of the present invention as a pretransfer charging device for giving electric charge to toner carried on a carrier, an image of high quality can be obtained while holding down the total cost. 
   In order to attain the first object, an image forming apparatus of the present invention is arranged so as to include an ion generating device of the present invention as a toner precharging device for giving electric charge to toner contained in a developing device for developing an electrostatic latent image. 
   By using an ion generating device of the present invention as a toner precharging device for giving electric charge to toner contained in a developing device for developing an electrostatic latent image, an image of high quality can be obtained while holding down the total cost. 
   It is a second object of the present invention to provide: an ion generating device easy to manufacture and capable of reducing the value of an alternating voltage to be applied between a discharge electrode and an inductive electrode; and an image forming apparatus including the ion generating device. 
   In order to attain the second object, an ion generating device of the present invention is an ion generating device for generating ions along with creeping discharge, the ion generating device including: a dielectric material; an inductive electrode disposed on a first surface of the dielectric material; a discharge electrode disposed on a second surface of the dielectric material; an alternating voltage applying section for applying an alternating voltage between the inductive electrode and the discharge electrode; and a DC bias voltage applying section for applying a DC bias voltage to the discharge electrode, the discharge electrode being constituted by a wire electrode member disposed in contact with the second surface of the dielectric material such that both ends of the electrode member in a longitudinal direction of the electrode member are supported outside of the dielectric material. 
   According to the foregoing arrangement, the discharge electrode is constituted by a wire electrode member disposed in contact with the second surface of the dielectric material such that both ends of the electrode member in a longitudinal direction of the electrode member are supported outside of the dielectric material. This makes it easier to manufacture a creeping discharge element and an ion generating device as compared with a conventional arrangement in which a discharge electrode has been formed directly on a surface of a dielectric material with use of a photolithographic technique. This makes it possible to reduce the cost of manufacturing. 
   Furthermore, the discharge electrode is in the shape of a wire. This makes it easy to efficiently cause creeping discharge entirely on an outer circumferential surface of the discharge electrode. As compared with a conventional arrangement in which a discharge electrode made by patterning an electrode film is provided, it becomes possible to increase the amount of discharge with respect to the alternating voltage applied between the discharge electrode and the inductive electrode. This makes it possible to decrease the alternating voltage applied between the discharge electrode and the inductive electrode. This makes it possible to save electric power and to extend the life of the ion generating device or the creeping discharge element by inhibiting the dielectric material from deteriorating. 
   Further, the discharge electrode is not formed directly on the dielectric material. This makes it easy to change the relative positions of the dielectric material and the discharge electrode. This makes it possible to further extend the life of the ion generating device or the creeping discharge element with efficient use of the dielectric material, for example, by switching the position of the discharge electrode with respect to the dielectric material. 
   With this, manufacturing is made easier than before and the value of an alternating voltage to be applied between the discharge electrode and the inductive electrode is made lower than before, so that the cost of manufacturing and the running cost can be made lower than before. 
   In order to attain the second object, an image forming apparatus of the present invention is arranged so as to include an ion generating device of the present invention as a pretransfer charging device for giving electric charge to toner carried on a carrier. 
   By using an ion generating device of the present invention as a pretransfer charging device for giving electric charge to toner carried on a carrier, an image of high quality can be obtained while holding down the cost of manufacturing and the running cost. 
   In order to attain the second object, an image forming apparatus of the present invention is arranged so as to include an ion generating device of the present invention as a toner precharging device for giving electric charge to toner contained in a developing device for developing an electrostatic latent image. 
   By using an ion generating device of the present invention as a toner precharging device for giving electric charge to toner contained in a developing device for developing an electrostatic latent image, an image of high quality can be obtained while holding down the cost of manufacturing and the running cost. 
   Additional objects, features, and strengths of the present invention will be made clear by the description below. Further, the advantages of the present invention will be evident from the following explanation in reference to the drawings. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
       FIG. 1 , showing Embodiment 1 of the present invention, is an explanatory diagram showing a creeping discharge element of an ion generating device as seen from a discharge electrode. 
       FIG. 2  is an explanatory diagram showing a main part of an image forming apparatus in which such ion generating devices as described above are used as a charging device for a latent image, a charging device for charging before a first transfer, and a charging device for charging before a second transfer. 
       FIG. 3  is an explanatory diagram showing an arrangement of the ion generating device used in the image forming apparatus as the charging device for charging before a second transfer. 
       FIG. 4  is a perspective view showing an arrangement of the creeping discharge element of the ion generating device as seen from an end of the creeping discharge element in a longitudinal direction of the creeping discharge element. 
       FIG. 5  is an explanatory diagram showing how the position of the discharge electrode with respect to a dielectric material is changed in the ion generating device by switching the discharge electrode from one pair of hook sections to another. 
       FIG. 6  is a graph showing a result obtained by using the ion generating device to examine a relationship of discharge time to discharge current and image quality. 
       FIG. 7  is an explanatory diagram showing a result obtained by using the ion generating device to examine the extent of deterioration of the dielectric material with respect to the discharge electrode. 
       FIG. 8 , showing an example for use in comparison with an embodiment of the present invention, is a graph showing a result obtained by examining a discharging characteristic. 
       FIG. 9 , showing Embodiment 2 of the present invention, is an explanatory diagram showing an arrangement of an ion generating device. 
       FIG. 10  is a flow chart showing the steps of a position switching process of automatically switching the position of a discharge electrode in the ion generating device. 
       FIG. 11 , showing Embodiment 3 of the present invention, is an explanatory diagram showing an arrangement of an ion generating device. 
       FIG. 12  is an explanatory diagram showing a preferred example of how a discharge electrode is disposed in the ion generating device. 
       FIG. 13(   a ) through  13 ( c ) are each an explanatory diagram showing a specific arrangement of a rotating roller having a discharge electrode provided on the inner circumference of a dielectric material. 
       FIG. 14 , showing Embodiment 4 of the present invention, is an explanatory diagram showing an arrangement of a main part of an image forming apparatus in which an ion generating device is used as a toner precharging device that is to be disposed in a developing device. 
       FIG. 15  is an explanatory diagram showing an arrangement of a main part of the developing device in which the ion generating device is used as the toner precharging device. 
       FIG. 16  is an explanatory diagram showing an arrangement of a conventional ion generating device. 
   

   DESCRIPTION OF THE EMBODIMENTS 
   The present invention can be used in an electrophotographic image forming apparatus as a charging device for performing (i) pretransfer charging by which a toner image to be formed on an image carrier such as a photoreceptor or an intermediate transfer body is charged before a transfer, (ii) electrostatic latent image charging by which a photoreceptor is charged, or (iii) toner precharging by which charging of toner contained in a developing device is supplemented. 
   Embodiment 1 
   An embodiment of the present invention will be described below with reference to  FIGS. 1 through 7 . The embodiment described below is a specific example of the present invention, and does not limit the technical scope of the present invention. 
   First, an overall arrangement of an electrophotographic image forming apparatus  100  according to the present embodiment will be described with reference to  FIG. 2 . The image forming apparatus  100  is a printer of a tandem type and of an intermediate transfer type, and can form a full-color image. 
   As shown in  FIG. 2 , the image forming apparatus  100  includes: four visible-image forming units  50   a  to  50   d  respectively corresponding to four colors (C, M, Y, and K); a transfer unit  40 ; and a fixing device  14 . 
   The transfer unit  40  includes an intermediate transfer belt  15  (image carrier), four first transfer devices  12   a  to  12   d , a charging device  3  for charging before a second transfer, a second transfer device  16 , and a cleaning device  17  for a transfer. The first transfer devices  12   a  to  12   d , the charging device  3 , the second transfer device  16 , and the cleaning device  17  are disposed around the intermediate transfer belt  15 . 
   The intermediate transfer belt  15  is a belt onto which toner images respectively visualized in their respective colors by the visual-image forming unit  50   a  to  50   d  are transferred so as to be superimposed onto one another and from which the toner images thus transferred are retransferred onto a sheet of recording paper. Specifically, the intermediate transfer belt  15  is an endless belt, stretched by a pair of a driving roller and an idling roller, which is driven at the time of image formation to rotate at a predetermined peripheral velocity. 
   The first transfer devices  12   a  to  12   d  are provided in the visible-image forming units  50   a  to  50   d , respectively. Each of the first transfer device  12   a  to  12   d  is disposed to face the corresponding one of the visible-image forming units  50   a  to  50   d  with the intermediate transfer belt  15  sandwiched therebetween. 
   The charging device  3  recharges the toner images so transferred onto the intermediate transfer belt  15  as to be superimposed onto one another. 
   The second transfer device  16  retransfers, onto the sheet of recording paper, the toner images transferred onto the intermediate transfer belt  15 . The second transfer device  16  is provided so as to make contact with the intermediate transfer belt  15 . The cleaning device  17  cleans a surface of the intermediate transfer belt  15  from which the toner images have been retransferred. 
   It should be noted that the first transfer devices  12   a  to  12   d , the charging device  3 , the second transfer device  16 , and the cleaning device  17  are disposed around the intermediate transfer belt  15  of the transfer unit  40  in this order from the upstream side of the rotation direction of the intermediate transfer belt  15 . 
   Provided on the downstream side of the second transfer device  16  in the recording-paper conveying direction is the fixing device  14 . The fixing device  14  fixes, onto the sheet of recording paper, the toner images transferred onto the sheet of recording paper by the second transfer device  16 . 
   Further provided along the rotation direction of the intermediate transfer belt  15  so as to make contact with the intermediate transfer belt  15  are the four visible-image forming units  50   a  to  50   d . The visible-image forming units  50   a  to  50   d  are identical in everything but color to one another, and the visible-image forming units  50   a  to  50   d  use yellow (Y) toner, magenta (M) toner, cyan (C) toner, and black toner (K), respectively. In the following, only the visible-image forming unit  50   a  is described, and the other visible-image forming units  50   b  to  50   d  are not described. 
   The visible-image forming unit  50   a  includes a photosensitive drum (image carrier)  7 , a latent image charging device  4 , a laser writing unit (not shown), a developing device  11 , a charging device  2  for charging before a first transfer, and a cleaning device  13 . The charging device  4 , the laser writing unit, the developing device  11 , the charging device  2 , and the cleaning device  13  are disposed around the photosensitive drum  7 . 
   The charging device  4  charges a surface of the photosensitive drum  7  so that the surface of the photosensitive drum  7  has a predetermined potential. 
   The laser writing unit irradiates (exposes) the photosensitive drum  7  with (to) laser light in accordance with image data received from an external device, scans an optical image on the photosensitive drum  7  that has been uniformly charged, and writes an electrostatic latent image onto the photosensitive drum  7 . 
   The developing device  11  supplies toner to the electrostatic latent image formed on the surface of the photosensitive drum  7 , and then forms a toner image by visualizing the electrostatic latent image. 
   Before the toner image formed on the surface of the photosensitive drum  7  is transferred, the charging device  2  recharges the toner image. 
   The cleaning device  13  removes and collects the toner remaining on the photosensitive drum  7  from which the toner image has been transferred onto the intermediate transfer belt  15 , thereby making it possible to record a new electrostatic latent image and a new toner image on the photosensitive drum  7 . 
   It should be noted that the charging device  4 , the laser writing unit, the developing device  11 , the charging device  2 , the first transfer device  12   a , and the cleaning device  13  are disposed around the photosensitive drum  7  of the visible-image forming unit  50   a  in this order from the upstream of the rotation direction of the photosensitive drum  7 . 
   In the image forming apparatus  100  of the present embodiment, such ion generating devices  1  as will be described below are used as the charging devices  2 ,  3 , and  4 . 
   The following describes an image forming operation of the image forming apparatus  100 . 
   First, the image forming apparatus  100  acquires image data from an external device. Further, a driving unit (not shown) of the image forming apparatus  100  causes the photosensitive drum  7  to rotate at a predetermined peripheral velocity in a direction indicated by an arrow in  FIG. 2 , and the charging device  4  charges a surface of the photosensitive drum  7  so that the surface of the photosensitive drum  7  has a predetermined potential. 
   Next, the laser writing unit exposes the surface of the photosensitive drum  7  to light in accordance with the image data thus acquired, and writes an electrostatic latent image onto the surface of the photosensitive drum  7  in accordance with the image data. Then, the developing device  11  supplies toner to the electrostatic latent image formed on the surface of the photosensitive drum  7 , thereby forming a toner image by causing the toner to adhere to the electrostatic latent image. 
   The toner image thus formed on the surface of the photosensitive drum  7  is recharged by the charging device  2 . The toner image thus recharged is transferred onto the intermediate transfer belt  15  by the first transfer device  12   a  applying a bias voltage opposite in polarity to the toner image formed on the surface of the photosensitive drum  7  (first transfer). 
   The visible-image forming units  50   a  to  50   d  take turns in performing this operation, so that four toner images respectively having four colors Y, M, C, and K are superimposed onto one another on the intermediate transfer belt  15 . 
   The toner images thus superimposed onto one another are conveyed to the charging device  3  by the intermediate transfer belt  15 , and the toner images thus conveyed are recharged by the charging device  3 . Then, the intermediate transfer belt  15  carrying the toner images thus recharged is pressed by the second transfer device  16  against a sheet of recording paper fed from a paper feeding unit (not shown), so that the toner images are transferred onto the sheet of recoding paper (second transfer). 
   Thereafter, the fixing device  14  fixes the toner images onto the sheet of recording paper, and the sheet of recording paper on which an image has been recorded is ejected onto a paper ejection unit (not shown). The toner remaining on the photosensitive drum  7  after the first transfer is removed and collected by the cleaning device  13 , and the toner remaining on the intermediate transfer belt  15  after the second transfer is removed and collected by the cleaning device  17 . The foregoing operation makes it possible to perform appropriate printing on a sheet of recording paper. 
   The following describes arrangements of the ion generating devices  1  used as the three charging devices  2 ,  3 , and  4 . 
     FIG. 3  shows the arrangement of the ion generating device  1  disposed as the charging device  3  near the intermediate transfer belt  15 . 
   As shown in  FIG. 3 , the charging device  3  includes a creeping discharge element  20 , an alternating voltage applying section  23 , a DC bias voltage applying section  22 , and a counter electrode  24 . 
   The creeping discharge element  20  includes a dielectric material  26 , an inductive electrode  25  disposed on one surface of the dielectric material  26 , and a discharge electrode  27  disposed on the other surface of the dielectric material  26 . 
   The inductive electrode  25  is constituted, for example, by a ribbon electrode layer formed on one surface of the dielectric material  26 . Such an electrode layer is processed with use of a photolithographic technique or the like. Moreover, the inductive electrode  25  is formed entirely on the dielectric material  26  except an outer portion of the dielectric material  26  so that discharge occurs only on the side of the discharge electrode  27 . The inductive electrode  25  can be made, for example, of tungsten wire, molybdenum, and stainless steel. 
   The discharge electrode  27  is constituted by a wire electrode member disposed so as to make contact with the other surface of the dielectric material  26 . Moreover, although described below in detail, the discharge electrode  27  is only fixed at both ends thereof so as to traverse the dielectric material  26  in a longitudinal direction of the dielectric material  26 . The discharge electrode  27  is arranged so as to freely move with respect to the dielectric material  26 . Such a discharge electrode  27  can be made, for example, of tungsten, molybdenum, and stainless steel. 
   Further, it is preferable that the diameter of the discharge electrode  27  fall within a range of not less than 20 μm to not more than 100 μm. When the diameter of the discharge electrode  27  is smaller than 20 μm, the discharge efficiency of the discharge electrode  27  increases, but the mechanical strength of the discharge electrode  27  becomes insufficient. This shortens the life of the discharge electrode  27 . On the other hand, when the diameter of the discharge electrode  27  exceeds 100 μm, the discharge efficiency of the discharge electrode  27  decreases, and the discharge electrode  27  requires strength of a member for fixing the discharge electrode  27 . This causes the device to be huge. 
   Moreover, in consideration of the mechanical strength, it is preferable that the lower limit fall within a range of not less than 30 μm. Further, in consideration of the discharge efficiency and the strength of the member for fixing the discharge electrode  27 , it is preferable that the upper limit fall within a range of not more than 70 μm. 
   It is desirable that each of the inductive electrode  25  and the discharge electrode  27  be plated with copper, gold, nickel, or the like. Plating makes it possible to extend the life of the electrode and increase the strength of the electrode. Among them, gold plating is most preferable. 
   The dielectric material  26  interposed between the inductive electrode  25  and the discharge electrode  27  is a long plate member, and can be made of mica material, ceramic, a resin film, or the like. Among them, mica paper made by joining pieces of mica on top of each other with resin is preferable in terms of price, insulating properties, and processability, 
   Usable examples of the mica paper include MICATITE MCT-BS manufactured by Okabe Mica Co., Ltd. The size of each mica flake of MICATITE MCT-BS is such that the thickness ranges from 1 μm to 10 μm and the mean diameter ranges from 100 μm to 200 μm. 
   In consideration of the dielectric breakdown voltage between the discharge electrode  27  and the inductive electrode  25 , it is necessary that the dielectric material  26  made of mica paper have a thickness of at least 1 mm. 
   Usable examples of the mica paper include MICATITE MCT-BS manufactured by Okabe Mica Co., Ltd. The size of each mica flake of MICATITE MCT-BS is such that the thickness ranges from 1 μm to 10 μm and the mean diameter ranges from 100 μm to 200 μm. 
   In consideration of the dielectric breakdown voltage between the discharge electrode  27  and the inductive electrode  25 , it is necessary that the dielectric material  26  made of mica paper have a thickness of at least 1 mm. 
   The counter electrode  24  is disposed so as to face the discharge electrode  27 . The counter electrode  24  is grounded. Such a counter electrode  24  is disposed so as to make it easy for the discharge electrode  27  to discharge. The counter electrode  24  is not an absolute necessity; that is, the counter electrode  24  can be omitted. 
   The DC bias voltage applying section  22  applies a DC bias voltage to the discharge electrode  27 . The alternating voltage applying section  23  applies an alternating voltage between the inductive electrode  25  and the discharge electrode  27 . 
   In the ion generating device  1  thus arranged, the discharge electrode  27  discharges when an alternating voltage having a frequency of several hundred Hz to several hundred kHz and a pulse height of 1 kV to 5 kV is applied between the discharge electrode  27  and the inductive electrode  25 . 
   When the discharge electrode  27  discharges, air between the electrodes is ionized in a discharge area, so that positive and negative corona ions are generated near the ionized air. The corona ions thus generated are taken out by applying a DC bias voltage to the discharge electrode  27 . 
   In cases where the DC bias voltage thus applied is a negative voltage, only negative ions are taken out from the ion generating device  1 . On the other hand, in cases where the DC bias voltage thus applied is a positive voltage, only positive ions are taken out from the ion generating device  1 . 
   The following fully describes the discharge electrode  27  in terms of being arranged so as to freely move with respect to the dielectric material  26 . 
     FIG. 1  is a plan view showing the creeping discharge element  20  of the ion generating device  1  as seen from the side on which the discharge electrode  27  is disposed. Further,  FIG. 4  is a perspective view showing the creeping discharge element  20  as seen from an end of the creeping discharge element  20  in a longitudinal direction of the dielectric material  26 . 
   As shown in  FIG. 1 , one end of the discharge electrode  27  is connected directly to a retaining member (supporting member)  33 , and the other end of the discharge electrode  27  is connected to a retaining member (supporting member) via an elastic member  32 . The elastic member  32  causes the discharge element  27  to be stretchable in the axial direction of the discharge element  27 . 
   Moreover, the ends of the discharge electrode  27  are fixed by the retaining members  33  so as be lower than a surface of the dielectric material  26  in contact with the discharge electrode  27 . Since the ends of the discharge electrode  27  are thus fixed by the retaining members  33  so as be lower than the surface of the dielectric material  26  in contact with the discharge electrode  27 , the stretchable discharge electrode  27  makes close contact with the dielectric material  26  entirely in the longitudinal direction of the dielectric material  26 . 
   As described above, the dielectric material  26  is a very thin plate member. Therefore, the dielectric material  26  may be deformed. If the dielectric material  26  is warped in such a manner that the surface of the dielectric material  26  in contact with the discharge electrode  27  is concaved across a central part of the longitudinal direction, the dielectric material  26  and the discharge electrode  27  are prevented from making contact with each other entirely in the longitudinal direction of the dielectric material  26 . 
   In view of this, it is preferable that the dielectric material  26  be warped from the beginning, rather than being shaped into a flat plate, in such a manner that the surface of the dielectric material  26  in contact with the discharge electrode  27  is concaved across the central part of the longitudinal direction. By thus warping the dielectric material  26  in advance, the discharge electrode  27  can be surely brought into close contact with the dielectric material  26 . 
   Disposed between the ends of the dielectric material  26  in the longitudinal direction and the retaining members  33  are positioning members  30 , respectively. The positioning members  30  serve to determine the position of the discharge electrode  27  with respect to the dielectric material  26 . Each of the positioning members  30  is provided with a plurality of (four in  FIG. 1 ) groove-like hook sections (holding sections)  31  on which the discharge electrode  27  is hung and held. The plurality of hook sections  31  are provided in parallel with one another along a direction, indicated by an arrow X, which is orthogonal to a longitudinal direction of the surface of the dielectric material  26  in contact with the discharge electrode  27 . 
   In such an arrangement, the position of the discharge electrode  27  with respect to the dielectric material  26  is changed in the direction of the arrow X by switching the discharge electrode  27  from one pair of hook sections  31  to another, so that those portions of the dielectric material  26  which make contact with the discharge electrode  27  can be switched. The hook sections  31  to be engaged can be easily switched with use of the stretchability attributed to the elastic member  32 . 
   In  FIG. 4 , that portion of the dielectric material  26  which makes contact with the discharge electrode  27  while the discharge electrode  27  is being held by the hook sections  31   a  and that portion of the dielectric material  26  which makes contact with the discharge electrode  27  when the discharge electrode  27  is being held by the hook sections  31   b  are already in a state of deterioration. In such a state, the discharge electrode  27  only needs to be switched to the adjoining hook sections  31   c  so as to make contact with a non-deteriorated part of the dielectric material  26 . 
     FIG. 5  shows how the discharge electrode  27  is switched from one pair of hook sections  31   b  to an adjoining pair of hook sections  31   c . It should be noted that  FIG. 5  is a cross-sectional view taken along the line A-A of  FIG. 4 . 
   By simple arithmetic, since the arrangement of  FIG. 4  is provided with four pairs of hook sections  31 , the arrangement has life approximately four times longer than the life of a conventional ion generating device. 
   The following explains a result obtained by examining a deterioration characteristic of the dielectric material  26  and a relationship between deterioration and image quality in order to determine a timing at which the position of the discharge electrode  27  is switched. 
   MICATITE MCT-BS manufactured by Okabe Mica Co., Ltd. was used as the dielectric material  26 , and a tungsten wire having a diameter of 60 μm was used as the discharge electrode  27 . A DC bias voltage of −1.5 kV was applied to the discharge electrode  27 , and an alternating voltage having a pulse height of 3.0 kV and a frequency of 500 Hz was applied between the inductive electrode  25  and the discharge electrode  27 . 
   Then, the amount of current flowing through the intermediate transfer belt  15  disposed between the discharge electrode  27  and the counter electrode  24  was measured every time five hours after the start of discharge by inserting a DC voltmeter between the counter electrode  24  and the ground. Similarly, an image was formed every five hours by charging toner on the intermediate transfer belt  15  before a transfer. The quality of the image was visually inspected. 
     FIG. 6  shows a result obtained by examining a relationship of discharge time to discharge current and image quality. As shown in  FIG. 6 , as the discharge time becomes longer, the discharge current gradually decreases while drawing a parabola. This shows that the dielectric material  26  deteriorates as the discharge time becomes longer. 
   Even as the discharge current (μA) decreased, the image quality had no trouble in cases where the discharge current exceeded −5 μA (in absolute value). However, when the discharge time reached 45 hours and the discharge current became not more than −5 μA (in absolute value), the image quality was influenced (area of influence on an image). Specifically, the density of a solid image became low, and the solid image came to have brush marks. This is because a decrease in amount of discharge from the discharge electrode  27  prevented the toner on the intermediate transfer belt  15  from being sufficiently charged and thereby caused a decrease in transfer efficiency. 
   In order to transfer the toner from the intermediate transfer belt  15  onto a sheet or the like with high transfer efficiency, it is necessary to charge the toner so that the toner has a required amount of charge. Pretransfer charging supplements the amount of charge. In order to transfer the toner from the intermediate transfer belt  15  onto a sheet or the like with high transfer efficiency, it is necessary to charge the toner so that the toner has an amount of charge of not less than −20 μq (in absolute value). 
   Therefore, the ion generating device  1  disposed as the charging device  3  requires a discharge current of more than −5 μA (in absolute value). At a point of time where the discharge current becomes not more than −5 μA (in absolute value), a portion of the dielectric material  26  in contact with the discharge electrode  27  can be judged to have deteriorated. However, in reality, it is only necessary to move the discharge electrode  27  after a discharge time of approximately 40 hours in view of the margin. 
   As shown above, the judgment of deterioration of a contact portion of the dielectric material  26  varies depending on the amount of discharge required of the ion generating device  1 . Further, the amount of discharge required of the ion generating device  1  is determined by how the ion generating device  1  is used. Therefore, the dielectric material  26  can be used for a longer period of time by changing the criterion for judgment of deterioration of a contact portion in accordance with how the ion generating device  1  is used. 
   The following explains a result obtained by examining, in order to determine the distance by which the discharge electrode  27  is moved, the extent of deterioration around a portion of the dielectric material  26  in contact with the discharge electrode  27 . 
   Also in this case, MICATITE MCT-BS manufactured by Okabe Mica Co., Ltd. was used as the dielectric material  26 , and a tungsten wire having a diameter of 60 μm was used as the discharge electrode  27 . A DC bias voltage of −1.5 kV was applied to the discharge electrode  27 , and an alternating voltage having a pulse height of 3.0 kV was applied between the inductive electrode  25  and the discharge electrode  27 . 
   Then, an area of the dielectric material where color has been changed was visually judged as a deteriorated area. 
     FIG. 7  shows a result obtained by examining, by brining the discharge electrode  27  into close contact with the dielectric material  26 , the extent of deterioration around the portion of the dielectric material  26  in contact with the discharge electrode  27 . As shown in  FIG. 7 , areas extending for approximately 300 μm on both sides of the axial direction of the discharge electrode  27  were found to have deteriorated. The same result was obtained even when a tungsten wire having a diameter of 30 μm was used as the discharge electrode  27 . 
   The extent of deterioration was examined while changing the value of the DC bias voltage and the alternating voltage. As a result, it was confirmed that although there was a difference in time taken for deteriorated areas to emerge on both sides of the axial direction of the discharge electrode  27 , there was no change in size of the deteriorated areas. 
   Furthermore, in addition to the wire discharge electrode  27 , a conventional creeping discharge element in which a ribbon discharge electrode is provided on one surface of a dielectric material made of MICATITE MCT-BS was examined for the purpose of seeing how a deteriorated area emerges. Also in this case, substantially the same result was obtained. 
   In view of this, the present embodiment sets a margin of 300 μm in consideration of the thickness of the discharge electrode  27 , and forms hook sections  31  so that the discharge electrode  27  can be moved by increments of 1 mm. 
   The following explains a result obtained by examining a discharging characteristic in the ion generating device  1  including the creeping discharge element  20  using the wire discharge electrode  27 . 
   As in the previous test, MICATITE MCT-BS manufactured by Okabe Mica Co., Ltd. was used as the dielectric material  26 , and a tungsten wire having a diameter of 60 μm (Example 1) and a tungsten wire having a diameter of 30 μm (Example 2) were each used as the discharge electrode  27 . A DC bias voltage of −1.5 kV was applied to the discharge electrode  27 , and an alternating voltage having a pulse height of 3.0 kV was applied between the inductive electrode  25  and the discharge electrode  27 . 
   Then, the amount of current flowing through a charged object disposed between the discharge electrode  27  and the counter electrode  24  was measured while changing the frequency of the alternating voltage applied between the inductive electrode  25  and the discharge electrode  27  and by inserting a DC voltmeter between the counter electrode  24  and the ground. 
   As Comparative Example, a ribbon electrode made of tungsten and having a width of 0.3 mm was formed as a discharge electrode on a surface of a similar dielectric material  26  made of MICATITE MCT-BS, and the discharge performance was examined under the same conditions. 
     FIG. 8  shows a result obtained by examining the discharge performance. As shown in  FIG. 8 , in each of (i) the creeping discharge element  20  of Example 1 whose discharge electrode  27  is made of a tungsten wire having a diameter of 60 μm, (ii) the creeping discharge element  20  of Example 2 whose discharge electrode  27  is made of a tungsten wire having a diameter of 30 μm, and (iii) the creeping discharge element of Comparative Example whose discharge electrode is made of a tungsten ribbon having a width of 0.3 mm, the amount of current increases as the frequency of the alternating voltage applied between the inductive electrode  25  and the discharge electrode  27  (ribbon discharge electrode in the case of Comparative Example) increases. 
   Example 1, Example 2, and Comparative Example show that as compared with a ribbon discharge electrode, a wire discharge electrode can better increase the amount of current flowing through a charged object, with the alternating voltage at the same frequency. 
   The reason for this is as follows. In the case of a ribbon discharge electrode, creeping discharge is performed mainly in an edge portion. On the other hand, in the case of an extra fine discharge electrode  27  having a diameter of 60 μm or 30 μm, creeping discharge occurs entirely on an outer circumferential surface. 
   This shows that as compared with a ribbon discharge electrode, a wire discharge electrode can better increase the amount of discharge and thereby reduce the value of an alternating voltage to be applied between the discharge electrode  27  and the inductive electrode  25 . 
   Furthermore, Examples 1 and 2 show that as a wire discharge electrode has a larger diameter, the wire discharge electrode can better increase the amount of current flowing through a charged object, with the alternating voltage at the same frequency. 
   This shows that as the diameter of a wire is made smaller, i.e., as the curvature is made smaller, the amount of discharge can be made larger, so that the value of an alternating voltage to be applied between the discharge electrode  27  and the inductive electrode  25  can be reduced. 
   As described above, in the image forming apparatus  100  of the present embodiment, each of the ion generating devices  1  provided as the charging devices  2 ,  3 , and  4  is arranged such that the discharge electrode  27  is movable with respect to the dielectric material  26  and that the relative positions of the discharge electrode  27  and the dielectric material  26  can be changed. 
   With this, even if the accumulated amount of discharge time becomes so large that a defect in discharge occurs due to deterioration caused in a portion of the dielectric material  26  in contact with the discharge electrode  27 , the position of the discharge electrode  27  with respect to the dielectric material  26  is changed, so that the portion of the dielectric material in contact with the discharge electrode  27  is switched to a new area free of deterioration. This makes it possible to continuously use the creeping discharge element  20  and the ion generating device  1  without replacing them. 
   This makes it possible to efficiently use a dielectric material that has conventionally been wastefully disposed with an undeteriorated area remaining thereon. This makes it possible to extend the life of a creeping discharge element or an ion generating device while using a dielectric material of the same size. Moreover, this makes it possible to reduce the total cost including maintenance cost for replacing the creeping discharge element  20  and the ion generating device  1 . 
   The present embodiment exemplifies, as the discharge electrode  27  provided so as to be movable with respect to the dielectric material  26 , an arrangement constituted by a wire electrode member. However, the discharge electrode  27  does not need to be fixed onto the dielectric material  26 , and the dielectric material  26  and the discharge electrode  27  only need to be movable with respect to each other. As for the shape of a dielectric electrode, the dielectric electrode may be a wide dielectric electrode patterned into a ribbon or the like as conventional. 
   However, in cases where a discharge electrode that moves with respect to the dielectric material  26  is shaped into a wire, a deteriorated area of the dielectric material  26  that extends from below the discharge electrode  27  to both sides of the discharge electrode  27  can be made smaller as compared with a conventional wide discharge electrode. Therefore, in the case of use of a dielectric material having the same amount of space, the number of times the position of a discharge electrode is switched can be made larger. This brings about an effect of extending the life of a creeping discharge element or an ion generating device. 
   Further, the present embodiment assumes an arrangement in which the dielectric material  26  is fixed and the discharge electrode  27  is moved with respect to the dielectric material  26 . However, such an arrangement is possible that the discharge electrode  27  is fixed and the dielectric material  26  is moved with respect to the discharge electrode  27 . 
   Furthermore, although the ion generating device  1  has the positioning members  30  disposed on both sides of the longitudinal direction of the dielectric material  26 , the position of the discharge electrode  27  with respect to the dielectric material  26  can be switched by disposing a positioning member  30  on at least one of the sides. 
   However, in an arrangement in which a positioning member  30  is disposed only on one of the sides, the discharge electrode  27  does not move in parallel, but moves so as to rotate on the retaining member  33  provided with no positioning member  30 . Therefore, in an arrangement in which a positioning member  30  is disposed on either of the sides, it is preferable to dispose a positioning member  30  on a side opposite to a side on which the provision of an elastic member  32  has increased the distance from the dielectric material  26  to the retaining member  33 . 
   Further, although the image forming apparatus  100  of the present embodiment is arranged so as to include ion generating devices  1  of the present invention as the charging devices  2  to  4 , the image forming apparatus  100  of the present embodiment can be of course arranged, for example, such that the charging device  4  is a scorotron corona charging device or a roller-type contact charging device. It is only necessary that at least one of the charging devices  2  to  4  be an ion generating device  1 . 
   Furthermore, such ion generating device  1  as described above can be used as an electricity-removing device or a transfer device of an image forming apparatus. 
   Further, it is preferable that the ion generating device  1  be disposed such that the discharge electrode  27  is located below the dielectric material  26  in a vertical direction. This makes it possible to prevent the surface of the dielectric material  26  in contact with the discharge electrode  27  from being greatly contaminated by toner and dust. 
   As described above, the ion generating device  1  makes effective use of the whole area of the dielectric material  26  by switching the relative positions of the discharge electrode  27  and the dielectric material  26 . In such an arrangement, it is undesirable that the surface of the dielectric material  26  be contaminated by toner and dust. Such disposition of the ion generating device  1  makes it possible to keep the surface of the dielectric material  26  clean. 
   In particular, since ions and ozone are heavier than air, ions and ozone are likely to flow downward. Therefore, such an arrangement makes it possible to cause generated ions to efficiently reach a charged object and the like. 
   Further, as described above, in the image forming apparatus  100  of the present embodiment, each of the ion generating devices provided as the charging devices  2  to  4  is arranged such that the discharge electrode  27  is a wire electrode member, that the discharge electrode  27  is not formed directly on a surface of the dielectric material  26 , and that the discharge electrode  27  is attached so as to be pressed against the dielectric material  26 . 
   Therefore, the arrangement is simpler than a conventional creeping discharge element and ion generating device in which a discharge electrode has been formed directly on a surface of the dielectric material  26  with use of a photolithographic technique. 
   Furthermore, the discharge electrode  27  is in the shape of a wire electrode member. Therefore as compared with a ribbon discharge electrode, the discharge electrode  27  can better increase the amount of discharge and thereby reduce the alternating voltage to be applied between the discharge electrode  27  and the inductive electrode  25 . This makes it possible to save electric power and to inhibit the dielectric material  26  from deteriorating. 
   Further, the discharge electrode  27  is not formed directly on the dielectric material  26 . This makes it possible to relatively change the positional relationship with the dielectric material  26  as described above. This makes it possible to extend the life of the ion generating device  1  or the creeping discharge element  20  with efficient use of the dielectric material  26 , for example, by sequentially switching contact portions of the dielectric material  26 . 
   Embodiment 2 
   Another embodiment of the present invention will be described below with reference to  FIGS. 9 and 10 . For convenience of explanation, members having the same functions as those used in Embodiment 1 will be given the same reference numerals and will not be described below. 
   An image forming apparatus of the present embodiment differs from the image forming apparatus  100  of Embodiment 1 in terms of ion generating devices that are used as a charging device  2  for charging before a first transfer, a charging device  3  for charging before a second transfer, and a latent image charging device  4 . In each ion generating device  28  provided in the image forming apparatus of the present embodiment, the position of a discharge electrode  27  with respect to a dielectric material  26  can be automatically switched. 
   The following describes only parts different from those of the ion generating device  1 . As shown in  FIG. 9 , the ion generating device  28  has a positioning member  35  provided with a single hook section  31 . Moreover, the positioning member  35  has a rack gear  39  formed on a lower surface thereof. Disposed below the positioning member  35  is a pinion gear  36  that engages with the rack gear  39 . 
   In such an arrangement, the rotation of the pinion gear  36  causes the positioning member  35  to slide in a direction indicated by an arrow X, so that the position of the discharge electrode  27  held by the hook section  31  is changed in the same direction with respect to the dielectric material  26 . The moving direction indicated by the arrow X is a direction orthogonal to a longitudinal direction of a surface of the dielectric material  26  in contact with the discharge electrode  27 . 
   Furthermore, in the present embodiment, the pinion gear  36  is designed to receive driving force of a motor  37  that is so controlled by a control section  38  as to be driven, so that the position of the discharge electrode  27  is switched at an appropriate timing in consideration of deterioration of the dielectric material  26 . 
     FIG. 10  shows the steps of a position switching process by which the control section  38  automatically switches the position of the discharge electrode  27 . 
   First, the control section  38  accumulates discharge time taken in the ion generating device  28  (S 1 ). Next, the control section  38  determines whether or not the accumulated amount of time has reached a threshold amount of time (S 2 ). The “threshold amount of time” here indicates the application limit of the dielectric material  26  as set in consideration of such an area of influence on an image as shown in  FIG. 6 . In cases where the control section  38  determines in S 2  that the accumulated amount of time has not reached the threshold amount of time, the control section  38  returns to S 1  and repeats the process of accumulating discharge time. 
   On the other hand, in cases where the control section  38  determines in S 2  that the accumulated amount of time has reached the threshold amount of time, the control section  38  determines whether or not the number of times of position switching has reached a threshold number of times (S 3 ). The “threshold number of times” here depends on the size of the dielectric material  26  and corresponds to the number of positions that the discharge electrode  27  can take with respect to the dielectric material  26 . If the dielectric material  26  has the same size as in the ion generating device  1  provided with four pairs of hook sections  31 , the number of times of switching is 4. 
   In cases where the control section  38  determines in S 3  that the number of times of position switching has not reached the threshold number of times, the control section  38  drives the motor  37  for a predetermined period of time (S 4 ). The motor  37  thus driven causes the positioning member  35  to move a predetermined distance, so that the position of the discharge electrode  27  is switched (S 4 ). After that, a counter that counts the number of times of switching counts up one (S 5 ), and a counter that counts the accumulated amount of time is cleared (S 6 ). Then, the control section  38  returns to S 1 . 
   On the other hand, in cases where the control section  38  determines in S 3  that the number of times of position switching has reached the threshold number of times, the control section  38  terminates the process. In so doing, the control section  38  may cause a display section (not shown) of the image forming apparatus, for example, to display a message that recommends replacing the ion generating device  28  or replacing a creeping discharge element of the ion generating device  28 . 
   As described above, in the ion generating device  28  provided in the image forming apparatus of the present embodiment, the position of the discharge electrode  27  with respect to the dielectric material  26  can be automatically switched. This makes it possible to automatically switch the position of the discharge electrode  27  with respect to the dielectric material  26 . This enables the device to have excellent maintainability. 
   Embodiment 3 
   Another embodiment of the present invention will be described below with reference to  FIGS. 11 through 13 . For convenience of explanation, members having the same functions as those used in Embodiments 1 and 2 will be given the same reference numerals and will not be described below. 
   An image forming apparatus of the present embodiment differs from the image forming apparatus  100  of Embodiment 1 in terms of ion generating devices that are used as a charging device  2  for charging before a first transfer, a charging device  3  for charging before a second transfer, and a latent image charging device  4 . In each ion generating device  42  provided in the image forming apparatus of the present embodiment, the position of a discharge electrode  27  with respect to a dielectric material  26  can be automatically switched as in the ion generating device  28  of Embodiment 2. 
   The following describes only parts different from those of the ion generating device  28 . As shown in  FIG. 11 , the ion generating device  42  has a cylindrical dielectric material  41  and a discharge electrode  27  disposed so as to make close contact with an outer circumferential surface of the dielectric material  41 . The cylindrical dielectric material  41  has an inductive electrode  43  provided on an inner circumferential surface thereof. The dielectric material  41  and the inductive electrode  43  are arranged so as to be freely rotated by a rotary shaft  44 . 
   In such an arrangement, the rotation of the rotary shaft  44  causes the dielectric material  41  to rotate, so that the position of the dielectric material  41  with respect to the discharge electrode  27  is changed. Also in the present embodiment, the rotary shaft  44  is designed to receive driving force of a motor  37  that is so controlled by a control section  38  as to be driven, so that the position of the dielectric material  41  with respect to the discharge electrode  27  is switched at an appropriate timing in consideration of deterioration of the dielectric material  41 . 
   In such an arrangement, even a small degree of rotation of the dielectric material  41  allows the discharge electrode  27  to face a new area free of deterioration, so that a large number of times of switching can be ensured. Therefore, even when a resin film that is inexpensive but more prone to dielectric breakdown than mica material and ceramic is used as the dielectric material  41 , the life of the ion generating device  42  or a creeping discharge element can be extended. 
   Further, in such an arrangement that the discharge electrode  27  is disposed on the outer circumferential surface of the cylindrical dielectric material  41 , it is preferable that, as shown in  FIG. 12 , the discharge electrode  27  be disposed so as to be skewed with respect to the axis of the dielectric material  41 . By thus disposing the discharge electrode  27  so that the discharge electrode  27  is skewed, the discharge electrode  27  is allowed to make close contact with the outer circumferential surface of the cylindrical dielectric material  41  entirely in the axial direction. 
   The cylindrical dielectric material  41  and the inductive electrode  43  formed on the inner circumferential surface thereof can be arranged, as shown in  FIG. 13(   a ) for example, by covering, with an insulating resin tube  46 , a metal cylinder  45  made of aluminum, stainless steel, or the like. 
   Alternatively, the dielectric material  41  and the inductive electrode  43  can be arranged, as shown in  FIG. 13(   b ), by coating, with ceramic  47 , a metal cylinder  45  made of aluminum, stainless steel, or the like. Alternatively, the dielectric material  41  and the inductive electrode  43  can be arranged, as shown in  FIG. 13(   c ), by coating an inner circumferential surface of a glass tube  48  with metal  49  such as gold. 
   Embodiment 4 
   Another embodiment of the present invention will be described below with reference to  FIGS. 14 and 15 . For convenience of explanation, members having the same functions as those used in Embodiments 1 to 3 will be given the same reference numerals and will not be described below. 
   As shown in  FIG. 14 , an image forming apparatus  101  of the present embodiment is an monochrome image forming apparatus including a single visible-image forming unit  50 . The image forming apparatus  101  does not include an intermediate transfer belt  15 . The image forming apparatus  101  is arranged such that a toner image formed on a photosensitive drum  7  is transferred directly onto a sheet of paper fed from a paper feed cassette device  53 . It should be noted that Reference Numeral  51  indicates an automatic document feeder. 
   The image forming apparatus  101  has a developing device  56  which contains a toner precharging device. As the toner precharging device, the aforementioned ion generating device  1 ,  28 , or  40  is used. 
     FIG. 15  shows an arrangement of the developing device  56  in detail.  FIG. 15  exemplifies an arrangement in which the ion generating device  1  is used as the toner precharging device. The developing device  56  deals with a two-component developer composed of toner and carrier and has a developer tank  57  in which the developer is contained. Disposed in the developer tank  57  are: a development roller  58 , which carries the developer to supply the developer to the photosensitive drum  7 ; a conveyer roller  59 ; a mixing roller  60 ; and a pumping roller  61 . 
   Further, the interior of the developer tank  57  is divided into upper and lower parts by a flow plate  62 , and the developer carried by the surface of the development roller and regulated by a layer-thickness regulating member  64  is returned to a space between the conveyer roller  59  and the mixing roller  60  via the flow plate  62 . 
   In the developing device  56  thus arranged, the toner precharging device  5  is disposed near the layer-thickness regulating member  64 . The toner precharging device  5  charges the developer, regulated by the layer-thickness regulating member  64 , which is returned to the space between the conveyer roller  59  and the mixing roller  60  via the flow plate  62 . With this, even if there is a shortage of triboelectric charging entailed by the conveying and stirring functions of the conveyer roller  59 , the mixing roller  60 , the pumping roller  61 , and the like, the amount of charge of the toner can be increased by making up for the shortage. 
   As described above, an ion generating device of the present invention is an ion generating device for generating ions along with creeping discharge, the ion generating device including: a dielectric material; an inductive electrode disposed on a first surface of the dielectric material; a discharge electrode disposed on a second surface of the dielectric material; an alternating voltage applying section for applying an alternating voltage between the inductive electrode and the discharge electrode; and a DC bias voltage applying section for applying a DC bias voltage to the discharge electrode, the discharge electrode and the dielectric material being provided so as to be movable with respect to each other. 
   According to the foregoing arrangement, the discharge electrode and the dielectric material are provided so as to be movable with respect to each other. Therefore, in cases where an increase in accumulated amount of discharge time causes the aforementioned deterioration in a portion of the dielectric material in contact with the discharge electrode and the deterioration causes a defect in discharge, the portion of the dielectric material in contact with the discharge electrode can be switched to a new area free of deterioration by changing the position of the discharge electrode with respect to the dielectric material. 
   This makes it possible to efficiently use a dielectric material that has conventionally been wastefully disposed with an undeteriorated area remaining thereon. This makes it possible to extend the life of a creeping discharge element or an ion generating device while using a dielectric material of the same size. Moreover, this makes it possible to reduce the total cost including maintenance cost for replacing the creeping discharge element and the ion generating device. 
   The ion generating device of the present invention may be further arranged such that: the inductive electrode is formed on the first surface of the dielectric material; and the discharge electrode is constituted by an electrode member that is narrower than the dielectric material, and is disposed in contact with the second surface of the dielectric material such that both ends of the electrode member in a longitudinal direction of the electrode member are supported by supporting members outside of the dielectric material. 
   This makes it possible to easily realize the ion generating device of the present invention in which the discharge electrode and the dielectric material are provided so as to be movable with respect to each other. 
   In this case, the ion generating device of the present invention may be further arranged such that: the dielectric material has a shape of a plate; one of the ends of the electrode member in the longitudinal direction is supported via an elastic member, the ion generating device further comprising a positioning member, provided between at least one of the supporting members supporting the ends of the electrode member and the dielectric material, which holds the electrode member and determines a position of the discharge electrode with respect to the dielectric material, the positioning member being provided with a plurality of hook sections, provided in parallel with one another along a direction orthogonal to the longitudinal direction of the electrode member on the second surface of the dielectric material, on which the electrode member is hung. 
   According to the foregoing arrangement, the position of the discharge electrode with respect to the plate dielectric material can be switched by a simple operation of switching the hook sections, provided on the positioning member, on which the electrode member constituting the discharge electrode is hung. Further, since one of the ends of the electrode member constituting the discharge electrode is supported via the elastic member, the electrode member has stretchability with respect to the support. This makes it possible to easily switch from one hook section to another. 
   In this case, the ion generating device of the present invention may be further arranged such that: the dielectric material has a shape of a plate; one of the ends of the electrode member in the longitudinal direction is supported via an elastic member, the ion generating device further comprising a positioning member, provided between at least one of the supporting members supporting the ends of the electrode member and the dielectric material, which holds the electrode member and determines a position of the discharge electrode with respect to the dielectric material, the positioning member including a hook section on which the electrode member is hung, the positioning member being arranged to be movable in a direction orthogonal to the longitudinal direction of the electrode member on the second surface of the dielectric material. 
   According to the foregoing arrangement, the position of the discharge electrode with respect to the plate dielectric material can be switched by moving the positioning member having the hook section on which the electrode member constituting the discharge electrode has been hung. Also in this case, since one of the ends of the electrode member constituting the discharge electrode is supported via the elastic member, the electrode member has stretchability. Therefore, the positioning member is not inhibited from moving. 
   Furthermore, when arranged such that the positioning member is movable, the ion generating device can be arranged so as to include a driving mechanism for moving the positioning member. This makes it possible to automatically switch the position of the discharge electrode with respect to the dielectric material. 
   Further, the ion generating device of the present invention can be characterized in that: the dielectric material and the inductive electrode constitute a rotating roller such that the dielectric material has a cylindrical shape and the inductive electrode is formed on an inner circumferential surface of the dielectric material; and the discharge electrode is disposed on an outer circumferential surface of the dielectric material. 
   According to the foregoing arrangement, the position of the dielectric material with respect to the discharge electrode can be changed by rotating the rotating roller. Moreover, the cylindrical dielectric material can ensure a larger amount of space than a plate dielectric material, so that a large number of times of switching can be ensured. Therefore, even when a resin film that is inexpensive but more prone to dielectric breakdown than mica material and ceramic is used as a dielectric material, a long-life ion generating device or a long-life creeping discharge element can be obtained. 
   In this case, the ion generating device is preferably arranged such that one of the ends of the electrode member in the longitudinal direction is supported via an elastic member. 
   According to the foregoing arrangement, since one of the ends of the electrode member constituting the discharge electrode is supported via the elastic member, the electrode member has stretchability with respect to the support. Therefore, although the rotating roller rotates in contact with the discharge electrode, the rotating roller rotates smoothly. 
   Furthermore, in the case of such a rotating roller dielectric material, the ion generating device can be arranged so as to include a driving mechanism for rotating the rotating roller. This makes it possible to automatically switch the position of the discharge electrode with respect to the dielectric material. 
   The rotating roller can be simply arranged, for example, by covering a metal cylinder with an insulating resin tube, by coating an outer circumferential surface of a metal cylinder with ceramic, or by coating an inner circumferential surface of a glass tube with metal. 
   Further, when arranged so as to include the driving mechanism for moving the positioning member or the driving mechanism for rotating the rotating roller, the ion generating device can be arranged so as to further include control means for accumulating discharge time, and for switching relative positions of the dielectric material and the discharge electrode by controlling the driving mechanism in accordance with the discharge time thus accumulated. 
   The dielectric material deteriorates as the accumulated amount of discharge time increases. Therefore, by thus controlling the switching in accordance with the accumulated amount of discharge time, for example, by switching the position of the discharge electrode with respect to the dielectric material when the accumulated amount of time reaches a predetermined amount of time, uniform ion generating performance can be ensured for a long period of time without any work done by a service person or a user. 
   Further, the ion generating device of the present invention is preferably arranged such that the discharge electrode is located below the dielectric material in a vertical direction. 
   In making effective use of the whole area of the dielectric material by switching the relative positions of the discharge electrode and the dielectric material, it is undesirable that the surface of the dielectric material be contaminated by toner and dust. By thus locating the discharge electrode below the dielectric material in a vertical direction, the surface of the dielectric material in contact with the discharge electrode faces downward in a vertical direction. This makes it possible to prevent the surface of the dielectric material in contact with the discharge electrode from being greatly contaminated by toner and dust, as compared with a case where the discharge electrode is located above the dielectric material in a vertical direction. 
   Further, since ions and ozone are heavier than air, ions and ozone are likely to flow downward. Therefore, such an arrangement makes it possible to cause generated ions to efficiently reach a charged object and the like. 
   An image forming apparatus of the present invention is arranged so as to include an ion generating device of the present invention as a pretransfer charging device for giving electric charge to toner carried on a carrier. 
   By using an ion generating device of the present invention as a pretransfer charging device for giving electric charge to toner carried on a carrier, an image of high quality can be obtained while holding down the total cost. 
   An image forming apparatus of the present invention is arranged so as to include an ion generating device of the present invention as a toner precharging device for giving electric charge to toner contained in a developing device for developing an electrostatic latent image. 
   By using an ion generating device of the present invention as a toner precharging device for giving electric charge to toner contained in a developing device for developing an electrostatic latent image, an image of high quality can be obtained while holding down the total cost. 
   As described above, an ion generating device of the present invention is an ion generating device for generating ions along with creeping discharge, the ion generating device including: a dielectric material; an inductive electrode disposed on a first surface of the dielectric material; a discharge electrode disposed on a second surface of the dielectric material; an alternating voltage applying section for applying an alternating voltage between the inductive electrode and the discharge electrode; and a DC bias voltage applying section for applying a DC bias voltage to the discharge electrode, the discharge electrode being constituted by a wire electrode member disposed in contact with the second surface of the dielectric material such that both ends of the electrode member in a longitudinal direction of the electrode member are supported outside of the dielectric material. 
   According to the foregoing arrangement, the discharge electrode is constituted by a wire electrode member disposed in contact with the second surface of the dielectric material such that both ends of the electrode member in a longitudinal direction of the electrode member are supported outside of the dielectric material. This makes it easier to manufacture a creeping discharge element and an ion generating device as compared with a conventional arrangement in which a discharge electrode has been formed directly on a surface of a dielectric material with use of a photolithographic technique. This makes it possible to reduce the cost of manufacturing. 
   Furthermore, the discharge electrode is in the shape of a wire. This makes it easy to efficiently cause creeping discharge entirely on an outer circumferential surface of the discharge electrode. As compared with a conventional arrangement in which a discharge electrode made by patterning an electrode film is provided, it becomes possible to increase the amount of discharge with respect to the alternating voltage applied between the discharge electrode and the inductive electrode. This makes it possible to decrease the alternating voltage applied between the discharge electrode and the inductive electrode. This makes it possible to save electric power and to extend the life of the ion generating device or the creeping discharge element by inhibiting the dielectric material from deteriorating. 
   Further, the discharge electrode is not formed directly on the dielectric material. This makes it easy to change the relative positions of the dielectric material and the discharge electrode. This makes it possible to further extend the life of the ion generating device or the creeping discharge element with efficient use of the dielectric material, for example, by switching the position of the discharge electrode with respect to the dielectric material. 
   Further, the ion generating device of the present invention is preferably arranged such that: the dielectric material has a shape of a flat plate, and is curved such that a surface of the dielectric material surface makes contact with the discharge electrode is convex. 
   The dielectric material is a very thin plate member. Therefore, the dielectric material may be deformed. If the dielectric material is warped in such a manner that the surface of the dielectric material in contact with the discharge electrode is concaved across a central part of the longitudinal direction, the dielectric material and the discharge electrode are prevented from making contact with each other entirely in the longitudinal direction. 
   On the other hand, the dielectric material is shaped from the beginning so as to be warped in such a manner that the surface of the dielectric material in contact with the discharge electrode is convexed across the central part of the longitudinal direction. This makes it possible to bring the discharge electrode into close contact with the dielectric material entirely. 
   Further, the ion generating device of the present invention can be further characterized in that one of the ends of the electrode member in the longitudinal direction is supported via an elastic member. 
   Such an arrangement imparts stretchability to the discharge electrode, so that the discharge electrode does not become loose and can make contact with the dielectric material with tension. 
   Further, the ion generating device of the present invention can be further characterized in that the electrode member has a circular cross-section as cut from a direction orthogonal to an axial direction of the electrode member. 
   The wire electrode member has a circular cross-section. This makes it possible to effectively cause discharge entirely on an outer circumferential surface of the electrode member. Moreover, in the case of the wire electrode member having a circular cross-section, as the curvature is made smaller, the amount of discharge can be made larger. Therefore, the smaller the diameter is, the lower the applied voltage can be. 
   Especially in this case, it is preferable that the diameter of the wire electrode member fall within a range of not less than 20 μm to not more than 100 μm. When the diameter is smaller than 20 μm, the discharge efficiency increases, but the mechanical strength becomes insufficient. This causes problems in terms of life. On the other hand, when the diameter exceeds 100 μm, the discharge efficiency decreases, and the strength of a member for fixing the discharge electrode is required. 
   Further, it is preferable that the electrode member have a discharge current of more than 5 μA in absolute value. For example, in cases where the ion generating device of the present invention is used as the after-mentioned pretransfer charging device for supplementing the amount of discharge of toner existing on the intermediate transfer belt, a discharge current of more than 5 μA in absolute value allows the toner on the intermediate transfer belt to be charged so as to have an amount of charge of not less than −20 μq (in absolute value). This makes is possible to realize high transfer efficiency. 
   Further, the ion generating device of the present invention can be further characterized in that the electrode member is plated with gold. This makes it possible to extend the life of the discharge electrode and to increase the strength of the discharge electrode. 
   The ion generating device of the present invention is preferably arranged such that the dielectric material is made of mica paper made by joining pieces of mica on top of each other with resin. Mica paper is preferable in terms of price, insulating properties, and processability together with such advantages that mica paper is more inexpensive than raw mica, that mica paper is more processable than ceramic, and that mica paper is more resistant to dielectric breakdown than a resin film. 
   An image forming apparatus of the present invention is arranged so as to include an ion generating device of the present invention as a pretransfer charging device for giving electric charge to toner carried on a carrier. 
   By using an ion generating device of the present invention as a pretransfer charging device for imparting appropriate charging properties to toner before a transfer, an image of high quality can be obtained while holding down the cost of manufacturing and the running cost. 
   An image forming apparatus of the present invention is arranged so as to include an ion generating device of the present invention as a toner precharging device for giving electric charge to toner contained in a developing device for developing an electrostatic latent image. 
   By using an ion generating device of the present invention as a toner precharging device for making up for a defect in charging of toner contained in a developing device, an image of high quality can be obtained while holding down the cost of manufacturing and the running cost. 
   The present invention is not limited to the description of the embodiments above, but may be altered by a skilled person within the scope of the claims. An embodiment based on a proper combination of technical means disclosed in different embodiments is encompassed in the technical scope of the present invention. 
   Numerical values falling outside the ranges indicated herein but falling within rational ranges that comply with the gist of the present invention are of course encompassed in the present invention.