Patent Publication Number: US-11378902-B2

Title: Image heating apparatus and heater for use therein

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application is a continuation of U.S. patent application Ser. No. 15/126,959, filed Sep. 16, 2016, which is a National Stage application of International Patent Application No. PCT/JP2015/001482, filed Mar. 17, 2015, which claims the benefit of Japanese Patent Application No. 2014-057058, filed Mar. 19, 2014, Japanese Patent Application No. 2015-012816, filed Jan. 26, 2015, Japanese Patent Application No. 2015-013726, filed Jan. 27, 2015, and Japanese Patent Application No. 2015-015750, filed Jan. 29, 2015, which are hereby incorporated by reference herein in their entirety. 
    
    
     TECHNICAL FIELD 
     The present invention relates to image heating apparatuses and heaters for use therein. More specifically, the present invention relates to an image heating apparatus, such as a fixing apparatus incorporated in an image forming apparatus of an electrophotographic recording type such as a copying machine or a printer, or a gloss applying apparatus for further heating a fixed toner image on a recording material to improve the glossiness of the toner image, and to a heater for use in the image heating apparatus. 
     BACKGROUND ART 
     One of the image heating apparatuses described above is an apparatus that includes an endless belt (also referred to as an endless film), a heater that comes into contact with an inner surface of the endless belt, and a roller cooperative with the heater to form a nip portion therebetween with the endless belt interposed therebetween. Continuous printing on small-size sheets using an image forming apparatus including such an image heating apparatus causes a phenomenon in which a gradual temperature rise occurs in an area of the nip portion through which the sheets do not pass in the longitudinal direction of the nip portion. This phenomenon is referred to as overheating in a no-media passage portion. Too high a temperature of the no-media passage portion may damage components in the apparatus, or may cause toner to be offset to the endless belt in an area of the large-size sheet which corresponds to the no-media passage portion. 
     One of the techniques to suppress the overheating in the no-media passage portion is as follows. A heating resistor (hereinafter referred to as a “heating element”) on a substrate of a heater is formed of a material having a positive temperature coefficient of resistance. Two conductors are disposed at opposite ends of the substrate in a transverse direction of the heater (a direction in which a recording sheet is conveyed) so that current flows through the heating element in the transverse direction (hereinafter referred to as the path of current in the conveyance direction) (see PTL 1). In the concept disclosed in PTL 1, as the temperature of the no-media passage portion increases, the resistance of the heating element in the no-media passage portion increases, suppressing current flowing through the heating element in the no-media passage portion and thus preventing the overheating in the no-media passage portion. The positive temperature coefficient of resistance is a characteristic in which the resistance increases as the temperature increases, and is hereinafter referred to as the PTC. 
     However, also in the heater described above, a certain amount of current flows through the heating element in the no-media passage portion. 
     CITATION LIST 
     Patent Literature 
     [PTL 1] 
     Japanese Patent Laid-Open No. 2011-151003 
     SUMMARY OF INVENTION 
     The present invention provides a heater and an image heating apparatus configured to suppress or at least reduce the overheating in a no-media passage portion of the heater without an increase in the size of the heater. 
     To this end, an aspect of the present invention provides an image heating apparatus which includes an endless belt; a heater configured to be in contact with an inner surface of the endless belt, the heater including a substrate, a first conductor disposed at a first position on the substrate so as to extend in a longitudinal direction of the substrate, a second conductor disposed at a second position on the substrate so as to extend in the longitudinal direction, the second position being different from the first position in a transverse direction of the substrate that is transverse to the longitudinal direction, and a heating element disposed between the first conductor and the second conductor and configured to generate heat by power supplied thereto via the first conductor and the second conductor; and electrical contacts configured to be in contact with electrodes of the heater to supply power to the heating element. The heater has a plurality of independently controllable heating blocks in the longitudinal direction, each of the plurality of independently controllable heating blocks including the first conductor, the second conductor, and the heating element. At least one of electrodes each corresponding to one of the plurality of heating blocks is disposed in an area where the heating element is located in the longitudinal direction on a second surface opposite to a first surface of the heater that comes into contact with the endless belt. The electrical contacts are arranged so as to face the second surface of the heater. 
     Another aspect of the present invention provides a heater which includes a substrate; a first conductor disposed at a first position on the substrate so as to extend in a longitudinal direction of the substrate; a second conductor disposed at a second position on the substrate so as to extend in the longitudinal direction, the second position being different from the first position in a transverse direction of the substrate that is transverse to the longitudinal direction; and a heating element disposed between the first conductor and the second conductor and configured to generate heat by power supplied thereto via the first conductor and the second conductor. The heater has a plurality of independently controllable heating blocks in the longitudinal direction, each of the plurality of independently controllable heating blocks including the first conductor, the second conductor, and the heating element. At least one of electrodes each corresponding to one of the plurality of heating blocks is disposed in an area where the heating element is located in the longitudinal direction. 
     Still another aspect of the present invention provides an image heating apparatus which includes an endless belt; and a heater configured to be in contact with an inner surface of the endless belt, the heater including a substrate, a first conductor disposed at a first position on the substrate so as to extend in a longitudinal direction of the substrate, a second conductor disposed at a second position on the substrate so as to extend in the longitudinal direction, the second position being different from the first position in a transverse direction of the substrate that is transverse to the longitudinal direction, and a heating element disposed between the first conductor and the second conductor and configured to generate heat by power supplied thereto via the first conductor and the second conductor. The heater has a plurality of independently controllable heating blocks in the longitudinal direction, each of the plurality of independently controllable heating blocks including the first conductor, the second conductor, and the heating element. Each of the plurality of heating blocks has a plurality of heating elements in the transverse direction of the substrate. The plurality of heating elements in each of the plurality of heating blocks are also independently controllable. 
     Still another aspect of the present invention provides a heater which includes a substrate; a first conductor disposed at a first position on the substrate so as to extend in a longitudinal direction of the substrate; a second conductor disposed at a second position on the substrate so as to extend in the longitudinal direction, the second position being different from the first position in a transverse direction of the substrate that is transverse to the longitudinal direction; and a heating element disposed between the first conductor and the second conductor and configured to generate heat by power supplied thereto via the first conductor and the second conductor. The heater has a plurality of independently controllable heating blocks in the longitudinal direction, each of the plurality of independently controllable heating blocks including the first conductor, the second conductor, and the heating element. Each of the plurality of heating blocks has a plurality of heating elements in the transverse direction of the substrate. The plurality of heating elements in each of the plurality of heating blocks are also independently controllable. 
     Still another aspect of the present invention provides an image heating apparatus which includes an endless belt; and a heater configured to be in contact with an inner surface of the endless belt, the heater including a substrate, a first heating block disposed on the substrate, and a second heating block disposed on the substrate at a position different from the position of the first heating block in a longitudinal direction of the substrate. The image heating apparatus has a first wire for the second heating block, the first wire being connected to a conductor for supplying power to the second heating block, and a second wire having a first end connected to the conductor to which the first wire for the second heating block is connected at a different position from a position at which the first wire for the second heating block is connected to the conductor, and having a second end connected to a conductor for the first heating block for supplying power to the first heating block. Power is supplied to the first heating block via the conductor to which the first wire for the second heating block is connected and via the second wire. 
     Advantageous Effects of Invention 
     According to some aspects of the present invention, a heater and an image heating apparatus may suppress or reduce the overheating in a no-media passage portion without an increase in the size of the heater. 
     Further features of the present invention will become apparent from the following description of exemplary embodiments with reference to the attached drawings. 
    
    
     
       BRIEF DESCRIPTION OF DRAWINGS 
         FIG. 1  is a cross-sectional view of an image forming apparatus. 
         FIG. 2  is a cross-sectional view of an image heating apparatus according to a first exemplary embodiment. 
         FIG. 3A  is a configuration diagram of a heater according to the first exemplary embodiment. 
         FIG. 3B  is a configuration diagram of the heater according to the first exemplary embodiment. 
         FIG. 3C  is a configuration diagram of the heater according to the first exemplary embodiment. 
         FIG. 4  is a circuit diagram of a control circuit for the heater according to the first exemplary embodiment. 
         FIG. 5  is a flowchart of a heater control process according to the first exemplary embodiment. 
         FIG. 6A  is a diagram depicting the effect of reducing the overheating in a no-media passage portion of the heater according to the first exemplary embodiment. 
         FIG. 6B  is a diagram depicting the effect of reducing the overheating in a no-media passage portion of the heater according to the first exemplary embodiment. 
         FIG. 7A  is a configuration diagram of a heater according to a second exemplary embodiment. 
         FIG. 7B  is a configuration diagram of the heater according to the second exemplary embodiment. 
         FIG. 7C  is a configuration diagram of the heater according to the second exemplary embodiment. 
         FIG. 8  is a circuit diagram of a control circuit for the heater according to the second exemplary embodiment. 
         FIG. 9  is a flowchart of a heater control process according to the second exemplary embodiment. 
         FIG. 10A  is a configuration diagram of a heater according to a third exemplary embodiment. 
         FIG. 10B  is a configuration diagram of the heater according to the third exemplary embodiment. 
         FIG. 11A  is a configuration diagram of a heater according to a fourth exemplary embodiment. 
         FIG. 11B  is a configuration diagram of the heater according to the fourth exemplary embodiment. 
         FIG. 12A  is a configuration diagram of a heater according to a fifth exemplary embodiment. 
         FIG. 12B  is a configuration diagram of the heater according to the fifth exemplary embodiment. 
         FIG. 13A  is a configuration diagram of a heater according to a sixth exemplary embodiment. 
         FIG. 13B  is a configuration diagram of the heater according to the sixth exemplary embodiment. 
         FIG. 13C  is a configuration diagram of the heater according to the sixth exemplary embodiment. 
         FIG. 14A  is a diagram depicting an advantage of a seventh exemplary embodiment. 
         FIG. 14B  is a diagram depicting an advantage of the seventh exemplary embodiment. 
         FIG. 15A  is a configuration diagram of a heater according to the seventh exemplary embodiment. 
         FIG. 15B  is a configuration diagram of the heater according to the seventh exemplary embodiment. 
         FIG. 16A  is a configuration diagram of a heater according to a modification of the seventh exemplary embodiment. 
         FIG. 16B  is a configuration diagram of the heater according to the modification of the seventh exemplary embodiment. 
         FIG. 17A  is a configuration diagram of a heater according to an eighth exemplary embodiment. 
         FIG. 17B  is a configuration diagram of the heater according to the eighth exemplary embodiment. 
         FIG. 18A  is a configuration diagram of a heater according to a ninth exemplary embodiment. 
         FIG. 18B  is a configuration diagram of the heater according to the ninth exemplary embodiment. 
         FIG. 19A  is a configuration diagram of a heater according to a tenth exemplary embodiment. 
         FIG. 19B  is a configuration diagram of the heater according to the tenth exemplary embodiment. 
         FIG. 20A  is a configuration diagram of a heater according to an eleventh exemplary embodiment. 
         FIG. 20B  is a configuration diagram of the heater according to the eleventh exemplary embodiment. 
         FIG. 21A  is a configuration diagram of a heater according to a twelfth exemplary embodiment. 
         FIG. 21B  is a configuration diagram of the heater according to the twelfth exemplary embodiment. 
         FIG. 21C  is a configuration diagram of the heater according to the twelfth exemplary embodiment. 
         FIG. 22  is a circuit diagram of a control circuit for the heater according to the twelfth exemplary embodiment. 
         FIG. 23A  illustrates heater control tables according to the twelfth exemplary embodiment. 
         FIG. 23B  illustrates a heater control table according to the twelfth exemplary embodiment. 
         FIG. 23C  illustrates a heater control table according to the twelfth exemplary embodiment. 
         FIG. 24  is a configuration diagram of a heater according to a thirteenth exemplary embodiment. 
         FIG. 25  is a circuit diagram of a control circuit for the heater according to the thirteenth exemplary embodiment. 
         FIG. 26  illustrates heater control tables according to the thirteenth exemplary embodiment. 
         FIG. 27  illustrates heater control tables according to a modification. 
         FIG. 28  illustrates heater control tables according to another modification. 
         FIG. 29  is a circuit diagram of a control circuit according to a fourteenth exemplary embodiment. 
         FIG. 30A  is a diagram depicting contact portions and wires of a heater according to the fourteenth exemplary embodiment. 
         FIG. 30B  is a diagram depicting the contact portions and wires of the heater according to the fourteenth exemplary embodiment. 
         FIG. 31  is a diagram of wiring according to Comparative Example 1. 
         FIG. 32A  is a configuration diagram of a heater according to a fifteenth exemplary embodiment. 
         FIG. 32B  is a diagram depicting contact portions and wires of the heater according to the fifteenth exemplary embodiment. 
         FIG. 32C  is a diagram depicting the contact portions and wires of the heater according to the fifteenth exemplary embodiment. 
         FIG. 32D  is a diagram depicting the contact portions and wires of the heater according to the fifteenth exemplary embodiment. 
     
    
    
     DESCRIPTION OF EMBODIMENTS 
     First Exemplary Embodiment 
       FIG. 1  is a cross-sectional view of a laser printer (an image forming apparatus)  100  that uses electrophotographic recording technology. In response to the generation of a print signal, laser light modulated in accordance with image information is emitted from a scanner unit  21 , and a photosensitive member  19  which is charged to a predetermined polarity by a charging roller  16  is scanned with the laser light. The laser light (dotted line) emitted from a laser diode  22  within the scanner unit  21  is caused to scan in a main scanning direction via a rotating polygon mirror  23  and a reflecting mirror  24 , and in a sub scanning direction by rotation of the photosensitive member  19 . Accordingly, an electrostatic latent image is formed on the photosensitive member  19 . Toner is supplied to the electrostatic latent image from a developing device  17 , and a toner image corresponding to the image information is formed on the photosensitive member  19 . Recording materials (recording sheets) P in a sheet feed cassette  11  are fed one-by-one by a pickup roller  12 , and a recording material P is conveyed toward a pair of registration rollers  14  by a pair of rollers  13 . The recording material P is further conveyed from the pair of registration rollers  14  to a transfer position at the timing of the toner image on the photosensitive member  19  arriving at the transfer position. The transfer position is located between the photosensitive member  19  and a transfer roller  20 . While the recording material P travels through the transfer position, the toner image on the photosensitive member  19  is transferred onto the recording material P. The recording material P is then heated by an image heating apparatus  200  so that the toner image is fixed to the recording material P by heat. The recording material P that carries the fixed toner image is fed by pairs of rollers  26  and  27  and is discharged into an upper tray of the laser printer  100 . A cleaner  18  cleans the photosensitive member  19 . A feed tray (manual feed tray)  28  has a pair of recording material regulating plates whose width is adjustable in accordance with the size of a recording material P. The feed tray  28  is provided to support recording materials P having non-standard sizes as well as standard sizes. A pair of pickup rollers  29  feeds a recording material P from the feed tray  28 . A motor  30  drives the image heating apparatus  200  and so on. A control circuit  400  is connected to a commercial alternating current (AC) power supply  401 , and power is supplied from the control circuit  400  to the image heating apparatus  200 . The photosensitive member  19 , the charging roller  16 , the scanner unit  21 , the developing device  17 , and the transfer roller  20  form an image forming unit that forms an unfixed image on a recording material P. A process cartridge  15  integrally includes the charging roller  16 , the developing device  17 , the cleaner  18 , and the photosensitive member  19 . 
     The laser printer  100  according to this exemplary embodiment supports a plurality of recording material sizes. The sheet feed cassette  11  is configured to hold sheets of letter size (approximately 216 mm×279 mm), legal size (approximately 216 mm×356 mm), A4 size (210 mm×297 mm), and executive size (approximately 184 mm×267 mm). The sheet feed cassette  11  is also configured to hold sheets of JIS (Japanese Industrial Standard) B5 size (182 mm×257 mm) and A5 size (148 mm×210 mm). 
     In addition, media in non-standard sizes including DL envelopes (110 mm×220 mm) and Commercial number 10 (COM-10) envelopes (approximately 105 mm×241 mm) may also be fed from the feed tray  28  and are printable. The printer  100  according to this exemplary embodiment is a basically vertical-feed laser printer (designed to convey a sheet in such a manner that the longer sides of the sheet are parallel to the conveyance direction of the sheet). A letter size sheet and a legal size sheet are recording materials having the largest width (or a large width) among the widths of recording materials in the standard sizes (nominal recording material widths) that the image forming apparatus  100  supports, and have a width of approximately 216 mm. In this exemplary embodiment, a recording material P having a smaller width than the maximum size that the image forming apparatus  100  supports is defined as a small-size sheet. 
       FIG. 2  is a cross-sectional view of the image heating apparatus  200 . The image heating apparatus  200  includes a cylindrical film (endless belt)  202 , a heater  300  that comes into contact with an inner surface of the film  202 , and a pressure roller (a nip portion forming member)  208  cooperative with the heater  300  to form a fixing nip portion N therebetween with the film  202  interposed therebetween. The film  202  has a base layer composed of heat-resistant resin such as polyimide or metal such as stainless steel. The film  202  also has a top layer which may be formed of an elastic layer of heat-resistant rubber or the like. The pressure roller  208  has a core metal  209  formed of a material such as iron or aluminum, and an elastic layer  210  formed of a material such as silicone rubber. The heater  300  is held in a holding member  201  made of heat-resistant resin. The holding member  201  has a guide function to guide the rotation of the film  202 . The pressure roller  208  is driven by the motor  30  to rotate in a direction indicated by an arrow. As the pressure roller  208  rotates, the film  202  rotates in association with the rotation of the pressure roller  208 . A recording material P that carries an unfixed toner image is conveyed while being held in the fixing nip portion N, and is heated to undergo fixing. 
     As illustrated in  FIG. 3A , the heater  300  includes a ceramic substrate  305  on which a heating element for use in heating is disposed. Thermistors TH 1 , TH 2 , TH 3 , and TH 4  serving as temperature sensing elements are disposed on a back surface of the substrate  305  in contact with a sheet (or media) passage area in the laser printer  100 . A safety element  212  activated in response to an abnormal temperature rise in the heater  300  to shut off the power supply to the heater  300 , such as a thermo-switch and a thermal fuse, is also disposed on the back surface of the substrate  305 . A metal stay  204  is disposed to apply the pressure exerted by a spring (not illustrated) to the holding member  201 . 
       FIGS. 3A to 3C  are configuration diagrams of the heater  300  according to the first exemplary embodiment. The configuration of the heater  300  and the effect of reducing the overheating in a no-media passage portion will be described with reference to  FIGS. 3A to 3C  and  FIGS. 6A and 6B . 
       FIG. 3A  is a diagram of a cross section of the heater  300  in its transverse direction. The heater  300  includes a first conductor  301  disposed on a first layer of a back surface thereof (i.e., the surface opposite to the surface that comes into contact with the endless belt  202 ) (hereinafter also referred to as the “first back surface layer”) so as to extend in the longitudinal direction of the heater  300  on the substrate  305 . The heater  300  further includes a second conductor  303  disposed on the substrate  305  at a position different from the position of the first conductor  301  in the transverse direction of the heater  300  so as to extend in the longitudinal direction of the heater  300 . The first conductor  301  is separated into a conductor  301   a  located upstream and a conductor  301   b  located downstream in the conveyance direction of the recording material P. 
     The heater  300  further includes a heating element  302  disposed between the first conductor  301  and the second conductor  303  for generating heat by power supplied via the first conductor  301  and the second conductor  303 . The heating element  302  is separated into a heating element  302   a  located upstream and a heating element  302   b  located downstream in the conveyance direction of the recording material P. 
     An asymmetric heat generation distribution in the transverse direction of the heater  300  (i.e., the conveyance direction of the recording material P) causes an increase in the stress generated in the substrate  305  while the heater  300  generates heat. The increased stress generated in the substrate  305  may crack the substrate  305 . To avoid cracking of the substrate  305 , the heating element  302  is separated into the heating element  302   a  located upstream and the heating element  302   b  located downstream in the conveyance direction to make the heat generation distribution symmetrical in the transverse direction of the heater  300 . 
     The heater  300  also includes an insulating (in this exemplary embodiment, glass) surface protective layer  307  disposed on a second layer of the back surface thereof (hereinafter also referred to as the “second back surface layer”) so as to cover the heating element  302 , the first conductor  301 , and the second conductor  303 . The heater  300  further includes a glass-coated or polyimide-coated slidable surface protective layer  308  disposed on a first layer of a sliding surface thereof (i.e., the surface that comes into contact with the endless belt  202 ) (hereinafter also referred to as the “first sliding surface layer”). 
       FIG. 3B  is a plan view of individual layers of the heater  300 . The heater  300  has a plurality of heating blocks on the first layer of the back surface thereof that are arranged in the longitudinal direction of the heater  300 , each heating block including the first conductor  301 , the second conductor  303 , and the heating element  302 . By way of example, the heater  300  according to this exemplary embodiment has a total of three heating blocks disposed in the center portion and opposite end portions thereof in the longitudinal direction of the heater  300 . A first heating block  302 - 1  includes heating elements  302   a - 1  and  302   b - 1  that are symmetrical to each other in the transverse direction of the heater  300 . Also, a second heating block  302 - 2  includes heating elements  302   a - 2  and  302   b - 2 , and a third heating block  302 - 3  includes heating elements  302   a - 3  and  302   b - 3 . 
     The first conductor  301  extends in the longitudinal direction of the heater  300 . The first conductor  301  is composed of the conductor  301   a , which is connected to the individual heating elements ( 302   a - 1 ,  302   a - 2 , and  302   a - 3 ), and the conductor  301   b , which is connected to the individual heating elements ( 302   b - 1 ,  302   b - 2 , and  302   b - 3 ). 
     The second conductor  303  extends in the longitudinal direction of the heater  300 , and is separated into three conductors  303 - 1 ,  303 - 2 , and  303 - 3 . 
     Electrodes E 1 , E 2 , E 3 , E 4 - 1 , and E 4 - 2  are each connected to an electrical contact for supplying power from the control circuit  400  for the heater  300 , described below. The electrode E 1  is an electrode for feeding electric power to the heating block  302 - 1  via the conductor  303 - 1 . The electrode E 2  is an electrode used to feed electric power to the heating block  302 - 2  via the conductor  303 - 2 . The electrode E 3  is an electrode for feeding electric power to the heating block  302 - 3  via the conductor  303 - 3 . The electrodes E 4 - 1  and E 4 - 2  are electrodes connected to a common electrical contact to feed electric power to the three heating blocks  302 - 1  to  302 - 3  via the conductor  301   a  and the conductor  301   b.    
     Since the resistance of the individual conductors is not zero, the conductors affect the heat generation distribution in the longitudinal direction of the heater  300 . Accordingly, the electrodes E 4 - 1  and E 4 - 2  are disposed at opposite ends of the heater  300  in the longitudinal direction of the heater  300  so that a heat generation distribution that is symmetrical in the longitudinal direction of the heater  300  can be obtained even when affected by the electrical resistance of the conductors  303 - 1 ,  303 - 2 ,  303 - 3 ,  301   a , and  301   b.    
     Further, the surface protective layer  307  on the second layer of the back surface of the heater  300  is formed to have openings at positions corresponding to the electrodes E 1 , E 2 , E 3 , E 4 - 1 , and E 4 - 2 , so that each of the electrodes E 1 , E 2 , E 3 , E 4 - 1 , and E 4 - 2  can be connected to the corresponding one of the electrical contacts from the back surface side of the heater  300 . In this exemplary embodiment, the electrodes E 1 , E 2 , E 3 , E 4 - 1 , and E 4 - 2  are disposed on the back surface of the heater  300  to enable power supply from the back surface side of the heater  300 . In addition, the ratio of the power to be supplied to at least one heating block among a plurality of heating blocks to the power to be supplied to the other heating blocks is made variable. Electrodes disposed on the back surface of the heater  300  do not require wiring of a conductive pattern on the substrate  305 , resulting in a reduction in the width of the substrate  305  in its transverse direction. This advantageously reduces the cost of the material of the substrate  305 , and reduces the warm-up time taken for the heater  300  increase its temperature due to the reduced heat capacity of the substrate  305 . The electrodes E 1 , E 2 , and E 3  are disposed in an area where heating elements are disposed in the longitudinal direction of the substrate  305 . Further, the surface protective layer  308  on the first layer of the sliding surface of the heater  300  is disposed in an area that is slidably engaged with the film  202 . 
     As illustrated in  FIG. 3C , the holding member  201  of the heater  300  has holes HTH 1  to HTH 4 , H 212 , HE 1 , HE 2 , HE 3 , HE 4 - 1 , and HE 4 - 2  for the thermistors (temperature sensing elements) TH 1  to TH 4 , the safety element  212 , and the electrical contacts of the electrodes E 1 , E 2 , E 3 , E 4 - 1 , and E 4 - 2 , respectively. 
     The thermistors (temperature sensing elements) TH 1  to TH 4 , the safety element  212 , and the electrical contacts that come into contact with the electrodes E 1 , E 2 , E 3 , E 4 - 1 , and E 4 - 2 , described above, are disposed between the stay  204  and the holding member  201 . The electrical contacts are represented by C 1 , C 2 , C 3 , C 4 - 1 , and C 4 - 2 . In  FIG. 3C , broken lines connected to the electrical contacts C 1  to C 3 , C 4 - 1 , and C 4 - 2  and broken lines connected to the safety element  212  indicate power feed cables (AC lines). Further, broken lines connected to the temperature sensing elements TH 1  to TH 4  indicate signal lines (DC lines). The individual elements and electrical contacts are arranged so as to face the back surface of the heater  300 . The electrical contacts C 1 , C 2 , C 3 , C 4 - 1 , and C 4 - 2  that come into contact with the electrodes E 1 , E 2 , E 3 , E 4 - 1 , and E 4 - 2  are electrically connected to electrode units of the heater  300  by being urged by a spring, welding, or any other suitable method. The electrical contacts C 1 , C 2 , C 3 , C 4 - 1 , and C 4 - 2  are connected to the control circuit  400  for the heater  300 , described below, via the cables (indicated by the broken lines described above) disposed between the stay  204  and the holding member  201  or via a conductive material such as a thin metal plate. 
     Power to the heater  300  is controlled in accordance with the output of the thermistor TH 1  disposed near the center of a media passage portion (i.e., near a conveyance reference position X described below). The thermistor TH 4  detects the temperature at an end of a heating area of the heating block  302 - 2  (i.e., the temperature at the end of the heating area in a state illustrated in  FIG. 6B ). The thermistor TH 2  detects the temperature at an end of a heating area of the heating block  302 - 1  (i.e., the temperature at the end of the heating area in a state illustrated in  FIG. 6A ). The thermistor TH 3  detects the temperature at an end of a heating area of the heating block  302 - 3  (i.e., the temperature at the end of the heating area in the state illustrated in  FIG. 6A ). 
     In the image heating apparatus  200  according to this exemplary embodiment, one or more thermistors are provided for each of the three heating blocks  302 - 1  to  302 - 3  to sense the state of power supply to only the single heating blocks due to failure or the like, in order to increase the safety of the image heating apparatus  200 . To take into account only failure of a triac  416  and a triac  426 , one or more thermistors may be provided for at least each of a plurality of independently controllable heating blocks (for example, in  FIG. 3C , only the thermistors TH 1  and TH 2  may be used). In this exemplary embodiment, one or more thermistors are provided for each of the three heating blocks  302 - 1  to  302 - 3  to take into account, in addition to failure of the triac  416  and the triac  426 , a defect of electrical contacts to individual electrodes. For example, if the connection of the electrical contact C 1  to the electrode E 1  is defective, no power is supplied to the heating block  302 - 1 , whereas power may be supplied to the heating block  302 - 3 . To suppress this inconvenience, the thermistors TH 2  and TH 3  are provided for the heating block  302 - 1  and the heating block  302 - 3 , respectively. 
     The safety element  212  is disposed in contact with a portion corresponding to an available minimum size media passage area set in the laser printer  100  (i.e., a portion near the center of the heating block  302 - 2 ), which is less affected by the overheating in the no-media passage portion, in order to prevent a malfunction caused by the overheating in the no-media passage portion. Accordingly, the temperature of the safety element  212  is low during the normal operation, and thus the operating temperature of the safety element  212  can be set low, providing an increase in the safety of the image heating apparatus  200 . 
     Next, the effect of reducing the overheating in the no-media passage portion of the heater  300  will be described with reference to  FIGS. 6A and 6B .  FIG. 6A  is a diagram depicting overheating in a no-media passage portion in a case where power is supplied to all the three heating blocks  302 - 1  to  302 - 3 . In the illustration, by way of example, a B5 size sheet is conveyed vertically with respect to the center portion of the heating area. A reference position for conveying the recording material P is defined as a conveyance reference position X of the recording material P. 
     The sheet feed cassette  11  has a position regulating plate for regulating the position of the recording material P, and is set in a predetermined position in accordance with each size of the recording material P loaded in the sheet feed cassette  11 , from which a recording material P is fed and conveyed so that the recording material P travels through a predetermined position in the image heating apparatus  200 . The feed tray  28  also has a position regulating plate for regulating the position of the recording material P, from which a recording material P is conveyed so that the recording material P travels through the predetermined position in the image heating apparatus  200 . 
     The heater  300  has a heating area length of 220 mm for a sheet width of approximately 216 mm in order to support the vertical conveyance of a letter size sheet. In a case where a B5 size sheet having a sheet width of 182 mm is vertically conveyed in the heater  300  that has a heating area length of 220 mm, 19-mm no-media passage areas are produced in opposite end portions of the heating area. While power supply to the heater  300  is controlled so that the sensing temperature of the thermistor TH 1  located near the center of the media passage portion is maintained at a target temperature, the temperature of the no-media passage portions increases compared to the media passage portion since the heat is not absorbed by the sheet in the no-media passage portions. As illustrated in  FIG. 6A , in the case of a B5 size sheet, the ends of the recording material P pass through portions of the heating block  302 - 1  and  302 - 3  located in the opposite end portions, resulting in no-media passage portions each having a length of 19 mm being produced in the opposite end portions. Since the heating element  302  is a PTC element, the resistance of the heating elements in the no-media passage portions becomes higher than that of the heating elements in the media passage portion, which impedes the flow of current. On the basis of this principle, overheating in the no-media passage portions may be suppressed or reduced. 
       FIG. 6B  is a diagram depicting overheating in a no-media passage portion in a case where power is supplied to only the heating block  302 - 2  located in the center portion of the heater  300 . In the illustration, by way of example, a DL size envelope having a width of 110 mm is conveyed vertically with respect to the center portion of the heating area. The heating block  302 - 2  of the heater  300  has a heating area length of 157 mm for sheets having a width of 148 mm in order to support the vertical conveyance of an A5 size sheet. In a case where a DL size envelope having a width of 110 mm is vertically conveyed in the heater  300  in which the heating block  302 - 2  located in the center has a length of 157 mm, 23.5-mm no-media passage areas are produced in opposite end portions of the center heating block  302 - 2 . The heater  300  is controlled based on the output of the thermistor TH 1  located near the center of the media passage portion, and the temperature of the no-media passage portions increases compared to the media passage portion since the heat is not absorbed by the sheet in the no-media passage portions. In the state illustrated in  FIG. 6B , power is initially supplied to only the heating block  302 - 2  to reduce the influence of the no-media passage areas. In general, the longer the no-media passage area, the higher the overheating in the no-media passage portions. Thus, only the effect of feeding electric power to the heating element  302 , which is a PTC element, in the conveyance direction would not sufficiently reduce the overheating in the no-media passage portion. Accordingly, as illustrated in  FIG. 6B , it is effective to reduce the length of the no-media passage areas as much as possible. In addition, overheating in the 23.5-mm no-media passage areas in the opposite end portions of the center heating block  302 - 2  may be suppressed or reduced on the basis of the principle similar to that described with reference to  FIG. 6A . 
     As illustrated in  FIG. 6B , the effect of reducing the overheating in a no-media passage portion in a case where power is supplied to only the heating block  302 - 2  located in the center portion of the heater  300  can also be obtained in a case where the heating element  302  is not a PTC element. Accordingly, this exemplary embodiment is not limited to the case where a PTC element is used as the heating element  302 . In addition, the configuration according to this exemplary embodiment is also applicable to the case where the heating element  302  has a zero temperature coefficient of resistance or has a negative temperature coefficient of resistance (NTC). 
       FIG. 4  is a circuit diagram of the control circuit  400  for the heater  300  according to the first exemplary embodiment. The commercial AC power supply  401  is connected to the laser printer  100 . Power to the heater  300  is controlled by conducting or non-conducting of the triac  416  and the triac  426 . The triac  416  and the triac  426  are controlled to make the heating blocks  302 - 1  and  302 - 3  and the heating block  302 - 2  controllable independently from each other. Power is supplied to the heater  300  via the electrodes E 1  to E 3 , E 4 - 1 , and E 4 - 2 . In this exemplary embodiment, by way of example, the heating elements  302   a - 1  and  302   b - 1  have a resistance of 140 ohms, the heating elements  302   a - 2  and  302   b - 2  have a resistance of 28 ohms, and the heating elements  302   a - 3  and  302   b - 3  have a resistance of 140 ohms. 
     A zero-crossing detection unit  430  is a circuit for detecting the zero crossing of the AC power supply  401 , and outputs a ZEROX signal to a central processing unit (CPU)  420 . The ZEROX signal is used to control the heater  300 . A relay  440  is used as a power shutoff unit for interrupting the supply of power to the heater  300 . The relay  440  is activated in accordance with the output from the thermistors TH 1  to TH 4  (to shut off power supply to the heater  300 ) in response to an excessive rise in the temperature of the heater  300  due to failure or the like. 
     When an RLON 440  signal is high, a transistor  443  is turned on, causing the secondary coil of the relay  440  to conduct current from a power supply voltage Vcc 2  to turn on the primary contact of the relay  440 . When the RLON 440  signal is Low, the transistor  443  is turned off, blocking the current flow to the secondary coil of the relay  440  from the power supply voltage Vcc 2  to turn off the primary contact of the relay  440 . 
     Next, the operation of a safety circuit that includes the relay  440  will be described. If one of the sensing temperatures obtained by the thermistors TH 1  to TH 4  exceeds a corresponding one of predetermined values that are individually set, a comparison unit  441  activates a latch unit  442 , and the latch unit  442  latches an RLOFF signal at a low level. When the RLOFF signal is low, the transistor  443  is maintained in an off condition even if the CPU  420  sets the RLON 440  signal high. Thus, the relay  440  is maintained in an off condition (or safe condition). 
     If none of the sensing temperatures obtained by the thermistors TH 1  to TH 4  exceeds the predetermined values that are individually set, the RLOFF signal of the latch unit  442  becomes open. Thus, the CPU  420  sets the RLON 440  signal high, thereby turning on the relay  440  to enable power supply to the heater  300 . 
     Next, the operation of the triac  416  will be described. Resistors  413  and  417  are bias resistors for the triac  416 , and a phototriac coupler  415  is a device for ensuring a primary-secondary creepage distance. A light-emitting diode of the phototriac coupler  415  is caused to conduct current to turn on the triac  416 . A resistor  418  is a resistor for limiting the current flow through the light-emitting diode of the phototriac coupler  415  from the power supply voltage Vcc, and the phototriac coupler  415  is turned on or off by a transistor  419 . The transistor  419  operates in accordance with a FUSER 1  signal from the CPU  420 . 
     When the triac  416  is in its conducting state, power is supplied to the heating elements  302   a - 2  and  302   b - 2 , and power is supplied to a resistor with a combined resistance of 14 ohms. Power control with the triac  416  and the triac  426  in a conduction ratio of 1:0 provides the state illustrated in  FIG. 6B  when only the heating elements  302   a - 2  and  302   b - 2  are supplied with power. 
     The circuit operation of the triac  426  is substantially the same as that of the triac  416 , and is not described herein. The triac  426  operates in accordance with a FUSER 2  signal from the CPU  420 . When the triac  426  is in its conducting state, power is supplied to the heating elements  302   a - 1 ,  302   b - 1 ,  302   a - 3 , and  302   b - 3 . Since the four heating elements  302   a - 1 ,  302   b - 1 ,  302   a - 3 , and  302   b - 3  are connected in parallel, power is supplied to a resistor with a combined resistance of 35 ohms. 
     In the state illustrated in  FIG. 6A , power is supplied using the triac  416  and the triac  426 . When the triac  416  and the triac  426  are in their conducting state, power is supplied to the heating elements  302   a - 1 ,  302   b - 1 ,  302   a - 2 ,  302   b - 2 ,  302   a - 3 , and  302   b - 3 . Since the six heating elements  302   a - 1 ,  302   b - 1 ,  302   a - 2 ,  302   b - 2 ,  302   a - 3 , and  302   b - 3  are connected in parallel, power is supplied to a resistor with a combined resistance of 10 ohms. Power control with the triac  416  and the triac  426  in a conduction ratio of 1:1 provides the state illustrated in  FIG. 6A . 
     The total resistance of the heater  300  is generally designed so as to support the power required for recording materials P having the maximum width available (in this exemplary embodiment, letter size sheets and legal size sheets). In the configuration according to this exemplary embodiment, a total resistance of 14 ohms is obtained in the state illustrated in  FIG. 6B , which is higher than a total resistance of 10 ohms which is obtained in the state illustrated in  FIG. 6A , and is more advantageous in terms of harmonic standards, flicker, and safety protection for the heater  300  (in general, the lower the resistance, the worse the problem). For example, it is assumed that the resistance of a heater including three heating blocks ( 302 - 1 ,  302 - 2 , and  302 - 3 ) which are connected in series is adjusted to 10 ohms. In this configuration, if power is supplied to only the heating block  302 - 2  in the center portion of the heater, the total resistance of the heater decreases, which is disadvantageous in terms of harmonic standards, flicker, and safety protection for the heater  300 . In the configuration according to this exemplary embodiment, a plurality of heating blocks (in this exemplary embodiment, three heating blocks) that are separate in the longitudinal direction of the heater  300  are connected in parallel, which is advantageous in reducing harmonics, flicker, and the like. 
     Next, a method for controlling the temperature of the heater  300  will be described. The temperature sensed by the thermistor TH 1  is sensed as a divided voltage of a resistor (not illustrated), and is supplied to the CPU  420  as a TH 1  signal (the temperatures sensed by the thermistors TH 2  to TH 4  are also sensed and supplied to the CPU  420  using a similar way). In the internal processing of the CPU (control unit)  420 , the power to be supplied is calculated based on the sensing temperature of the thermistor TH 1  and the set temperature of the heater  300  in accordance with, for example, proportion-integral (PI) control. The power to be supplied is further converted into a control level of a phase angle (phase control) or a wave number (wave-number control) corresponding to the power to be supplied, and the triac  416  and the triac  426  are controlled in accordance with this control condition. In this exemplary embodiment, the heater temperature sensed by the thermistor TH 1  is used for temperature control of the heater  300 . The temperature of the film  202  may also be sensed by a thermistor or a thermopile, and the sensed temperature may be used for temperature control of the heater  300 . 
       FIG. 5  is a flowchart depicting the control sequence for the image heating apparatus  200 , which is performed by the CPU  420 . In response to the occurrence of a print request in S 501 , in S 502 , the relay  440  is turned on. Then, in S 503 , it is determined whether or not the recording material has a width greater than or equal to 157 mm. In the laser printer  100  according to this exemplary embodiment, the process proceeds to S 504  if the recording material is a letter size sheet, a legal size sheet, an A4 size sheet, an executive size sheet, a B5 size sheet, or a non-standard size medium having a width greater than or equal to 157 mm which is fed from the feed tray  28 . Then, the conduction ratio of the triac  416  to the triac  426  is set to 1:1 (the state illustrated in  FIG. 6A . 
     If the recording material has a width less than 157 mm (in this exemplary embodiment, an A5 size sheet, a DL envelope, a COM-10 envelope, or a non-standard size medium having a width less than 157 mm), the process proceeds to S 505 . Then, the conduction ratio of the triac  416  to the triac  426  is set to 1:0 (the state illustrated in  FIG. 6B ). 
     The determination of the width of the recording material in S 503  may be based on any method, for example, using sheet-width sensors provided for the sheet feed cassette  11  and the feed tray  28 , or using a sensor such as a flag provided on the path along which the recording material P is conveyed. Other methods available are based on width information on the recording material P which is set by a user, image information for forming an image on the recording material P, or the like. 
     In S 506 , the process speed for forming an image is set to full speed by using the set conduction ratio, and a fixing process is performed at a target temperature of 200 degrees Celsius which is set for the thermistor TH 1 . 
     In S 507 , it is determined whether a maximum temperature TH 2 Max of the thermistor TH 2 , a maximum temperature TH 3 Max of the thermistor TH 3 , and a maximum temperature TH 4 Max of the thermistor TH 4 , which are set in the CPU  420 , are not exceeded. If it is detected that the temperature at an end of the heating area exceeds the corresponding one of the predetermined upper limit values on the basis of the thermistor signals TH 2  to TH 4  due to the deterioration of the overheating in a no-media passage portion, the process proceeds to S 509 . In S 509 , the process speed for forming an image is set to half speed, and a fixing process is performed at a target temperature of 170 degrees Celsius which is set for the thermistor TH 1 . The processing of S 509  is iterated to continue the fixing process until the completion of the print job is sensed in S 510 . Setting the process speed for forming an image to half speed achieves fixability at a lower temperature than that for full speed. Thus, the target temperature for fixing operation can be reduced, and the temperature at the no-media passage portions can be reduced. If it is determined in S 507  that none of the temperatures of the respective thermistors exceeds the associated maximum temperature, the process proceeds to S 508 . Until the print job is completed in S 508 , the processing from S 506  is iterated to continue the fixing process. 
     The process described above is repeatedly performed. If the completion of the print job is detected in S 508  or S 510 , then, in S 511 , the relay  440  is turned off. In S 512 , the control sequence of image formation ends. 
     In the control according to this exemplary embodiment, the conduction ratio of the triac  416  to the triac  426  is set based on width information on the recording material P to control a heat generation distribution in the longitudinal direction of the heater  300 . Other methods are also available, examples of which include controlling a heat generation distribution in the longitudinal direction of the heater  300  on the basis of the temperatures sensed by the individual thermistors associated with the respective heating blocks. In a specific example, power to the heating block  302 - 2  may be controlled based on the temperature sensed by the thermistor TH 1 , by using the triac  416  in accordance with PI control or the like. Alternatively, power to the heating block  302 - 1  and the heating block  302 - 3  may be controlled based on the temperature sensed by the thermistor TH 2  or the thermistor TH 3 , by using the triac  426  in accordance with PI control or the like. An optimum control method may be used in accordance with the configuration of the image heating apparatus  200  (such as the number of heating blocks of the heater  300  and the positions of the thermistors) and the specification of the image forming apparatus  100  (such as a type of recording material that the image forming apparatus  100  supports). 
     As described above, the use of the heater  300  and the image heating apparatus  200  according to the first exemplary embodiment may suppress or reduce the overheating in a no-media passage portion in a case where a sheet having a smaller size than the maximum size that the image forming apparatus  100  supports is to be printed. In addition, the symmetry of the heat generation distribution in the transverse direction of the heater  300  may be improved to reduce the thermal stress of the substrate  305 . In addition, the symmetry of the heat generation distribution in the longitudinal direction of the heater  300  may be improved to reduce the non-uniformity in the heat generation distribution in the longitudinal direction of the heater  300 . In the heater  300  according to this exemplary embodiment, furthermore, electrodes disposed on the back surface of the heater  300  do not require wiring of a conductive pattern on the substrate  305 . Accordingly, the number of heating blocks in the longitudinal direction of the heater  300 , the number of electrodes, and the number of triacs for controlling the heat generation distribution in the longitudinal direction of the heater  300  may be increased without an increase in the width of the heater  300  in its transverse direction. In addition, the number of ways in which the heat generation distribution in the longitudinal direction of the heater is switchable may be increased to obtain a heat generation distribution in the longitudinal direction of the heater that is optimized for a larger number of widths of recording materials P. Thus, the heater  300  may reduce the width of the substrate  305  in its transverse direction, and, advantageously, reduce the cost of the material of the substrate  305  and reduce the warm-up time of the image heating apparatus  200  due to the reduction in the heat capacity of the substrate  305 . Moreover, one or more thermistors provided for each of a plurality of heating blocks may increase safety while the image heating apparatus  200  is in a failure state. 
     Second Exemplary Embodiment 
     Next, a second exemplary embodiment will be described. In the second exemplary embodiment, the heater  300  described in the first exemplary embodiment, which is incorporated in the image heating apparatus  200  of the laser printer  100 , the holding member  201  of the heater  300 , and the control circuit  400  for the heater  300  are modified. Components similar to those in the first exemplary embodiment are assigned the same numerals and are not described herein. A heater  700  according to the second exemplary embodiment is configured to switch the heat generation distribution in the longitudinal direction of the heater  700  in four ways.  FIGS. 7A to 7C  are configuration diagrams of the heater  700  according to the second exemplary embodiment.  FIG. 7A  is a diagram of a cross section of the heater  700  in its transverse direction. 
     The heater  700  includes a first conductor  701  disposed on the substrate  305  so as to extend in the longitudinal direction of the heater  700 , and a second conductor  703  disposed on the substrate  305  at a different position from the position of the first conductor  701  in the transverse direction of the heater  700  so as to extend in the longitudinal direction of the heater  700 . The first conductor  701  is separated into a conductor  701   a  located upstream and a conductor  701   b  located downstream in the conveyance direction of the recording material P. 
     The heater  700  further includes a heating element  702  disposed between the first conductor  701  and the second conductor  703  for generating heat by power supplied via the first conductor  701  and the second conductor  703 . The heating element  702  is separated into a heating element  702   a  located upstream and a heating element  702   b  located downstream in the conveyance direction of the recording material P. 
       FIG. 7B  is a plan view of individual layers of the heater  700 . The heater  700  has a plurality of heating blocks on the first layer of the back surface thereof that are arranged in the longitudinal direction of the heater  700 , each heating block including the first conductor  701 , the second conductor  703 , and the heating element  702 . By way of example, the heater  700  according to this exemplary embodiment has a total of seven heating blocks  702 - 1  to  702 - 7  disposed in the center portion and opposite end portions thereof in the longitudinal direction of the heater  700 . 
     The heating blocks  702 - 1  to  702 - 7  include heating elements  702   a - 1  to  702   a - 7  and heating elements  702   b - 1  to  702   b - 7  that are symmetrical in the transverse direction of the heater  700 . The first conductor  701  is composed of the conductor  701   a , which is connected to the individual heating elements ( 702   a - 1  to  702   a - 7 ), and the conductor  701   b , which is connected to the individual heating elements ( 702   b - 1  to  702   b - 7 ). Similarly, the second conductor  703  is separated into seven conductors  703 - 1  to  703 - 7 . 
     Electrodes E 1  to E 7 , E 8 - 1 , and E 8 - 2  are each used to connect to an electrical contact used to supply power from a control circuit  800  for the heater  700 , described below. The electrodes E 1  to E 7  are electrodes for supplying power to the heating blocks  702 - 1  to  702 - 7  via the conductors  703 - 1  to  703 - 7 , respectively. The electrodes E 8 - 1  and E 8 - 2  are electrodes used to connect to a common electrical contact to feed electric power to the seven heating blocks  702 - 1  to  702 - 7  via the conductor  701   a  and the conductor  701   b , respectively. 
     The heater  700  further includes a surface protective layer  707  on the second layer of the back surface thereof. The surface protective layer  707  is formed to have openings at positions corresponding to the electrodes E 1 , E 2 , E 3 , E 4 , E 5 , E 6 , E 7 , E 8 - 1 , and E 8 - 2 , so that the electrodes E 1 , E 2 , E 3 , E 4 , E 5 , E 6 , E 7 , E 8 - 1 , and E 8 - 2  can be connected to the electrical contacts from the back surface side of the heater  700 . 
     In this exemplary embodiment, the electrodes E 1 , E 2 , E 3 , E 4 , E 5 , E 6 , E 7 , E 8 - 1 , and E 8 - 2  are disposed on the back surface of the heater  700  to enable power supply from the back surface side of the heater  700 . In addition, the ratio of the power to be supplied to at least one heating block among the heating blocks to the power to be supplied to the other heating blocks is made controllable. 
     As illustrated in  FIG. 7C , a holding member  712  of the heater  700  has holes for a thermistor (temperature sensing element) TH 1 , and the safety element  212 , and the electrical contacts of the electrodes E 1 , E 2 , E 3 , E 4 , E 5 , E 6 , E 7 , E 8 - 1 , and E 8 - 2 . 
     The thermistor (temperature sensing element) TH 1 , the safety element  212 , and the electrical contacts of the electrodes E 1 , E 2 , E 3 , E 4 , E 5 , E 6 , E 7 , E 8 - 1 , and E 8 - 2 , described above, are disposed between the stay  204  and the holding member  712 , and are disposed in contact with the back surface of the heater  700 . The configuration of the electrical contacts that come into contact with the electrodes E 1 , E 2 , E 3 , E 4 , E 5 , E 6 , E 7 , E 8 - 1 , and E 8 - 2  is substantially the same as that in the first exemplary embodiment, and is not described herein. 
       FIG. 8  is a circuit diagram of the control circuit  800  for the heater  700  according to the second exemplary embodiment. In  FIG. 4 , which illustrates the first exemplary embodiment, two triacs are used to control power and control the heat generation distribution in the longitudinal direction of the heater  300 . In the second exemplary embodiment, a single triac is used to control power, and three relays  851  to  853  are used to control the heat generation distribution in the longitudinal direction of the heater  700 . In this exemplary embodiment, the relays  851  to  853  are controlled to select a heating block to which power is to be supplied from among a plurality of heating blocks. The plurality of heating blocks include a heating block to which power is to be supplied and a heating block to which no power is to be supplied, and are thus referred to as independently controllable heating blocks. 
     The relays  851  to  853  operate in accordance with an RLON 851  signal, an RLON 852  signal, and an RLON 853  signal (hereinafter referred to as the “RLON 851  to RLON 853  signals”) from the CPU  420 , respectively. When the RLON 851  to RLON 853  signals are high, transistors  861  to  863  are turned on, causing the secondary coils of the relays  851  to  853  to conduct current from the power supply voltage Vcc 2  to turn on the primary contacts of the relays  851  to  853 . When the RLON 851  to RLON 853  signals are low, the transistors  861  to  863  are turned off, blocking the current flow to the secondary coils of the relays  851  to  853  from the power supply voltage Vcc 2  to turn off the primary contacts of the relays  851  to  853 . 
     Next, the relationship between the state of the relays  851  to  853  and the heat generation distribution in the longitudinal direction of the heater  700  will be described. When all of the relays  851  to  853  are in an off state, the heating block  702 - 4  is supplied with power. As illustrated in  FIG. 7B , a portion of the heater  700  having a width of 115 mm generates heat, yielding a heat generation distribution for DL envelopes and COM-10 envelopes. When the relay  851  is in an on state and the relays  852  and  853  are in an off state, the heating blocks  702 - 3  to  702 - 5  are supplied with power. As illustrated in  FIG. 7B , a portion of the heater  700  having a width of 157 mm generates heat, yielding a heat generation distribution for A5 size sheets. When the relays  851  and  852  are in an on state and the relay  853  is in an off state, the heating blocks  702 - 2  to  702 - 6  are supplied with power. As illustrated in  FIG. 7B , a portion of the heater  700  having a width of 190 mm generates heat, yielding a heat generation distribution for executive size sheets and B5 size sheets. When all the relays  851  to  853  are in an on state, the heating blocks  702 - 1  to  702 - 7  are supplied with power. As illustrated in  FIG. 7B , a portion of the heater  700  having a width of 220 mm generates heat, yielding a heat generation distribution for letter size sheets, legal size sheets, and A4 size sheets. In the way described above, using the three relays  851  to  853 , the control circuit  800  according to this exemplary embodiment can control the heat generation distribution in the longitudinal direction of the heater  700  in four ways. 
     Power to the heater  700  is controlled by conducting or non-conducting of a triac  816 . The circuit operation of the triac  816  is substantially the same as that of the triac  416  described in the first exemplary embodiment, and is not described herein. The triac  816  is provided on a common conducting path for the current flowing through all the heating blocks  702 - 1  to  702 - 7 . Accordingly, in any of the above-described four ways of controlling the heat generation distribution of the heater  700 , the power to be supplied to the heater  700  may be controlled by the conducting or non-conducting of the triac  816 . 
     Next, a method for controlling the temperature of the heater  700  will be described. The temperature sensed by the thermistor TH 1  is sensed as a divided voltage of a resistor (not illustrated), and is supplied to the CPU  420  as a TH 1  signal. In the internal processing of the CPU (control unit)  420 , the power to be supplied is calculated based on the sensing temperature of the thermistor TH 1  and the set temperature of the heater  700  in accordance with, for example, PI control. The power to be supplied is further converted into a control level of a phase angle (phase control) or a wave number (wave-number control) corresponding to the power to be supplied, and the triac  816  is controlled in accordance with the control condition. 
     In addition, since a temperature sensing element is provided for the heating block  702 - 4  connected to a power supply without the intervention of the relays  851  to  853 , the temperature of the heater  700  may be sensed regardless of the operating condition of the relays  851  to  853 . Similarly to the first exemplary embodiment, control may be based on a film temperature rather than a heater temperature. 
     In the configuration described in the second exemplary embodiment, power supply to only the heating blocks  702 - 1  to  702 - 3  and  702 - 5  to  702 - 7  located in the opposite end portions of the heater  700  may be prevented regardless of the operating condition (assuming the short-circuit failure and open-circuit failure states) of the relays  851  to  853 . When the heating blocks  702 - 1  to  702 - 3  and  702 - 5  to  702 - 7  located in the opposite end portions of the heater  700  may be supplied with power, the heating block  702 - 2  located in the center portion of the heater  700  is also supplied with power regardless of the operating condition of the relays  851  to  853 . To this end, in this exemplary embodiment, the thermistor TH 1  and the safety element  212  are disposed in contact with a position corresponding to the heating block  702 - 4 , resulting in a safety circuit (a safety circuit of the relay  440  or the safety element  212 ) functioning regardless of the operating condition of the relays  851  to  853 . 
       FIG. 9  is a flowchart depicting the control sequence for the image heating apparatus  200 , which is performed by the CPU  420 . In response to the occurrence of a print request in S 901 , in S 902 , the relay  440  is turned on. 
     In S 903 , it is determined whether the recording material P has a width greater than or equal to 115 mm. If the recording material P has a width greater than or equal to 115 mm, the process proceeds to S 904 . In S 904 , the relay  851  is kept in an on state. If the recording material P has a width less than 115 mm, the process proceeds to S 905 . In S 905 , the relay  851  is kept in an off state. In S 906 , it is determined whether the recording material P has a width greater than or equal to 157 mm. 
     If the recording material P has a width greater than or equal to 157 mm, the process proceeds to S 907 . In S 907 , the relay  852  is kept in an on state. If the recording material P has a width less than 157 mm, the process proceeds to S 908 . In S 908 , the relay  852  is kept in an off state. 
     In S 909 , it is determined whether the recording material P has a width greater than or equal to 190 mm. If the recording material P has a width greater than or equal to 190 mm, the process proceeds to S 910 . In S 910 , the relay  853  is kept in an on state. If the recording material P has a width less than 190 mm, the process proceeds to S 911 . In S 911 , the relay  853  is kept in an off state. 
     In S 912 , the process speed for forming an image is set to full speed while the set states of the relays  851  to  853  is maintained, and an image forming operation is performed at a target temperature of 200 degrees Celsius which is set for the thermistor TH 1 . The processing of S 912  is iterated to continue the fixing process until the print job is completed in S 913 . The process described above is repeatedly performed. If the completion of the print job is detected in S 913 , then, in S 914 , the relay  440  is turned off. In S 915 , the control sequence of image formation ends. 
     The heater  700  according to this exemplary embodiment may also increase the number of ways in which the heat generation distribution in the longitudinal direction of the heater  700  is switchable, without an increase in the width of the heater  700  in its transverse direction. 
     The control circuit  800  described in the second exemplary embodiment is applicable to the heater  300  by adjusting the number of relays that control the heat generation distribution for the heater  300  (i.e., by switching the heat generation distribution in the heater longitudinal direction in two ways by using one relay). Also, the control circuit  400  described in the first exemplary embodiment is applicable to the heater  700  by adjusting the number of triacs that control the heat generation distribution in the heater longitudinal direction for the heater  700  (i.e., by switching the heat generation distribution in the heater longitudinal direction in four ways by using four triacs). Either the control method performed by the control circuit  400  or the control method performed by the control circuit  800  may be used for heaters illustrated in  FIGS. 10A and 10B, 11A and 11B, 12A and 12B , and  FIGS. 13A to 13C , which will be described in the following exemplary embodiments. 
     Third Exemplary Embodiment 
       FIGS. 10A and 10B  are diagrams depicting the configuration of a heater  1000  applicable to a third exemplary embodiment. Components similar to those in the first exemplary embodiment are assigned the same numerals and are not described herein. The heater  1000  illustrated in  FIGS. 10A and 10B  has a feature to feed electric power to the heating element  302  disposed on the sliding surface of the substrate  305  from an electrode on the back surface of the heater  1000  via a through hole T. 
       FIG. 10A  is a diagram of a cross section of the heater  1000  in its transverse direction. As illustrated in  FIG. 10A , the heater  1000  includes a first conductor  301 , a second conductor  303 , and a heating element  302  that are disposed on a first layer of the sliding surface of the substrate  305 . 
       FIG. 10B  is a plan view of individual layers of the heater  1000 . An electrode E 1  formed on the back surface of the heater  1000  is connected to a conductor  303 - 1  via a conductor  1004 - 1  and a through hole T 1 . Likewise, an electrode E 2  is connected to a conductor  303 - 2  via a conductor  1004 - 2  and through holes T 2 - 1  and T 2 - 2 . An electrode E 3  is connected to a conductor  303 - 3  via a conductor  1004 - 3  and a through hole T 3 . An electrode E 4 - 1  is connected to conductors  301   a  and  301   b  via a conductor  1004 - 4 - 1  and through holes T 4 - 1   a  and T 4 - 1   b . An electrode E 4 - 2  is connected to the conductors  301   a  and  301   b  via a conductor  1004 - 4 - 2  and through holes T 4 - 2   a  and T 4 - 2   b.    
     The heater  1000  further includes a surface protective layer  1008  on a second layer of the sliding surface thereof. The surface protective layer  1008  is an insulating glass layer for protecting the first conductor  301 , the second conductor  303 , and the heating element  302 , and improving the capability of being slidably engaged with the film  202 . 
     As in the heater  1000 , the configuration of the heating element  302  disposed on the sliding surface of the substrate  305  provides the advantages disclosed herein. 
     Fourth Exemplary Embodiment 
       FIGS. 11A and 11B  are diagrams depicting the configuration of a heater  1100  applicable to a fourth exemplary embodiment. Components similar to those in the first and third exemplary embodiments are assigned the same numerals and are not described herein. 
     The heater  1100  illustrated in  FIGS. 11A and 11B  has a feature in which heating blocks  1102 - 1  to  1102 - 3  are not separated in the transverse direction of the heater  1100 , and a first conductor  1101  is not also separated in the transverse direction of the heater  1100 . The number of electrodes is smaller than that in the heater  300  and the heater  1000  since the electrode E 1  and the electrode E 3  are connected to each other on the substrate  305 , and the electrode E 4 - 1  and the electrode E 4 - 2  are connected to each other on the substrate  305 . 
       FIG. 11A  is a diagram of a cross section of the heater  1100  in its transverse direction.  FIG. 11B  is a plan view of individual layers of the heater  1100 . 
     The electrode E 1  formed on the back surface of the heater  1100  is connected to a conductor  1103 - 1  via a conductor  1104 - 1  and a through hole T 1 . Also, the electrode E 2  is connected to a conductor  1103 - 2  via a conductor  1104 - 2  and through holes T 2 - 1  and T 2 - 2 . The electrode E 4  is connected to a conductor  1101  via a conductor  1104 - 4  and a through hole T 4 . A conductor  1103 - 3  is connected to the electrode E 1  via the conductor  1104 - 1  and a through hole T 3 . In the configuration described above with reference to the control circuit  400  illustrated in  FIG. 4 , the electrode E 1  and the electrode E 3  need to be connected to each other outside the heater  300 . In the configuration described above, in contrast, the electrode E 1  and the electrode E 3  do not need to be connected to each other outside the heater  1100 . In the configuration described above, furthermore, the electrode E 4 - 1  and the electrode E 4 - 2  do not also need to be connected to each other outside the heater  1100 . Accordingly, a protective layer  1107  is formed on the second layer of the back surface of the heater  1100 , except for the portions corresponding to the electrodes E 1 , E 2 , and E 4 . 
     In the heater  1100  according to this exemplary embodiment, second conductors connected to heating blocks that do not need to be controlled independently (i.e., the heating blocks  1102 - 1  and  1102 - 3 ) are connected to each other on the substrate  305 , thereby removing the electrode E 3 . In addition, one of electrodes disposed in the right and left portions on the substrate  305  (i.e., E 4 - 1  and E 4 - 2  in  FIG. 3B ), which are connected to first conductors, is removed. Accordingly, the number of electrodes required may be reduced. As in the heater  1100 , the configuration in which the heating element  1102  is not separated in the transverse direction of the heater  1100  provides the advantages disclosed herein. 
     Fifth Exemplary Embodiment 
       FIGS. 12A and 12B  are diagrams depicting the configuration of a heater  600  applicable to a fifth exemplary embodiment. Components similar to those in the first exemplary embodiment are assigned the same numerals and are not described herein. 
     The heater  600  illustrated in  FIGS. 12A and 12B  has a feature in which heating elements  602   a - 1 ,  602   b - 1 ,  602   a - 2 ,  602   b - 2 ,  602   a - 3 , and  602   b - 3  are each further divided into a plurality of heating elements that are connected in parallel with each other. 
       FIG. 12A  is a diagram of a cross section of the heater  600  in its transverse direction.  FIG. 12B  is a plan view of individual layers of the heater  600 . 
     The heating element  602   a - 1  divided into a plurality of heating elements is connected between a conductor  603 - 1  and a conductor  601   a , and is supplied with power. The heating element  602   b - 1 , the heating element  602   a - 2 , the heating element  602   b - 2 , the heating element  602   a - 3 , and the heating element  602   b - 3  have a similar configuration to that of the heating element  602   a - 1 , and are not described herein. 
     The plurality of parallel connected heating elements of the heating element  602   a - 1  are arranged to be inclined with respect to the longitudinal and transverse directions of the heater  600 . The plurality of parallel connected heating elements of the heating element  602   a - 1  further overlap each other in the longitudinal direction. This may reduce the influence of gaps between the plurality of heating elements, and improve the uniformity of the heat generation distribution in the longitudinal direction of the heater  600 . In the heater  600  according to this exemplary embodiment, furthermore, the influence of gaps between heating blocks may also be reduced since endmost heating elements in adjacent heating blocks overlap each other in the longitudinal direction, and the heat generation distribution may be made more uniform. The endmost heating elements of adjacent heating blocks are a combination of the heating element at the right end of the heating element  602   a - 1  and the heating element at the left end of the heating element  602   a - 2 , and a combination of the heating element at the right end of the heating element  602   a - 2  and the heating element at the left end of the heating element  602   a - 3 . 
     In addition, the resistance values of the plurality of parallel connected heating elements of the heating elements  602   a - 1  to  602   a - 3  and  602   b - 1  to  602   b - 3  may be adjusted to make the temperature distribution in one heating block uniform. Also, the resistance values of the plurality of parallel connected heating elements of the heating elements  602   a - 1  to  602   a - 3  and  602   b - 1  to  602   b - 3  may be adjusted so that the heat generation distribution in the longitudinal direction of the heater  600  is uniform across a plurality of heating blocks (e.g., the heating blocks  602 - 1  to  602 - 3 ). 
     The resistance values of the plurality of parallel connected heating elements of the heating elements  602   a - 1  to  602   a - 3  and  602   b - 1  to  602   b - 3  may be adjusted by adjusting the widths, lengths, intervals, inclinations, and the like of the individual heating elements. The use of the heater  600  according to this exemplary embodiment may suppress or reduce temperature variations in gaps between a plurality of heating blocks. 
     Sixth Exemplary Embodiment 
       FIGS. 13A to 13C  are diagrams depicting the configuration of a heater  1300  applicable to a sixth exemplary embodiment. Components similar to those in the first and third exemplary embodiments are assigned the same numerals and are not described herein. 
     The heater  1300  illustrated in  FIGS. 13A to 13C  has a feature to feed electric power to only some heating blocks via an electrode on the back surface of the heater  1300 . 
       FIG. 13A  is a diagram of a cross section of the heater  1300  in its transverse direction. As illustrated in  FIG. 13A , the heater  1300  includes a first conductor  1301 , a second conductor  1303 , and a heating element  302  that are disposed on a first layer of the sliding surface of the substrate  305 . 
       FIG. 13B  is a plan view of individual layers of the heater  1300 . An electrode E 2  formed on the first layer of the back surface of the substrate  305  is connected to a conductor  1303 - 2  formed on the first layer of the sliding surface via a conductor  1304  and through holes T 2 - 1  and T 2 - 2 . An electrode E 1  is connected to a conductor  1303 - 1 , an electrode E 3  is connected to a conductor  1303 - 3 , and an electrode E 4 - 1  and an electrode E 4 - 2  are connected to conductors  1301   a  and  1301   b , respectively. The electrode E 1 , the electrode E 3 , the electrode E 4 - 1 , and the electrode E 4 - 2  are located outside the portions at the opposite ends of the heater  1300  in its longitudinal direction that are slidably engaged with the film  202 . Thus, electrical contacts are disposed on the sliding surface at the opposite ends of the heater  1300  in its longitudinal direction so that the electrical contacts are connected to the electrode E 1 , the electrode E 3 , the electrode E 4 - 1 , and the electrode E 4 - 2 . Thus, a holding member  1312  in the heater  1300  has no holes for the electrode E 1 , the electrode E 3 , the electrode E 4 - 1 , and the electrode E 4 - 2 . 
     The heater  1300  is configured to feed electric power to only some heating blocks (e.g., the heating block  302 - 2 ) via the electrode on the back surface. In order to feed electric power to a heating block that is not in contact with the opposite end portions of the heater  1300  in its longitudinal direction from the opposite ends of the heater  1300  in its longitudinal direction, it is necessary to increase the width of the heater  1300  in its transverse direction and to dispose an additional conductor on the substrate  305 . Examples of the heating block that is not in contact with the opposite end portions of the heater in its longitudinal direction include the heating block  302 - 2  in the heater  1300  according to this exemplary embodiment, and the heating blocks  702 - 2  to  702 - 6  in the heater  700  described in the second exemplary embodiment. Accordingly, it may be sufficient to provide a configuration that enables electric power feed to one or more heating blocks that are not in contact with at least the opposite end portions of the heater  1300  in its longitudinal direction from an electrode provided for a second conductor or from an electrode connected via the through hole T. 
     Seventh Exemplary Embodiment 
       FIGS. 15A and 15B  are diagrams depicting the configuration of a heater  1500  applicable to a seventh exemplary embodiment. The heater  1500  illustrated in  FIG. 15A  is configured such that electrodes E 1 , E 2 , E 4 , and E 5  are located at positions in the respective heating blocks that are nearer the center of the heater  1500  in its longitudinal direction (i.e., a location indicated by a broken line X in  FIGS. 15A and 15B ). The illustrated configuration may suppress or reduce the non-uniformity in heat generation of the heater  1500 . The effect will be described hereinbelow. 
     First, the non-uniformity in heat generation, which is caused in a heater in which current flows in parallel to the recording material conveyance direction will be described with reference to a heater  1400  illustrated in  FIGS. 14A and 14B  to illustrate the non-uniformity in heat generation.  FIG. 14A  is a plan view of a first layer of the back surface of the heater  1400 . The cross-sectional configuration of the heater  1400 , that is, the configuration of the back surface layers, the sliding surface layer, and the substrate, is similar to that in the first exemplary embodiment. For ease of understanding, in the heater  1400 , a first conductor ( 1401  and  1402 ), a second conductor  1403 , and a heating element ( 1404  and  1405 ) are not separated in the longitudinal direction of the heater  1400 . Further, the first and second conductors and the heating element have a uniform resistance. Electrodes E 1 , E 2   a , and E 2   b  are connected to electrical contacts for supplying power. The electrode E 1  is located at the center in the longitudinal direction, and a voltage is applied between the electrodes E 1  and E 2   a  and between the electrodes E 1  and E 2   b  to cause the heating element ( 1404  and  1405 ) to generate heat. 
       FIG. 14B  illustrates a potential distribution of the conductors  1401  and  1403  in the longitudinal direction of the heater  1400  when a voltage of +100 V is applied to the electrode E 1  and a voltage of 0 V is applied to the electrodes E 2   a  and E 2   b . The conductor  1402  has the same potential distribution as the conductor  1401 , and is not illustrated. The conductor  1403  has a potential that exhibits a maximum value in the center portion in the longitudinal direction and that decreases toward the opposite ends. The electrical resistance of the conductor  1403  causes a voltage drop. Further, the magnitude of the voltage drop varies depending on the ratio of the resistance of the conductor  1403  to the resistance of the heating element  1404 . The potential distribution of the conductor  1401  also has a voltage drop from the center to the ends. The magnitude of the voltage drop also varies depending on the ratio of the resistance of the conductor  1401  to the resistance of the heating element  1405 . 
     The conductors and the heating elements of the heater  1400  are formed on a ceramic substrate by screen printing, and have a thickness in the range of 4 to 10 micrometers. The conductors ( 1401 ,  1402 , and  1403 ) are composed of Ag, and have a specific resistance of 2×10 −8  ohm-meters. The heating elements ( 1404  and  1405 ) are composed of RuO 2 , and have a specific resistance of 3×10 −2  ohm-meters. 
     The voltage to be applied to the heating element  1404  is equal to the potential difference between the conductor  1403  and the conductor  1401 . Thus, the distribution indicated by the broken line in  FIG. 14B  is obtained. That is, the voltage to be applied to the heating element  1404  is non-uniform in the longitudinal direction, resulting in the heat generation distribution of the heating element  1404  being also non-uniform. The heat generation distribution of the heating element  1405  is also non-uniform. Thus, non-uniformity in heat generation occurs in the heater  1400 . 
     Next, the configuration of the heater  1500  according to the seventh exemplary embodiment will be described.  FIG. 15A  is a plan view of a first layer of the back surface of the heater  1500 . The cross-sectional configuration of the heater  1500 , that is, the configuration of the second layer of the back surface, the sliding surface layer, and the substrate, is similar to that is the first exemplary embodiment. The following eighth exemplary embodiment and other exemplary embodiments are also the same as the first exemplary embodiment, except for the first layer of the back surface and the configuration of the electrodes, and the layers other than the first layer of the back surface are not described herein. 
     A conductor  1503  and heating elements ( 1504  and  1505 ) are each separated in to five pieces in the longitudinal direction of the heater  1500 , and individual blocks are supplied with power via electrodes E 1 , E 2 , E 3 , E 4 , and E 5 , respectively. The electrodes E 1 , E 2 , E 4 , and E 5  are located at positions that are nearer the center of the heater  1500  (indicated by the broken line X), rather than the center of the respective blocks, in the longitudinal direction of the heater  1500 . 
       FIG. 15B  illustrates a potential distribution of conductors  1501  and  1503  when a voltage of +100 V is applied to the electrodes E 1 , E 2 , E 3 , E 4 , and E 5  of the heater  1500  and a voltage of 0 V is applied to electrodes E 6   a  and E 6   b . The potential distribution of a conductor  1502  is similar to that of the conductor  1501 , and is not illustrated. The conductors  1501  and  1503  have a potential that decreases toward the ends of a block in the longitudinal direction from the respective electrode positions. This phenomenon is similar to that related to the voltage drop described with reference to the heater  1400  in  FIGS. 14A and 14B . Further, a distribution of the potential difference between the conductor  1503  and the conductor  1501  is indicated by the broken line in  FIG. 15B , and the potential difference has a maximum value of 97 V and a minimum value of 92 V. That is, the voltage to be applied to the heating elements ( 1504  and  1505 ) has a variation (range) of 5 V. 
       FIGS. 16A and 16B  illustrate an example of a heater different from the heater  1500  in the positions of electrodes. A heater  1600  has a structure in which the electrodes E 1 , E 2 , E 4 , and E 5  are located at positions that are nearer the ends of the heater  1600 , rather than the center of the respective blocks. 
       FIG. 16B  illustrates a potential distribution of conductors  1601  and  1603  when a voltage of +100 V is applied to the electrodes E 1 , E 2 , E 3 , E 4 , and E 5  of the heater  1600  and a voltage of 0 V is applied to electrodes E 6   a  and E 6   b . The potential distribution of a conductor  1602  is similar to that of the conductor  1601 , and is not illustrated. A distribution of the potential difference between the conductor  1603  and the conductor  1601  is indicated by the broken line in  FIG. 16B , and the potential difference has a maximum value of 99 V and a minimum value of 90 V. That is, the voltage to be applied to heating elements ( 1604  and  1605 ) has a variation of 9 V. 
     Table 1 shows maximum values and minimum values of potential differences between conductors of the heater  1500  and the heater  1600 , and ranges of the potential differences. 
     
       
         
           
               
               
               
               
             
               
                   
                 TABLE 1 
               
               
                   
                   
               
               
                   
                   
                 Minimum value 
                 Range (maximum 
               
               
                   
                 Maximum value of 
                 of potential 
                 value − minimum 
               
               
                   
                 potential difference 
                 difference 
                 value) 
               
               
                   
                   
               
             
            
               
                   
               
            
           
           
               
               
               
               
            
               
                 Heater 1500 
                 97 V 
                 92 V 
                 5 V 
               
               
                 Heater 1600 
                 99 V 
                 90 V 
                 9 V 
               
               
                   
               
            
           
         
       
     
     Accordingly, preferably, as in the heater  1500 , the position of an electrode in each block is located nearer the center of the heater (indicated by the broken line X), rather than the center of the associated block, in the longitudinal direction of the heater in order to reduce the non-uniformity in heat generation of the heater in the longitudinal direction of the heater. 
     Eighth Exemplary Embodiment 
       FIGS. 17A and 17B  are diagrams depicting the configuration of a heater  1700  applicable to an eighth exemplary embodiment. The heater  1700  is configured such that each heating block has a plurality of electrodes. 
       FIG. 17A  is a plan view of the first layer of the back surface of the heater  1700 . A conductor  1703  and heating elements ( 1704  and  1705 ) are each separated into three pieces in the longitudinal direction of the heater  1700 . Heating elements  1704   a  and  1705   a  are supplied with power from electrodes E 1  and E 2 , heating elements  1704   b  and  1705   b  are supplied with power from electrodes E 3  and E 4 , and heating elements  1704   c  and  1705   c  are supplied with power from electrodes E 5  and E 6 . 
     All the electrodes E 1 , E 2 , E 3 , E 4 , E 5 , and E 6  have the same potential, and all electrodes E 11 , E 12 , E 13 , E 14 , E 21 , E 22 , E 23 , and E 24  also have the same potential.  FIG. 17B  illustrates a potential distribution of conductors  1701  and  1703  when a voltage of +100 V is applied to the electrodes E 1 , E 2 , E 3 , E 4 , E 5 , and E 6  and a voltage of 0 V is applied to the electrodes E 11 , E 12 , E 13 , E 14 , E 21 , E 22 , E 23 , and E 24 . The potential distribution of a conductor  1702  is similar to that of the conductor  1701 , and is not illustrated. In the potential distribution of the conductor  1703 , the potential exhibits a maximum value at the positions of the six electrodes E 1  to E 6 , and decreases in periods between the electrodes. Note that the amount by which the potential decreases is smaller than that of the heater  1600  illustrated in  FIG. 16A . The reason for this is that, for example, in the case of a path of the current flowing from the electrode E 1  to the electrode E 11 , the two electrodes E 1  and E 2  in the block associated with a conductor  1703   a  reduces the distance between the electrodes E 1  and E 11 . That is, the apparent resistance value of the conductor in the current paths for the electrodes E 1  and E 11  is small, resulting in a reduction in the amount of decrease in the potential of the conductor  1703   a . Likewise, the conductor  1701  also has a plurality of electrodes (E 11 , E 12 , E 13 , and E 14 ), resulting in a reduction in the variation of the potential of the conductor  1701 . 
     Accordingly, the potential difference between the conductors  1703  and  1701  indicated by the broken line in  FIG. 17B  has a maximum value of 99 V and a minimum value of 98 V, and the range of the potential difference is small. In this manner, one heating block including a plurality of electrodes having the same potential may suppress or reduce the variation of the potential difference in the longitudinal direction of the heater. This makes the voltages to be applied to the heating elements  1704  and  1705  uniform in the longitudinal direction of the heater  1700 , and suppresses or reduces the non-uniformity in heat generation of the heater  1700 . 
     Ninth Exemplary Embodiment 
       FIGS. 18A and 18B  are diagrams depicting the configuration of a heater  1800  applicable to a ninth exemplary embodiment. The heater  1800  includes heating elements  1804  and  1805  each of which is consecutive (i.e., is not separated) in the longitudinal direction of the heater  1800 . 
       FIG. 18A  is a plan view of the first layer of the back surface of the heater  1800 . A conductor  1803  is separated into three conductors  1803   a ,  1803   b , and  1803   c  in the longitudinal direction. The conductor  1803   a  is supplied with power from an electrode E 1 , the conductor  1803   b  is supplied with power from an electrode E 2 , and the conductor  1803   c  is supplied with power from an electrode E 3 . 
       FIG. 18B  illustrates a potential distribution of the heating elements  1804  and  1805 , and conductors  1801  and  1802  when a voltage of +100 V is applied to the electrodes E 1 , E 2 , and E 3  of the heater  1800  and a voltage of 0 V is applied to electrodes E 4   a  and E 4   b . The potential distributions of the heating elements  1804  and  1805  are obtained at positions indicated by broken lines A and B in  FIG. 18A , respectively. In this exemplary embodiment, the heating elements  1804  and  1805  are not separated. Thus, the potentials of the heating elements  1804  and  1805  are not equal to 0 V at positions corresponding to the positions at which the conductor  1803  is separated. Accordingly, the heating elements  1804  and  1805  continuously generate heat in the longitudinal direction, and there is no area where the amount of heat generated is 0, making the heat generation distribution of the heater  1800  more uniform. 
     Tenth Exemplary Embodiment 
       FIGS. 19A and 19B  are diagrams depicting the configuration of a heater  1900 A and a heater  1900 B applicable to a tenth exemplary embodiment.  FIG. 19A  illustrates a first layer of the back surface of the heater  1900 A, and a conductor  1903 A is separated into conductors  1903 Aa,  1903 Ab, and  1903 Ac in the longitudinal direction of the heater  1900 A. The boundary between the conductor  1903 Aa and the conductor  1903 Ab is inclined with respect to the longitudinal direction of the heater  1900 A and the recording material conveyance direction. The boundary between the conductor  1903 Ab and the conductor  1903 Ac is also inclined with respect to the longitudinal direction of the heater  1900 A and the recording material conveyance direction. 
     A heating element  1904 A and a heating element  1905 A are not separated in the longitudinal direction. As described in the ninth exemplary embodiment, the amount of heat generated is low in portions where the heating element  1904 A is in contact with the gap areas between the pieces into which the conductor  1903 A is separated. The portions where the amount of heat generated by the heating element  1904 A is low and the portions where the amount of heat generated by the heating element  1905 A is low are shifted in the longitudinal direction of the heater  1900 A because the boundaries in the conductor  1903 A are inclined. 
     Shifting the portions where the amount of heat generated by the heating element  1904 A is low and the portions where the amount of heat generated by the heating element  1905 A is low in the longitudinal direction makes the heat generation distribution of the overall heater more uniform. 
     As illustrated in  FIG. 19B , a conductor  1903 B may be separated by step-shaped boundaries. The configuration of a conductor  1903 B illustrated in  FIG. 19B  other than the shape is similar to that in  FIG. 19A , and is not described in detail herein. 
     Eleventh Exemplary Embodiment 
       FIGS. 20A and 20B  are diagrams depicting the configuration of a heater  2000  applicable to an eleventh exemplary embodiment. The heater  2000  illustrated in  FIGS. 20A and 20B  is the same as the heater  1900 A or  1900 B according to the tenth exemplary embodiment in that a heating element is not separated but a conductor is separated to form individual blocks. The difference is that electrodes are disposed outside an area (maximum size media passage area) where a heating element is disposed in the longitudinal direction of the heater  2000 . 
       FIG. 20A  is a cross-sectional view of the heater  2000 . As illustrated in  FIG. 20A , the heater  2000  includes first conductors  2001  and  2002 , a second conductor  2003 , a heating element  2004 , and a heating element  2005  that are disposed on a first layer of a sliding surface of a substrate  2010 . 
       FIG. 20B  is a plan view of the first layer of the sliding surface. As illustrated in  FIG. 20B , the heating elements  2004  and  2005  are not separated in the longitudinal direction of the heater  2000 . The conductor  2001  is separated into three conductors  2001   a ,  2001   b , and  2001   c  in the longitudinal direction of the heater  2000 , and the conductor  2002  is separated into three conductors  2002   a ,  2002   b , and  2002   c  in the longitudinal direction of the heater  2000 . Electrodes E 1 , E 2 , E 3 , and E 4  connected to the conductors  2001 ,  2002 , and  2003  are disposed outside a recording material passage area. Also in the heater  2000 , the direction in which current flows through the heating elements  2004  and  2005  is parallel to the recording material conveyance direction. A second layer of the sliding surface (surface protective layer  2012 ) is an insulating glass layer for protecting the conductors  2001  and  2002  and the heating elements  2004  and  2005  and improving the capability of being slidably engaged with the film  202 . The boundary position between the conductors  2001   a  and  2001   b  and the boundary position between the conductors  2002   a  and  2002   b  may be different in the longitudinal direction of the heater  2000 . The boundary position between the conductors  2001   b  and  2001   c  and the boundary position between the conductors  2002   b  and  2002   c  may also be different in the longitudinal direction of the heater  2000 . 
     Twelfth Exemplary Embodiment 
     Next, a heater and an image heating apparatus configured to suppress or reduce the overheating in the no-media passage portion and also to suppress or reduce harmonics will be described. 
       FIGS. 21A to 21C  are configuration diagrams of a heater  2100 . As illustrated in  FIG. 21A , the heater  2100  has a heating element on a ceramic substrate  305  thereof. A thermistor TH 1  serving as a temperature sensing element is disposed on the back surface of the substrate  305  in contact with a passage area of the laser printer  100 . A safety element  212  activated in response to an abnormal temperature rise in the heater  2100  to shut off the power supply to the heater  2100 , such as a thermo-switch and a thermal fuse, is also disposed on the back surface of the substrate  305 . A metal stay  204  is disposed to apply the pressure exerted by a spring (not illustrated) to a holding member  2112 . Power to the heater  2100  is controlled in accordance with the output of the thermistor TH 1  disposed near the center of a media passage portion (i.e., near the conveyance reference position X). The printer  100  according to this exemplary embodiment is configured to convey a recording material in such a manner that the center of the recording material in its widthwise direction is aligned with the reference position X. 
     The heater  2100  is configured such that the heat generation distribution in the longitudinal direction is switchable in four ways, and an upstream heating element  702   a  and a downstream heating element  702   b  are independently controllable. 
       FIG. 21A  is a cross-sectional view of the heater  2100 .  FIG. 21B  is a plan view of individual layers of the heater  2100 . The heater  2100  has the ceramic substrate  305 , a first sliding surface layer that comes into contact with the endless belt  202 , a first back surface layer having a heating element and a conductor described below disposed thereon, and a second back surface layer that covers the first back surface layer. The first sliding surface layer has a glass-coated or polyimide-coated surface protective layer  308 . The second back surface layer has an insulating (in this exemplary embodiment, glass) surface protective layer  1407 . 
     The first back surface layer on the substrate  305  has a first conductor  701  ( 701   a  and  701   b ) extending in the longitudinal direction of the heater  2100 . The first back surface layer also has a second conductor  703  ( 703 - 1  to  703 - 7 ) at a different position from the position of the first conductor  701  in the transverse direction of the heater  2100  so as to extend in the longitudinal direction of the heater  2100 . The first conductor  701  is separated into a conductor  701   a  located upstream and a conductor  701   b  located downstream in the conveyance direction of the recording material P. 
     The first back surface layer also has a heating element  702  disposed thereon between the first conductor  701  and the second conductor  703  for generating heat by power supplied via the first conductor  701  and the second conductor  703 . The heating element  702  is separated into a heating element  702   a  ( 702   a - 1  to  702   a - 7 ) located upstream and a heating element  702   b  ( 702   b - 1  to  702   b - 7 ) and located downstream in the conveyance direction of the recording material P. The heating element  702  has a positive temperature coefficient of resistance. Due to the positive temperature coefficient of resistance, even if an end of a recording material in its widthwise direction travels through part of one heating block (described below), the overheating in a no-media passage portion may be suppressed or reduced. 
     The first layer back surface has a plurality of heating blocks disposed thereon in the longitudinal direction of the heater  2100 . Each of the plurality of heating blocks includes the first conductor  701   a , the second conductor  703  ( 703 - 1  to  703 - 7 ), and the heating element  702   a  ( 702   a - 1  to  702   a - 7 ). The sequence of heating block is referred to as a first heating block line L 1 . The first layer back surface also has a plurality of heating blocks disposed thereon in the longitudinal direction of the heater  2100 . Each of the plurality of heating blocks includes the first conductor  701   b , the second conductor  703  ( 703 - 1  to  703 - 7 ), and the heating element  702   b  ( 702   b - 1  to  702   b - 7 ). The sequence of heating blocks is referred to as a second heating block line L 2 . In the heater  2100  according to this exemplary embodiment, each of the first heating block line L 1  and the second heating block line L 2  includes seven heating blocks (BL 1  to BL 7 ). 
     Electrodes E 8   a - 1 , E 8   a - 2 , E 8   b - 1 , and E 8   b - 2  are disposed at ends of the heater  2100  in its longitudinal direction. The electrodes E 8   a - 1  and E 8   a - 2  are electrodes for feeding electric power to the heating elements  702   a - 1  to  702   a - 7  of the first heating block line L 1  via the first conductor  701   a . The electrodes E 8   b - 1  and E 8   b - 2  are electrodes for feeding electric power to the heating elements  702   b - 1  to  702   b - 7  of the second heating block line L 2  via the first conductor  701   b . Electrodes E 1  to E 7  are electrodes common to the first heating block line L 1  and the second heating block line L 2 . As illustrated in  FIG. 21B , the electrodes E 1  to E 7  are disposed in an area where the heating elements  702   a - 1  to  702   a - 7  and  702   b - 1  to  702   b - 7  are disposed in the longitudinal direction of the heater  2100 . 
     The surface protective layer  1407  is formed to have openings at positions corresponding to the electrodes E 1  to E 7 , E 8   a - 1 , E 8   a - 2 , E 8   b - 1  and E 8   b - 2 . Thus, each of the electrodes E 1  to E 7 , E 8   a - 1 , E 8   a - 2 , E 8   b - 1  and E 8   b - 2  can be connected to an electrical contact for power supply from the back surface side of the heater  2100 . 
     As illustrated in  FIG. 21C , the holding member  2112  has holes HTH 1 , H 212 , HE 1  to HE 7 , HE 8   a - 1 , HE 8   a - 2 , HE 8   b - 1 , and HE 8   b - 2  for the thermistor (temperature sensing element) TH 1 , the safety element  212 , such as a thermo-switch or a thermal fuse, and the electrodes E 1  to E 7 , E 8   a - 1 , E 8   a - 2 , E 8   b - l , and E 8   b - 2 , respectively. The temperature sensing element TH 1 , the safety element  212 , and the electrical contacts that come into contact with the electrodes E 1  to E 7 , E 8   a - 1 , E 8   a - 2 , E 8   b - l , and E 8   b - 2  are disposed between the stay  204  and the holding member  2112 . The electrical contacts are represented by C 1  to C 7 , C 8   a - 1 , C 8   a - 2 , C 8   b - 1 , and C 8   b - 2 . In  FIG. 21C , broken lines connected to the electrical contacts C 1  to C 7 , C 8   a - 1 , C 8   a - 2 , C 8   b - 1 , and C 8   b - 2  and broken lines connected to the safety element  212  indicate power feed cables (AC lines). Further, broken lines connected to the temperature sensing element TH 1  indicates a signal line (DC line). Since the electrodes E 1  to E 7  are disposed in an area where the heating elements  702   a - 1  to  702   a - 7  and  702   b - 1  to  702   b - 7  are disposed in the longitudinal direction of the heater  2100 , an increase in the size of the image heating apparatus  200  may be avoided although the number of electrodes is large. 
       FIG. 22  illustrates a control circuit  2500  for the heater  2100 . The control circuit  2500  is capable of switching the heat generation distribution in the longitudinal direction of the heater  2100  by using three relays  851  to  853 . In addition, two triacs  816   a  and  816   b  are independently driven to reduce the harmonic currents or reduce flicker. The operation of the control circuit  2500  will be described hereinafter. 
     A commercial AC power supply  401  is provided. A zero-crossing detection unit  430  is a circuit for detecting the zero-crossing of the AC power supply  401 , and outputs a ZEROX signal to the CPU  420 . The ZEROX signal is used to control the heater  2100 . A relay  440  is used as a power shutoff unit for interrupting the supply of power to the heater  2100 . The relay  440  is activated in accordance with the output from the thermistor TH 1  (to shut off power supply to the heater  2100 ) in response to an excessive rise in the temperature of the heater  2100  due to failure or the like. 
     When an RLON 440  signal is high, a transistor  443  is turned on, causing the secondary coil of the relay  440  to conduct current from a power supply Vcc 2  to turn on the primary contact of the relay  440 . When the RLON 440  signal is low, the transistor  443  is turned off, blocking the current flow to the secondary coil of the relay  440  from the power supply Vcc 2  to turn off the primary contact of the relay  440 . A resistor  444  is a current limiting resistor. 
     Next, the operation of a safety circuit that includes the relay  440  will be described. If the sensing temperature (TH 1  signal) obtained by the thermistor TH 1  exceeds a predetermined value, the comparison unit  441  activates the latch unit  442 , and the latch unit  442  latches an RLOFF signal at a low level. When the RLOFF signal is low, the transistor  443  is maintained in an off condition even if the CPU  420  sets the RLON 440  signal high. Thus, the relay  440  is maintained in an off condition (or safe condition). Further, power to the secondary coil of the relay  440  is fed via the safety element  212 . Accordingly, in response to an excessive rise in the temperature of the heater  2100  due to failure or the like, the safety element  212  is activated to shut off power supply to the secondary coil of the relay  440 , thereby turning off the primary contact of the relay  440 . 
     If the sensing temperature obtained by the thermistor TH 1  does not exceed the predetermined value, the RLOFF signal of the latch unit  442  becomes open. Thus, the CPU  420  sets the RLON 440  signal high, thereby turning on the relay  440  to enable power supply to the heater  2100 . 
     Next, the operation of a circuit for driving the triac  816   a  will be described. The triac  816   a  is disposed in a power supply path to the first heating block line L 1 . Resistors  813   a  and  817   a  are bias resistors for the triac  816   a , and a phototriac coupler  815   a  is a device for ensuring a primary-secondary creepage distance. A light-emitting diode of the phototriac coupler  815   a  is caused to conduct current to turn on the triac  816   a . A resistor  818   a  is a resistor for limiting the current flow through the light-emitting diode of the phototriac coupler  815   a  from the power supply Vcc, and the phototriac coupler  815   a  is turned on or off by a transistor  819   a . The transistor  819   a  operates in accordance with a FUSER-a signal sent from the CPU  420  via a current limiting resistor  812   a.    
     The operation of a circuit for driving the triac  816   b  is substantially the same as that of the circuit for driving the triac  816   a , and is not described herein. The triac  816   b  is disposed in a power supply path to the second heating block line L 2 . 
     Next, switching of the heat generation distribution in the longitudinal direction of the heater  2100  will be described. In this exemplary embodiment, the relays  851  to  853  are controlled to select a heating block to which power is to be supplied from among a plurality of heating blocks. That is, all of the heating blocks may be supplied with power or only some of them may be supplied with power. 
     The relays  851  to  853  operate in accordance with an RLON 851  signal, an RLON 852  signal, and an RLON 853  signal (hereinafter referred to as the “RLON 851  to RLON 853  signals”) from the CPU  420 . When the RLON 851  to RLON 853  signals are high, transistors  861  to  863  are turned on, causing the secondary coil of the relays  851  to  853  to conduct current from the power supply Vcc 2  to turn on the primary contact of the relays  851  to  853 . When the RLON 851  to RLON 853  signals are low, the transistors  861  to  863  are turned off, blocking the current flow to the secondary coil of the relays  851  to  853  from the power supply Vcc 2  to turn off the primary contact of the relays  851  to  853 . Resistors  871  to  873  are current limiting resistors. 
     Next, the relationship between the relays  851  to  853  and the heat generation distribution in the longitudinal direction of the heater  2100  will be described. When all of the relays  851  to  853  are in an off state, the heating block BL 4  is supplied with power. Then, a portion having a width of 115 mm illustrated in  FIG. 21B  generates heat, yielding a heat generation distribution for DL envelopes and COM-10 envelopes. When the relay  851  is in an on state and the relays  852  and  853  are in an off state, the heating blocks BL 3  to BL 5  can be supplied with power. Then, a portion having a width of 157 mm illustrated in  FIG. 21B  generates heat, yielding a heat generation distribution for A5 size sheets. When the relays  851  and  852  are in an on state and the relay  853  is in an off state, the heating blocks BL 2  to BL 6  can be supplied with power. Then, a portion having a width of 190 mm illustrated in  FIG. 21B  generates heat, yielding a heat generation distribution for executive size sheets and B5 size sheets. When all of the relays  851  to  853  are in an on state, the heating blocks BL 1  to BL 7  can be supplied with power. Then, a portion having a width of 220 mm illustrated in  FIG. 21B  generates heat, yielding a heat generation distribution for letter size sheets, legal size sheets, and A4 size sheets. In the manner described above, the control circuit  2500  according to this exemplary embodiment controls the three relays  851  to  853  in accordance with recording material width information (or information on the width of the area where an image is to be formed) input to the CPU  420 , enabling the selection of heat generation distributions in four ways (heat generation widths). Accordingly, a block to generate heat is selected in accordance with the size of the recording material, suppressing heat from generated in an area in the heater  2100  through which the recording material does not pass. In this exemplary embodiment, furthermore, each heating element has a positive temperature coefficient of resistance. Thus, even if an end of the recording material in its widthwise direction passes through an area corresponding to one heating block, rather than a boundary between adjacent heating blocks, the portion of the heating block that falls outside the end of the recording material may be suppressed from generating heat. The individual heating elements may not necessarily have a positive temperature coefficient of resistance, and it may be sufficient that the individual heating elements have a temperature coefficient of resistance of resistor greater than or equal to zero. 
     As described above, the triac  816   a  is disposed in a power supply path to the first heating block line L 1 . Accordingly, by controlling turning on or off of the triac  816   a , it is possible to control power supply to a heating element block corresponding to the selected heat generation width within the first heating block line L 1 . Also, by controlling turning on or off of the triac  816   b , it is possible to control power supply to a heating element block corresponding to the selected heat generation width within the second heating block line L 2 . 
     Next, a method for controlling the temperature of the heater  2100  will be described. The temperature sensed by the thermistor TH 1  is input to the CPU  420  as a TH 1  signal. The CPU (control unit)  420  calculates the power to be supplied (control level) based on the sensing temperature of the thermistor TH 1  and the control target temperature of the heater  2100  in accordance with, for example, PI control. Further, the CPU  420  transmits a FUSER-a signal and a FUSER-b signal so that the current to flow through the heater  2100  is equal to the phase angle or wave number corresponding to the calculated control level, thereby controlling the triacs  816   a  and  816   b , respectively. 
       FIG. 23A  illustrates the waveform of the current (table A) flowing through heating elements in the first heating block line L 1  using the triac  816   a , and the waveform of the current (table B) flowing heating elements in the second heating block line L 2  using the triac  816   b . The first half-wave of the table A and the first half-wave of the table B are in-phase half-waves. The same applies to the half-waves of the other numbers. The tables A and B (the relationships between of the duty cycles and the waveforms) are set in the CPU  420 . The duty cycle is the percentage of ON period in one control period. The CPU  420  drives the triacs  816   a  and  816   b  so that the sensing temperature TH 1  is equal to a control target temperature. Further, the CPU  420  sets a duty cycle per control period in accordance with the sensing temperature TH 1 , where the control period is a period taken to update the control and is four consecutive half-waves (two cycles) of the AC waveform. As illustrated in  FIG. 23A , each of the two tables shows a waveform including both a phase control waveform and a wave-number control waveform within one control period. The phase control waveform is a waveform in which part of a half-wave is turned on, and the wave-number control waveform is a waveform in which the whole of a half-wave is turned on. Since the waveforms include both a phase control waveform and a wave-number control waveform within one control period, harmonics and flicker may be suppressed or reduced. In control periods having the same phase, the FUSER-a signal and the FUSER-b signal are signals having the same duty cycle. For example, in a case where the control level (duty cycle) calculated in accordance with the sensing temperature is 50%, current having the waveform with a 50% duty cycle in the table A flows through heating elements in the first heating block line L 1 , and current having the waveform with a 50% duty cycle in the table B flows through heating elements in the second heating block line L 2 . 
     As described above, each of the heating blocks BL 1  to BL 7  includes a plurality of heating elements (in this exemplary embodiment, two heating elements) in the transverse direction of the heater  2100  (the substrate  305 ), and a plurality of heating elements in each heating block are also independently controllable. 
     Next, the effect of independently controlling the first heating block line L 1  and the second heating block line L 2  will be described. For simplicity of description, it is assumed that the combined resistance of the heating elements  702   a - 1  to  702   a - 7  of the first heating block line L 1  is 20 ohms, the combined resistance of the heating elements  702   b - 1  to  702   b - 7  of the second heating block line L 2  is 20 ohms, and the total resistance of the heater  2100  is 10 ohms. Furthermore, the effective voltage value of the AC power supply  401  is 100 Vrms. 
     First, a description will be given of the case of a duty cycle of 25%. In the table A for the triac  816   a , the first two half-waves are controlled with a phase angle of 90 degrees to supply 50% power, and the second two half-waves are switched off. Accordingly, heating elements in a heating block selected by a relay from within the first heating block line L 1  are supplied with 25% power on average. Also, in the table B for the triac  816   b , the first two half-waves are switched off and the second two half-waves are controlled with a phase angle of 90 degrees to supply 50% power. Accordingly, heating elements in a heating block selected by a relay from within the second heating block line L 2  are supplied with 25% power on average. Therefore, 25% power is supplied to the heater  2100  as a whole. As can be understood with reference to  FIG. 23A , the table A and the table B are set so as to prevent current having a phase control waveform from flowing through the first heating block line L 1  and the second heating block line L 2  during in-phase half-waves. That is, the control unit  420  performs control so that current having a phase control waveform does not flow through a plurality of heating elements in one heating block at the same timing. The waveform in the table B illustrated in  FIG. 23A  is a waveform whose phase is shifted from the waveform in the table A by one cycle, resulting in no phase control waveforms overlapping in the two tables. Setting the relationship between the tables A and B in the way described above prevents current having a phase control waveform from flowing through the first heating block line L 1  and the second heating block line L 2  during in-phase half-waves. 
     As described above, a waveform including both a phase control waveform and a wave-number control waveform within one control period allows a reduction in harmonics and flicker. In this exemplary embodiment, furthermore, current having a phase control waveform is not caused to flow through the first heating block line L 1  and the second heating block line L 2  at the same time during in-phase half-waves, which would further reduce harmonics. Degradation of harmonic current occurs because current having a phase control waveform having a large amplitude flows. Note that, when a wave-number control waveform and a phase control waveform overlaps, degradation of harmonic current is not greater than when phase control waveforms overlap. Since a wave-number control waveform is a waveform that does not cause degradation of harmonic current, degradation of harmonic current does not also occur when wave-number control waveforms overlap. 
     As described above, the combined resistance of heating elements in each of the first and second heating block lines L 1  and L 2  is 20 ohms, and the effective voltage value of the AC power supply  401  is 100 Vrms. The current flowing through each heating element has a waveform obtained by controlling a sine wave having an effective current value of 5 Arms, and the phase control waveform of current flowing through each heating element is also a waveform obtained through the phase control of a sine wave having an effective current value of 5 Arms. As described above, furthermore, current having a phase control waveform is not caused to flow through the first heating block line L 1  and the second heating block line L 2  during in-phase half-waves. Thus, within the combined waveform of the current flowing through the first heating block line L 1  and the current flowing through the second heating block line L 2 , a half-wave only for a phase control waveform has a waveform obtained through phase control of a sine wave having an effective current value of 5 Arms (see  FIG. 23C ). 
     In a heater configured such that the first heating block line L 1  and the second heating block line L 2  are not independently controllable, similarly to this exemplary embodiment, the phase control waveform of current flowing through each heating element is a waveform obtained through phase control of a sine wave having an effective current value of 5 Arms. During in-phase half-waves, however, current having a phase control waveform flows through the first heating block line L 1  and the second heating block line L 2 . Thus, within the combined waveform of the current flowing through the first heating block line L 1  and the current flowing through the second heating block line L 2 , a half-wave only for a phase control waveform has a waveform obtained through phase control of a sine wave having an effective current value of 10 Arms, which will reduce the harmonic reducing effect (see  FIG. 23B ). 
     In the manner described above, independently controlling the first heating block line L 1  and the second heating block line L 2  can reduce the peak current value or the variation in current value, and can suppress or reduce harmonic or flicker. 
     For the other duty cycles, independently controlling the first heating block line L 1  and the second heating block line L 2  can reduce the peak current value or the variation in current value. For example, for a duty cycle of 75%, a the variation in current value caused by controlling the triacs  816   a  and  816   b  with a phase angle of 90 degrees can be reduced. In this way, the harmonic current and flicker can be reduced. 
     A reduction in the harmonic current and flicker allows the harmonic current and flicker standards to be met even if the total resistance of the heater  2100  is set low. A reduction in the total resistance of the heater  2100  can increase the maximum power that can be supplied from the AC power supply  401  to the heater  2100 . 
     As described above, the heater  2100  according to this exemplary embodiment includes a plurality of independently controllable heating blocks in the longitudinal direction thereof, each of the independently controllable heating blocks including a first conductor, a second conductor, and a heating element. Each heating block includes a plurality of heating elements in the transverse direction of the substrate  305 , and a plurality of heating elements in each heating block are also independently controllable. This enables the heat generation distribution in the longitudinal direction of the heater  2100  to be controlled in a plurality of ways, and also enables a reduction in harmonic current and flicker. In addition, in addition to the effect of reducing the overheating in the no-media passage portion of the heater  2100 , the warm-up time required by the image heating apparatus  200  (to increase the temperature of the image heating apparatus  200  to a temperature at which fixing occurs) may also be reduced. 
     Thirteenth Exemplary Embodiment 
       FIG. 24  is a configuration diagram of a heater  2400 . Components similar to those in the twelfth exemplary embodiment are assigned the same numerals and are not described herein. 
     Similarly to the twelfth exemplary embodiment, the heater  2400  is also configured to make the heat generation distribution in the longitudinal direction switchable in four ways. The difference from the twelfth exemplary embodiment is that the first and second heating block lines L 1  and L 2  are each divided into two groups in the longitudinal direction of the heater  2400 , so that power supply to four groups in total is independently controllable. The cross section of the heater  2400  and the shape of a holding member that holds the heater  2400  are substantially the same as those in the twelfth exemplary embodiment, and are not illustrated. 
     The first heating block line L 1  includes a left group  1  ( 702   a - 1  to  702   a - 3 , and  702   a - 4 - 1 ) and a right group  2  ( 702   a - 5  to  702   a - 7 , and  702   a - 4 - 2 ). The second heating block line L 2  includes a left group  3  ( 702   b - 1  to  702   b - 3 , and  702   b - 4 - 1 ) and a right group  4  ( 702   b - 5  to  702   b - 7 , and  702   b - 4 - 2 ). Thus, the heating block BL 4  is separated into two segments BL 4 - 1  and BL 4 - 2 , and the number of heating blocks in the longitudinal direction of the heater  2400  is eight. 
     The electrode E 8   a - 1  is an electrode for supplying power to the group  1  via the conductor  701   a - 1 . The electrode E 8   a - 2  is an electrode for supplying power to the group  2  via the conductor  701   a - 2 . The electrode E 8   b - 1  is an electrode for supplying power to the group  3  via the conductor  701   b - 1 . The electrode E 8   b - 2  is an electrode for supplying power to the group  4  via the conductor  701   b - 2 . 
       FIG. 25  illustrates a control circuit  2800  for the heater  2400 . In this exemplary embodiment, four triacs  816   a   1 ,  816   a   2 ,  816   b   1 , and  816   b   2  are used for power control to reduce the harmonic current or reduce flicker. The method for selecting a heating block by using the relays  851  to  853  may be substantially the same as that in the twelfth exemplary embodiment, and is not described herein. The circuit operation of the triacs  816   a   1 ,  816   a   2 ,  816   b   1 , and  816   b   2  is also substantially the same as that of the triacs  816   a  and  816   b  described in the first exemplary embodiment, and is not described herein. In  FIG. 25 , circuits for driving the triacs  816   a   1 ,  816   a   2 ,  816   b   1 , and  816   b   2  are not illustrated. 
     The triac  816   a   1  is an element for controlling the power to be supplied to heating blocks in the group  1 . The triac  816   a   2  is an element for controlling the power to be supplied to heating blocks in the group  2 . The triac  816   b   1  is an element for controlling the power to be supplied to heating blocks in the group  3 . The triac  816   b   2  is an element for controlling the power to be supplied to heating blocks in the group  4 . Driving signals (FUSER-a 1 , FUSER-a 2 , FUSER-b 1 , and FUSER-b 2 ) are transmitted from the CPU  420  to the triacs  816   a   1 ,  816   a   2 ,  816   b   1 , and  816   b   2 , respectively. 
       FIG. 26  illustrates the waveforms of the current (tables) to flow through the four groups. A table A 1  shows the waveform of the current flowing through heating elements in the group  1  within the first heating block line L 1  by using the triac  816   a   1 . A table A 2  shows the waveform of the current flowing through heating elements in the group  2  within the first heating block line L 1  by using the triac  816   a   2 . A table B 1  shows the waveform of the current flowing through heating elements in the group  3  within the second heating block line L 2  by using the triac  816   b   1 . A table B 2  shows the waveform of the current flowing through heating elements in the group  4  within the second heating block line L 2  by using the triac  816   b   2 . In the four tables, one control period is eight half-waves (four cycles). Furthermore, the four tables show a waveform including both a phase control waveform and a wave-number control waveform within one control period. Moreover, the four tables are set so as to prevent current having a phase control waveform from flowing through the four groups at the same time during in-phase half-waves. The four tables illustrated in  FIG. 26  show waveforms whose phase is shifted by one cycle. Setting the waveforms in the tables prevents current having a phase control waveform from flowing through the four groups at the same time during in-phase half-waves. Similarly to the twelfth exemplary embodiment, in control periods having the same phase, the FUSER-a 1  signal, the FUSER-a 2  signal, the FUSER-b 1  signal, and the FUSER-b 2  signal are signals having the same duty cycle. 
     Next, the effect of independently controlling the four groups will be described. For simplicity of description, it is assumed that the effective voltage value of the AC power supply  401  is 100 Vrms, the combined resistance of each group is 40 ohms, and the total resistance value of the heater  2400  is 10 ohms. 
     First, a description will be given of the case of a duty cycle of 12.5%, by way of example. In the table A 1  for the triac  816   a   1 , the first and second half-waves are controlled with a phase angle of 90 degrees to supply 50% power, and the third through eighth half-waves are switched off. Thus, the group  1  is supplied with power with 12.5% on average. In the table A 2  for the triac  816   a   2 , the third and fourth half-waves are controlled with a phase angle of 90 degrees to supply 50% power, and the other half-waves are switched off. Thus, the group  2  is supplied with power with 12.5% on average. Therefore, the heating element  702   a  in the first heating block line L 1  is supplied with power with 12.5% on average. 
     Also, in the table B 1  for the triac  816   b   1 , the fifth and sixth half-waves are controlled with a phase angle of 90 degrees to supply 50% power, and the other half-waves are switched off. Thus, the group  3  is supplied with power with 12.5% on average. In the table B 2  for the triac  816   b   2 , the seventh and eighth half-waves are controlled with a phase angle of 90 degrees to supply 50% power, and the other half-waves are switched off. Thus, the group  4  is supplied with power with 12.5% on average. Therefore, the heating element  702   b  in the second heating block line L 2  is supplied with power with 12.5% on average. 
     Since the combined resistance of each of the groups  1  to  4  is 40 ohms, the current flowing through heating elements in each group has a waveform obtained through phase control of a sine wave having an effective current value of 2.5 Arms, and the phase control waveform of the current flowing through each heating element is also a waveform obtained through phase control of a sine wave having an effective current value of 2.5 Arms. As described above, current having a phase control waveform is not caused to flow through the four groups during in-phase half-waves. Accordingly, within the combined waveform of the current flowing through the overall heater, a half-wave only for a phase control waveform has a waveform obtained through phase control of a sine wave having an effective current value of 2.5 Arms. For the other duty cycles, independently controlling the four groups can reduce the peak current value or the variation in current value. Thus, harmonic current and flicker may further be reduced compared to the twelfth exemplary embodiment. 
     In the waveforms illustrated in  FIG. 26 , subsequently to the group  1  (after one cycle), current flows through the group  2  included in the first heating block line L 1 , which also includes the group  1 . Subsequently to the group  3  (after one cycle), current flows through the group  4  included in the second heating block line L 2 , which also includes the group  3 . This also reduces temperature variations in the longitudinal direction of the heater  2400 . 
     Alternatively, as illustrated in  FIG. 27 , the relationship between the four tables may be such that current flows through the group  1 , the group  4 , the group  3 , and the group  2  in this order. 
     Alternatively, as illustrated in  FIG. 28 , switching between the groups may be controlled every half-wave. Switching between the groups at intervals of a short time period in the manner as illustrated in  FIG. 28  can reduce temperature variations in the longitudinal direction and transverse direction of the heater  2400 . 
     The number of heating block lines and the number of groups may be larger than those in this exemplary embodiment. 
     Fourteenth Exemplary Embodiment 
     Next, a fourteenth exemplary embodiment will be described. A heater according to the fourteenth exemplary embodiment has substantially the same configuration as that of the heater  700  illustrated in  FIGS. 7A to 7C , and is not illustrated herein. The fourteenth and fifteenth exemplary embodiments relate to power supply wires to be connected to a heater. 
     As illustrated in  FIGS. 7A to 7C , the heating blocks BL 1  and BL 7  are arranged to be symmetrical to each other with respect to the conveyance reference position X of the recording material in the longitudinal direction of the heater  700  (the longitudinal direction of the substrate  305 ). In this exemplary embodiment, the two heating blocks symmetrical to each other with respect to the conveyance reference position X are referred to as a first heating block and a second heating block. That is, the heating block BL 1  is a first heating block, and the heating block BL 7  is a second heating block. Also, the heating block BL 2  is a first heating block, and the heating block BL 6  is a second heating block. Further, the heating block BL 3  is a first heating block, and the heating block BL 5  is a second heating block. In the manner described above, the heater  700  includes a plurality of sets of heating blocks, each having a first heating block and a second heating block. Note that no heating block is paired with the heating block BL 4  located at the conveyance reference position X. In the following description, however, the heating block BL 4  is also regarded as one set, for simplicity. 
       FIG. 29  illustrates a control circuit  2900  for the heater  700 . A commercial AC power supply  401  is connected to the laser printer  100 . The control circuit  2900  includes four triacs (drive elements)  416 ,  426 ,  436 , and  446 . Each of the triacs  416 ,  426 ,  436 , and  446  is an element for controlling the power to be supplied to one of the sets of heating blocks. Conducting or non-conducting of each triac allows independent control of the set of heating blocks connected to this triac on a set-by-set basis. The switching between heat generation distributions in the longitudinal direction of the heater  700  may be achieved with a configuration other than the configuration illustrated in  FIG. 29  in which a dedicated triac is provided for each set of heating blocks. For example, one or more relays may be used to select sets of heating blocks to be used, and all the selected sets may be controlled by using a single drive element (triac). 
     The triac  416  is connected to the electrode E 4 , and is used to control the heating block BL 4 . The triac  426  is connected to the electrode E 5 , and is used to control the set of heating blocks BL 3  and BL 5 . The triac  436  is connected to the electrode E 6 , and is used to control the set of heating blocks BL 2  and BL 6 . The triac  446  is connected to the electrode E 7 , and is used to control the set of heating blocks BL 1  and BL 7 . 
     A zero-crossing detection unit  430  is a circuit for detecting the zero-crossing of the AC power supply  401 , and outputs a ZEROX signal to the CPU  420 . The ZEROX signal is used to control the heater  700 . 
     A relay  450  is used as a power shutoff unit for interrupting the supply of power to the heater  700 . The relay  450  is activated in accordance with the output from the thermistors TH 1  to TH 4  (to shut off power supply to the heater  700 ) in response to an excessive rise in the temperature of the heater  700  due to failure or the like. 
     When an RLON 450  signal is high, a transistor  453  is turned on, causing the secondary coil of the relay  450  to conduct current from the power supply voltage Vcc 2  to turn on the primary contact of the relay  450 . When the RLON 450  signal is low, the transistor  453  is turned off, blocking the current flow to the secondary coil of the relay  450  from the power supply voltage Vcc to turn off the primary contact of the relay  450 . A resistor  454  is a current limiting resistor. 
     Next, the operation of a safety circuit  455  that includes the relay  450  will be described. If one of the sensing temperatures obtained by the thermistors TH 1  to TH 4  exceeds a corresponding one of predetermined values that are individually set, a comparison unit  451  activates a latch unit  452 , and the latch unit  452  latches an RLOFF signal at a low level. When the RLOFF signal is low, the transistor  453  is maintained in an off condition even if the CPU  420  sets the RLON 450  signal high. Thus, the relay  450  is maintained in an off condition (or safe condition). 
     If none of the sensing temperatures obtained by the thermistors TH 1  to TH 4  exceeds the predetermined values that are individually set, the RLOFF signal of the latch unit  452  becomes open. Thus, the CPU  420  sets the RLON 450  signal high, thereby turning on the relay  450  to enable power supply to the heater  700 . 
     Next, the operation of the triac  416  will be described. Resistors  413  and  417  are bias resistor for the triac  416 , and a phototriac coupler  415  is a device for ensuring a primary-secondary creepage distance. A light-emitting diode of the phototriac coupler  415  is caused to conduct current to turn on the triac  416 . A resistor  418  is a resistor for limiting the current flow through the light-emitting diode of the phototriac coupler  415  from a power supply voltage Vcc, and the phototriac coupler  415  is turned on or off by a transistor  419 . The transistor  419  operates in accordance with a FUSER 1  signal from the CPU  420 . 
     When the triac  416  is in its conducting state, power is supplied to the heating elements  702   a - 4  and  702   b - 4 . 
     The circuit operation of the triacs  426 ,  436 , and  446  is substantially the same as that of the triac  416 , and is not described herein. The triac  426  operates in accordance with a FUSER 2  signal from the CPU  420  to control the power to be supplied to the heating elements  702   a - 5 ,  702   b - 5 ,  702   a - 3 , and  702   b - 3 . The triac  436  operates in accordance with a FUSER 3  signal from the CPU  420  to control the power to be supplied to the heating elements  702   a - 6 ,  702   b - 6 ,  702   a - 2 , and  702   b - 2 . The triac  446  operates in accordance with a FUSER 4  signal from the CPU  420  to control the power to be supplied to the heating elements  702   a - 7 ,  702   b - 7 ,  702   a - 1 , and  702   b - 1 . 
     Next, a method for controlling the temperature of the heater  700  will be described. The temperature sensed by the thermistor TH 1  located in the area responding to the heating block BL 4 , which includes the conveyance reference position X, is input to the CPU (control unit)  420  as a TH 1  signal. The CPU  420  also receives recording material size information as input to select a set of heating blocks to be caused to generate heat. Further, the CPU  420  calculates the power to be supplied (control level) based on the sensing temperature of the thermistor TH 1  and the control target temperature of the heater  700  in accordance with, for example, PI control. The CPU  420  transmits a FUSER signal (any of the FUSER 1  to FUSER 4  signals) to one of the triacs  416 ,  426 ,  436 , and  446  associated with the selected set so that the current to flow through the heater  700  is equal to the phase angle or wave number corresponding to the calculated control level. 
     In this exemplary embodiment, the heater temperature sensed by the thermistor TH 1  is used to control the temperature of the heater  700 . Alternatively, the thermistor TH 1  may be configured to sense the temperature of the film  202 , and the temperature of the film  202  may be used to control the temperature of the heater  700 . 
     Next, the connection configuration of power supply wires will be described.  FIG. 30A  is a plan view of the holding member  201 . As described with reference to  FIG. 2 , a second layer of the back surface of the heater  700  is beneath the holding member  201  in contact with the holding member  201 . The holding member  201  has holes at positions that overlap the electrodes E 1  to E 7 , E 8 - 1 , and E 8 - 2  of the heater  700  and at positions which the thermistors TH 1  to TH 4  are in contact with. 
     Wires  501   a ,  501   b ,  502   a  to  505   a , and  503   b  to  505   b  are connected to the control circuit  2900 , and are connected to the respective electrodes of the heater  700  through the holes formed in the holding member  201 . The electrodes are portions that connect the wires to the corresponding conductors, and may be regarded as part of the conductors. 
     The image heating apparatus  200  according to this exemplary embodiment includes a first wire for a second heating block, the first wire being connected to a conductor for supplying power to the second heating block. The image heating apparatus  200  further includes a second wire having a first end connected to the conductor, to which the first wire for the second heating block is connected, at a different position from the position at which the first wire is connected, and a second end connected to a second wire for a first heating block, the second wire being connected to a conductor for supplying power to the first heating block. The image heating apparatus  200  is configured such that power is supplied to the first heating block via the conductor to which the first wire for the second heating block is connected and also via the second wire. A specific description will be given hereinafter. 
     The wire  501   a  is connected to the electrode E 8 - 2 , and the wire  501   b  is connected to the electrode E 8 - 1 . The wire  502   a  connected to the triac  416  is connected to the electrode E 4 . 
     The wire  503   a  (first wire) connected to the triac  426  is connected to the electrode E 5 , which is an electrode for, within the set of heating blocks BL 3  (first heating block) and BL 5  (second heating block), the second heating block BL 5 . That is, the wire  503   a  (first wire) is equivalent to being connected to the conductor  703 - 5  of the second heating block BL 5 . The wire  503   b  (second wire) has a first end connected to the electrode E 5  for the second heating block BL 5 , to which the first wire  503   a  is connected, and a second end connected to the electrode E 3  for the first heating block BL 3 . That is, the second wire  503   b  is equivalent to having a first connected to the conductor  703 - 5  for the second heating block BL 5 , to which the first wire  503   a  is connected, and a second end connected to the conductor  703 - 3  for the first heating block BL 3 . The position at which the second wire  503   b  is connected to the electrode E 5  is different from the position at which the first wire  503   a  is connected to the electrode E 5 . In the manner described above, the second wire  503   b  is connected to the electrode E 3  with the electrode E 5  acting as a relay node. The temperature sensing element TH 2  is located at the position at which the temperature of the second heating block BL 5  is sensed, and no temperature sensing element is located at the position corresponding to the first heating block BL 3 . 
     The set of heating blocks BL 2  and BL 6  controlled using the triac  436 , and the set of heating blocks BL 1  and BL 7  controlled using the triac  446  also have a similar wiring configuration to the wiring configuration of the set of heating blocks BL 3  and BL 5  controlled using the triac  426 . Specifically, the second wire  504   b  is connected to the electrode E 2  with the electrode E 6  acting as a relay node. The second wire  505   b  is connected to the electrode E 1  with the electrode E 7  acting as a relay node. The temperature sensing element TH 3  is placed at the position at which the temperature of the second heating block BL 6  is sensed, that is, at the position of the heating block where the relay node E 6  is located. The temperature sensing element TH 4  is placed at the position at which the temperature of the second heating block BL 7  is sensed, that is, at the position of the heating block where the relay node E 7  is located. 
     In the manner described above, in a set of two heating blocks, power is supply to a first heating block via a conductor connected to a first wire for a second heating block and via a second wire. Further, a temperature sensing element that monitors the temperature of a heating block is provided only for a second heating block in which an electrode acting as a relay node is located, among a first heating block and the second heating block. 
       FIG. 30B  is a cross-sectional view of the holding member  201  illustrated in  FIG. 30A  taken along the line XXXB-XXXB. The wires  503   a  and  503   b  are connected to the surface of the electrode E 5  at independent contacts “a” and “b”, respectively. That is, power is supplied to the heating block BL 3 , which is a second heating block, via the electrode E 5  (the conductor  703 - 5 ) of the heating block BL 5 , which is a first heating block. Also, the wires  504   a  and  504   b  are connected to the electrode E 6  at independent contacts, and the wires  505   a  and  505   b  are connected to the electrode E 7  at independent contacts. 
     Next, the advantage of two wires being independently connected to one conductor of a second heating block will be described. For example, the following two configurations are considered: In the first configuration, the wire  503   b  branches off midway from the wire  503   a  and is connected to the heating block BL 3  (Comparative Example 1). In the second configuration, the wire  503   a  and the wire  503   b  are connected to the electrode E 5  at the same position (contact) on the electrode E 5  (Comparative Example 2).  FIG. 31  is a circuit diagram of Comparative Example 1. In  FIG. 31 , heating blocks other than the heating blocks BL 3 , BL 4 , and BL 5  are not illustrated. 
     In Comparative Example 1, if the wire  503   a  is disconnected from the electrode E 5 , the wire  503   b  is still connected to the electrode E 3 . Thus, by taking into account abnormal heat generation that the heating block BL 3  will undergo due to the failure of the CPU  420  or the like, a temperature sensing element at the position of the heating block BL 3  is also required to sense an abnormal temperature rise of the heating block BL 3 . That is, a temperature sensing element at the position of the heating block BL 3  is required in addition to a temperature sensing element at the position of the heating block BL 5 . 
     In Comparative Example 2, when the wire  503   a  is disconnected from the electrode E 5 , the wire  503   b  may also be disconnected from the electrode E 5  while being electrically connected to the wire  503   a . In this case, the heating block BL 5  generates no heat, whereas the heating block BL 3  generates heat. Accordingly, similarly to Comparative Example 1, taking into account an abnormal temperature rise of the heating block BL 3  due to the failure of the CPU  420  or the like, a temperature sensing element at the position of the heating block BL 3  is also required to sense an abnormal temperature rise. That is, a temperature sensing element at the position of the heating block BL 3  is required in addition to a temperature sensing element at the position of the heating block BL 5 . 
     In connection configuration according to this exemplary embodiment, in contrast, even if the contact “a” (the wire  503   a ) is erroneously disconnected, the contact “b” is not disconnected while the wire  503   a  and the wire  503   b  are electrically connected. In this case, since the wire  503   a  is disconnected from the electrode E 5 , no abnormal temperature rise will occur in the heating block BL 5 . In addition, no an abnormal temperature rise will also occur in the heating block BL 3 . If the wire  503   b  (contact “b”) is disconnected from the electrode E 5 , the heating block BL 3  does not generate heat, and only the heating block BL 5  might undergo abnormal heat generation. Such abnormal heat generation can be detected by the temperature sensing element TH 2  disposed at the position of the heating block BL 5 . With the wiring configuration according to this exemplary embodiment, in a set of heating blocks including the heating block BL 3  and the heating block BL 5 , only the heating block BL 3  will not generate heat. This does not require a temperature sensing element at the position of the heating block BL 3 . Accordingly, in a set of two heating blocks, power is supplied to a first heating block (BL 3 ) via a conductor ( 703 - 5 ) to which a first wire ( 503   a ) for a second heating block (BL 5 ) is connected to and via a second wire ( 503   b ). The above-described configuration can reduce the cost of the image heating apparatus  200 . 
     Fifteenth Exemplary Embodiment 
       FIGS. 32A to 32D  are diagrams illustrating the configuration of a heater and the wiring configuration of power supply wires according to this exemplary embodiment. This exemplary embodiment is different from the fourteenth exemplary embodiment in that a conductor to which both a first wire and a second wire are connected is provided with electrodes for the respective wires. Other configuration is similar to that in the fourteenth exemplary embodiment. 
     As illustrated in  FIG. 32A , a heater  770  according to this exemplary embodiment includes electrodes E 5 - 1  and E 5 - 2  for a conductor  703 - 5 . The heater  770  further includes electrodes E 6 - 1  and E 6 - 2  for a conductor  703 - 6 , and electrodes E 7 - 1  and E 7 - 2  for a conductor  703 - 7 . Since the heater  770  has a larger number of electrodes than the heater  700  according to the fourteenth exemplary embodiment, as illustrated in  FIG. 32B , a holding member  2201  that holds the heater  770  has a larger number of holes for the respective electrodes. 
     As illustrated in  FIG. 32B , the wire  503   a  is connected to the electrode E 5 - 1 , and the wire  503   b  is connected to the electrode E 5 - 2  and the electrode E 3 . The wire  504   a  is connected to the electrode E 6 - 1 , and the wire  504   b  is connected to the electrode E 6 - 2  and the electrode E 2 . The wire  505   a  is connected to the electrode E 7 - 1 , and the wire  505   b  is connected to the electrode E 7 - 2  and the electrode E 1 . 
       FIG. 32C  is a cross-sectional view of the holding member  2201  illustrated in  FIG. 32B  taken along the line XXXIIC-XXXIIC, and  FIG. 32D  is a cross-sectional view of the holding member  2201  illustrated in  FIG. 32B  taken along the line XXXIID-XXXIID. The wire  503   a  is in contact with the electrode E 5 - 1  at a contact “c”, and the wire  503   b  is in contact with the electrode E 5 - 2  at a contact “d”. As described above, the electrode E 5 - 1  and the electrode E 5 - 2  are electrodes for the conductor  703 - 5 . The configuration of wires and contacts for the other sets of heating blocks are similar to those described above, and are not described herein. 
     Similarly to the fourteenth exemplary embodiment, also in the configuration according to this exemplary embodiment, power is supplied to a first heating block (BL 3 ) via a conductor ( 703 - 5 ) to which a first wire ( 503   a ) for a second heating block (BL 5 ) is connected and via a second wire ( 503   b ). Further, the electrode E 5 - 1  for the conductor  703 - 5  to which the first wire  503   a  is connected, and the electrode E 5 - 2  for the conductor  703 - 5  to which the second the wire  503   b  is connected are separately disposed. Thus, similarly to the fourteenth exemplary embodiment, no disconnection will occur while the wire  503   a  and the wire  503   b  are electrically connected, and only the heating block BL 3  within the set of the heating blocks BL 3  and BL 5  does not generate heat. This does not require a temperature sensing element disposed at the position of the heating block BL 3 . 
     In addition, the wire length can be reduced by an amount corresponding to the distance L between the electrode E 5 - 1  (at the position indicated by the line XXXIIC-XXXIIC) and the electrode E 5 - 2  (at the position indicated by the line XXXIID-XXXIID), resulting in a reduction in cost. 
     In the fourteenth and fifteenth exemplary embodiments, each wire is implemented as a cable with an insulating coating, and is connected to an electrode by welding. Any other type of cable or any other connection method may be used. 
     While the present invention has been described with reference to exemplary embodiments, it is to be understood that the invention is not limited to the disclosed exemplary embodiments. The scope of the following claims is to be accorded the broadest interpretation so as to encompass all such modifications and equivalent structures and functions.