Patent Publication Number: US-10768558-B2

Title: Image formation apparatus

Description:
The entire disclosure of Japanese Patent Application No. 2017-122036 filed on Jun. 22, 2017 is incorporated herein by reference in its entirety. 
     BACKGROUND 
     Technological Field 
     The present invention relates to an image formation apparatus. 
     Description of the Related Art 
     In transfer of a toner image to paper in an electrophotographic process, in general, a bias is applied across an image carrier or an intermediate transfer element and paper so that toner is transferred owing to static electricity. Intensity of electric field applied to the toner has been known to be affected by electrical properties of paper such as a capacitance and an electrical resistance. 
     In order to adjust a transfer bias in accordance with difference in paper, a method of setting a transfer bias in accordance with paper information (a type or a basis weight) set by a user has been put into practical use. In recent years, in order to improve convenience of a user, a method of mounting a sensor configured to sense physical properties of paper and setting a transfer bias in accordance with sensing information from the sensor has been put into practical use. 
     These methods, however, suffer from the following problems. Though paper information such as a basis weight set by a user relates to some extent to electrical properties of paper which affect transfer, it is not directly relevant to a dielectric constant or a resistance and hence an appropriate transfer bias cannot be set in some cases. In some cases, since a user is unable to determine paper information or does not set paper information, an appropriate transfer bias cannot be set. 
     When a sensor configured to sense physical properties of paper is employed, cost for sensing increases and a space for installation of the sensor is required as compared with a conventional example. 
     In order to address the problems above, an image formation apparatus disclosed in Japanese Laid-Open Patent Publication No. 2003-287966 estimates an electrical resistance or a capacitance of paper based on a transfer current. 
     An image formation apparatus disclosed in Japanese Laid-Open Patent Publication No. 2010-276668 estimates an electrical resistance or a capacitance of paper based on a discharged current. 
     SUMMARY 
     In the image formation apparatus disclosed in Japanese Laid-Open Patent Publication No. 2003-287966, however, when a current is low, one cannot distinguish simply based on a transfer current alone, whether the low current is caused by a high resistance of paper or a low capacitance of the paper. 
     In the image formation apparatus disclosed in Japanese Laid-Open Patent Publication No. 2010-276668, a voltage to be applied is determined based on a sensed discharged current by preparing a table for each of a selected type of paper (plain paper/cardboard) and a humidity. The discharged current, however, is affected by a secondary transfer current, and therefore, under constant voltage control generally employed in secondary transfer, the secondary transfer current varies depending on a type of paper. Then, the discharged current also varies. Therefore, depending on a type of paper, a proper voltage cannot be set based on the prepared table. 
     The present invention was made in view of the problems as above, and an object of the present invention is to provide an image formation apparatus capable of accurately setting a transfer condition in transfer of a toner image to a recording medium. 
     To achieve at least one of the abovementioned objects, according to an aspect of the present invention, an image formation apparatus reflecting one aspect of the present invention comprises a first electrode portion including a contact electrode and a counter electrode, the contact electrode being in contact with a transported recording medium, the counter electrode being arranged to be opposed to the contact electrode such that the transported recording medium lies between the contact electrode and the counter electrode, a second electrode portion arranged as not being in contact with the recording medium such that charges applied to the recording medium are movable, a first sensing unit configured to sense a first current which flows to the first electrode portion as a result of application of a voltage across the contact electrode and the counter electrode while the recording medium lies between the contact electrode and the counter electrode, a second sensing unit configured to sense a second current which flows from the charged recording medium to the second electrode portion, and a control unit configured to receive input of results of sensing by the first sensing unit and the second sensing unit. The control unit is configured to set a transfer condition for transferring a toner image to the recording medium based on the first current sensed by the first sensing unit and the second current sensed by the second sensing unit. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The advantages and features provided by one or more embodiments of the invention will become more fully understood from the detailed description given hereinbelow and the appended drawings which are given by way of illustration only, and thus are not intended as a definition of the limits of the present invention. 
         FIG. 1  is a schematic diagram of an image formation apparatus according to an embodiment. 
         FIG. 2  is a schematic diagram showing a peripheral structure of a secondary transfer apparatus according to the embodiment. 
         FIG. 3  is a diagram showing relation between a secondary transfer current which flows to the secondary transfer apparatus and an electrical resistance of paper according to the embodiment. 
         FIG. 4  is a diagram showing one example of a thickness of paper and an electrical resistance of the paper when a secondary transfer current is set to 145 μA in the relation shown in  FIG. 3 . 
         FIG. 5  is a schematic diagram showing a peripheral structure of an electricity removal electrode according to the embodiment. 
         FIG. 6  is a diagram showing relation between a discharged current which flows to the electricity removal electrode and an electrical resistance of paper according to the embodiment. 
         FIG. 7  is a diagram showing one example of a thickness of paper and an electrical resistance of the paper which are specified from a secondary transfer current and a discharged current. 
         FIG. 8  is a diagram showing one example of a table used in calculation of physical properties of paper in the image formation apparatus according to the embodiment. 
         FIG. 9  is a schematic diagram showing a first state of a cooling apparatus according to the embodiment. 
         FIG. 10  is a schematic diagram showing a second state of the cooling apparatus according to the embodiment. 
         FIG. 11  is a diagram showing one example of physical properties of paper calculated from a secondary transfer current and a discharged current measured with a first sensing unit and a second sensing unit as well as a transfer condition and a cooling condition determined based on the physical properties of the paper according to the embodiment. 
         FIG. 12  is a diagram showing a first example of a flow in which a transfer condition is determined in the image formation apparatus according to the embodiment. 
         FIG. 13  is a diagram showing a second example of the flow in which a transfer condition is determined in the image formation apparatus according to the embodiment. 
         FIG. 14  is a diagram showing a third example of the flow in which a transfer condition and a cooling condition are determined in the image formation apparatus according to the embodiment. 
         FIG. 15  is a diagram showing one example of a table that is used in determining physical properties of paper, a transfer condition, and a cooling condition based on a secondary transfer current and a discharged current sensed by the first sensing unit and the second sensing unit according to the embodiment. 
         FIG. 16  is a diagram showing a fourth example of the flow in which a transfer condition and a cooling condition are determined in the image formation apparatus according to the embodiment. 
     
    
    
     DETAILED DESCRIPTION OF EMBODIMENTS 
     Hereinafter, one or more embodiments of the present invention will be described with reference to the drawings. However, the scope of the invention is not limited to the disclosed embodiments. 
     The same or common elements in the embodiment shown below have the same reference characters allotted in the drawings and description thereof will not be repeated. 
       FIG. 1  is a schematic diagram of an image formation apparatus according to an embodiment. An image formation apparatus  100  will be described with reference to  FIG. 1 . 
       FIG. 1  shows image formation apparatus  100  as a color printer. Though image formation apparatus  100  as the color printer is described below, image formation apparatus  100  is not limited to the color printer. For example, image formation apparatus  100  may be a monochrome printer, a facsimile machine, or a multi-functional peripheral (M P) of a monochrome printer, a color printer, and a facsimile machine as being combined. 
     Image formation apparatus  100  includes image formation units  1 Y,  1 M,  1 C, and  1 K, an intermediate transfer belt  30 , a primary transfer roller  31 , a secondary transfer roller  33 , a cassette  37 , a driven roller  38 , a drive roller  39 , a timing roller  40 , a fixation apparatus  50 , a cooling apparatus  80 , a housing  90 , and a control unit  101 . Secondary transfer roller  33  and drive roller  39  function as a secondary transfer apparatus  61 . 
     Image formation apparatus  100  includes secondary transfer apparatus  61  as a first electrode portion, an electricity removal electrode  62  as a second electrode portion, a first sensing unit  71 , and a second sensing unit  72 . 
     Secondary transfer apparatus  61  is set to sandwich a transported recording medium and configured to receive application of a voltage. Electricity removal electrode  62  is arranged as not being in contact with the recording medium such that applied charges are movable. Electricity removal electrode  62  is arranged downstream from secondary transfer apparatus  61  in a direction of transportation of the recording medium. 
     First sensing unit  71  senses a first current which flows to secondary transfer apparatus  61 . Second sensing unit  72  senses a second current which flows from the recording medium to electricity removal electrode  62 . First sensing unit  71  and second sensing unit  72  are each implemented, for example, by a current sensor. Results of sensing by first sensing unit  71  and second sensing unit  72  are input to control unit  101 . 
     An image formation portion is constituted of image formation units  1 Y,  1 M,  1 C, and  1 K, intermediate transfer belt  30 , primary transfer roller  31 , secondary transfer roller  33 , cassette  37 , driven roller  38 , drive roller  39 , and timing roller  40 . The image formation portion forms a toner image on paper S as a recording medium which is transported along a transportation path  41  which will be described later. 
     Image formation units  1 Y,  1 M,  1 C, and  1 K are sequentially aligned along intermediate transfer belt  30 . Image formation unit  1 Y forms a toner image of yellow (Y) upon receiving supply of toner from a toner bottle  15 Y. Image formation unit  1 M forms a toner image of magenta (M) upon receiving supply of toner from a toner bottle  15 M. Image formation unit  1 C forms a toner image of cyan (C) upon receiving supply of toner from a toner bottle  15 C. Image formation unit  1 K forms a toner image of black (BK) upon receiving supply of toner from a toner bottle  15 K. 
     Image formation units  1 Y,  1 M,  1 C, and  1 K are arranged sequentially in a direction of rotation of intermediate transfer belt  30  along intermediate transfer belt  30 . Each of image formation units  1 Y,  1 M,  1 C, and  1 K includes a photoconductor  10 , a charging apparatus  11 , an exposure apparatus  12 , a development apparatus  13 , and a cleaning apparatus  17 . 
     Charging apparatus  11  evenly charges a surface of photoconductor  10 . Exposure apparatus  12  irradiates photoconductor  10  with laser beams in response to a control signal from control unit  101  and exposes the surface of photoconductor  10  in accordance with an input image pattern. An electrostatic latent image in accordance with an input image is thus formed on photoconductor  10 . 
     Development apparatus  13  applies a development bias to a development roller  14  while it rotates development roller  14 , to thereby attach toner onto a surface of development roller  14 . The toner is thus transferred from development roller  14  to photoconductor  10  and a toner image in accordance with the electrostatic latent image is developed on the surface of photoconductor  10 . 
     Photoconductor  10  and intermediate transfer belt  30  are in contact with each other at a portion where primary transfer roller  31  is provided. Primary transfer roller  31  is in a shape of a roller and configured to be rotatable. A transfer voltage opposite in polarity to the toner image is applied to primary transfer roller  31  so that the toner image is transferred from photoconductor  10  to intermediate transfer belt  30 . The toner image of yellow (Y), the toner image of magenta (M), the toner image of cyan (C), and the toner image of black (BK) are successively layered and transferred from photoconductor  10  to intermediate transfer belt  30 . The color toner image is thus formed on intermediate transfer belt  30 . 
     Intermediate transfer belt  30  is looped around driven roller  38  and drive roller  39 . Drive roller  39  is rotationally driven, for example, by a motor (not shown). Intermediate transfer belt  30  and driven roller  38  rotate in coordination with drive roller  39 . A toner image on intermediate transfer belt  30  is thus transported to secondary transfer roller  33 . 
     Cleaning apparatus  17  is pressed against photoconductor  10  as being in contact therewith. Cleaning apparatus  17  recovers toner which remains on the surface of photoconductor  10  after transfer of the toner image. 
     Paper S is set in cassette  37 . Paper S is sent from cassette  37  to secondary transfer roller  33  one by one along transportation path  41  by timing roller  40 . Secondary transfer roller  33  is in a shape of a roller and configured to be rotatable. Secondary transfer roller  33  applies a transfer voltage opposite in polarity to the toner image to transported paper S. 
     The toner image is thus attracted from intermediate transfer belt  30  to secondary transfer roller  33  and the toner image on intermediate transfer belt  30  is transferred to paper S. Timing of transportation of paper S to secondary transfer roller  33  is adjusted by timing roller  40  in accordance with a position of the toner image on intermediate transfer belt  30 . Owing to timing roller  40 , the toner image on intermediate transfer belt  30  is transferred to an appropriate position on paper S. 
     When a voltage is applied across drive roller  39  and secondary transfer roller  33  in thus transferring a toner image to paper S, a first current flows from secondary transfer roller  33  through paper S toward drive roller  39 . This first current is sensed by first sensing unit  71 . 
     In secondary transfer, charges are accumulated in paper S. Charges accumulated in paper S move toward the second electrode arranged in proximity to paper S while the paper is transported along the transportation path. A second current thus flows to electricity removal electrode  62 . The second current is sensed by second sensing unit  72 . 
     Control unit  101  sets a transfer condition for transfer of a toner image to paper S based on the first current sensed by first sensing unit  71  and the second current sensed by second sensing unit  72 . 
     Fixation apparatus  50  pressurizes and heats paper S which passes therethrough. The toner image is thus fixed onto paper S. Fixation apparatus  50  thus fixes the toner image onto paper S transported along transportation path  41 . Paper S on which the toner image has been fixed is ejected onto a tray  48 . 
     Though image formation apparatus  100  adopting a tandem system as a printing method has been described above, a printing method of image formation apparatus  100  is not limited to the tandem system. Arrangement of each feature in image formation apparatus  100  can be modified as appropriate in accordance with an adopted printing method. A rotary system or a direct transfer system may be adopted as the printing method of image formation apparatus  100 . In the rotary system, image formation apparatus  100  is constituted of a single photoconductor  10  and a plurality of development apparatuses  13  configured to be rotatable on the same axis. Image formation apparatus  100  sequentially guides each development apparatus  13  to photoconductor  10  during printing and develops a toner image of each color. In the direct transfer system, image formation apparatus  100  directly transfers a toner image formed on photoconductor  10  onto paper S. 
       FIG. 2  is a schematic diagram showing a peripheral structure of the secondary transfer apparatus according to the embodiment. The peripheral structure of the secondary transfer apparatus according to the embodiment will be described with reference to  FIG. 2 . 
     As shown in  FIG. 2 , the secondary transfer apparatus is constituted of secondary transfer roller  33  as a contact electrode and drive roller  39  as a counter electrode. Secondary transfer roller  33  is in contact with a transported recording medium. Secondary transfer roller  33  functions as the contact electrode. Drive roller  39  is arranged to be opposed to secondary transfer roller  33  such that the recording medium lies between the drive roller and the secondary transfer roller. 
     Secondary transfer roller  33  and drive roller  39  are each constituted, for example, of a core metal and a surface layer. The core metal is made of aluminum or iron and is in a shape of a pipe. The surface layer is made, for example, of an ion conductive rubber material. For example, nitrile rubber (NBR) and epichlorohydrin rubber (ECO) can be used as being blended as the ion conductive rubber material. Drive roller  39  is rotationally driven by a drive source (not shown). Intermediate transfer belt  30  is made, for example, of a polyimide film. 
     By applying a voltage across secondary transfer roller  33  and drive roller  39  while paper lies between secondary transfer roller  33  and drive roller  39 , a secondary transfer current as the first current flows to secondary transfer roller  33  and drive roller  39 . 
     As an electrical resistance of paper S is lower, more secondary transfer current flows. As a capacitance of paper S is higher, more secondary transfer current flows. 
     The secondary transfer current flows through two paths below. The first path is a path which passes through a transfer nip portion defined by secondary transfer roller  33  and drive roller  39 . The second path is a path which passes through a gap provided between secondary transfer roller  33  and drive roller  39  upstream and downstream from the transfer nip portion in the direction of transportation of paper S. 
     The current which flows through the first path flows in accordance with an electrical resistance on the first path (an electrical resistance of each roller, an electrical resistance of paper S, and an electrical resistance of intermediate transfer belt  30 ) with respect to a transfer bias applied to secondary transfer roller  33 . Namely, the current flows under the Ohm&#39;s law. Therefore, as an electrical resistance of paper S is lower, more current flows. 
     The current which flows through the second path flows owing to discharging. In order to cause discharging in the gap, a voltage not lower than a certain level (the Paschen&#39;s law) is required, and as a capacitance of paper is higher, a potential loss in paper is less. A potential difference in the gap is thus greater and discharging is more likely to occur. Therefore, as a capacitance of paper is higher, more current flows. 
       FIG. 3  is a diagram showing relation between a secondary transfer current which flows to the secondary transfer apparatus and an electrical resistance of paper according to the embodiment. Relation between a secondary transfer current which flows to the secondary transfer apparatus and an electrical resistance of paper will be described with reference to  FIG. 3 . 
       FIG. 3  shows a secondary transfer current sensed by first sensing unit  71  when paper S passes through the secondary transfer apparatus, with electrical resistances of three types of paper S having thicknesses of 50 μm, 100 μm, and 150 μm, respectively, being varied as appropriate. 
     A process speed in passage through the secondary transfer apparatus is set, for example, to 100 mm/s, and a secondary transfer voltage applied to the secondary transfer apparatus (more specifically, across secondary transfer roller  33  and drive roller  39 ) is set, for example, to approximately 3000 V. Intermediate transfer belt  30  made of a polyimide film has a thickness, for example, of 130 μm and has a volume resistance of approximately 1E3Ω. Ion conductive rubber used for a surface layer of secondary transfer roller  33  has a thickness, for example, of 3 mm and has a volume resistance of approximately 1E5Ω. 
     As shown in  FIG. 3 , in a region where an electrical resistance of paper S is low, a secondary transfer current varies in accordance with an electrical resistance. In the region where an electrical resistance of paper S is low, a current which flows to a secondary transfer nip portion is dominant. 
     In a region where an electrical resistance of paper S is relatively low, a constant current flows regardless of a thickness of paper S. In this case, an electrical resistance of paper S is low, and an electrical resistance of intermediate transfer belt  30  and electrical resistances of secondary transfer roller  33  and drive roller  30  are relatively high, which are hence dominant. 
     On the other hand, in a region where an electrical resistance of paper S is high, the secondary transfer current is not much dependent on various electrical resistances but is dependent on a thickness, that is, on a capacitance, of paper S. 
     When the secondary transfer nip portion has a width of 5 mm, with the process speed of 100 mm/s, paper S passes through the secondary transfer nip portion in 1/20 second. Therefore, even though a bias of 3000 V set as a secondary transfer voltage is applied to paper S, with an electrical resistance not lower than 1E9Ω, only at most 3 μA of current flows. The current resulting from discharging is rather dominant in the region where the electrical resistance of paper S is high. Therefore, the secondary transfer current is significantly affected by a thickness of paper, that is, by a capacitance of the paper. 
       FIG. 4  is a diagram showing one example of a thickness of paper and an electrical resistance of the paper when the secondary transfer current is set to 145 μA in the relation shown in  FIG. 3 . One example of a thickness of paper and an electrical resistance of the paper when the secondary transfer current is set to a prescribed value will be described with reference to  FIG. 4 . 
     Three sets as shown in  FIG. 4  represent examples of a thickness of paper (a capacitance of paper) and an electrical resistance of the paper when a secondary transfer current is set to 145 μA. Therefore, it is difficult, only with a secondary transfer current sensed by first sensing unit  71 , to accurately estimate physical properties (a capacitance and an electrical resistance) of paper. 
     In the present embodiment, second sensing unit  72  is configured to sense a discharged current as the second current which flows to electricity removal electrode  62 . 
       FIG. 5  is a schematic diagram showing a peripheral structure of the electricity removal electrode according to the embodiment. The peripheral structure of the electricity removal electrode according to the embodiment will be described with reference to  FIG. 5 . 
     As shown in  FIG. 5 , paper S which has passed through the secondary transfer nip portion defined between secondary transfer roller  33  and drive roller  39  is charged as a result of application of a high voltage across secondary transfer roller  33  and drive roller  39  in secondary transfer. 
     The discharged current which flows from charged paper S to electricity removal electrode  62  owing to discharging is dependent on an amount of charges in paper S and a capacitance of the paper. Since an amount of charges in paper S resulting from charging is basically equal to an amount of charges applied in secondary transfer, it is dependent on an amount of secondary transfer current. The amount of charges in paper S owing to charging is dependent on an electrical resistance and a capacitance of paper S. 
     An amount of charges held in paper S owing to charging until the charges reach electricity removal electrode  62  is important. Since secondary transfer apparatus  61  according to the embodiment is of a transfer roller type, charges substantially equal in amount and different in polarity are applied to a front surface and a rear surface of paper in the secondary transfer nip portion. 
     Therefore, when the electrical resistance of paper S is low, charges on the front surface and the rear surface are removed by the time the charges reach electricity removal electrode  62  after secondary transfer and hence the discharged current becomes less. 
     Electricity removal electrode  62  is arranged on a rear surface side of paper S, and discharging to electricity removal electrode  62  does not occur until a potential difference between the rear surface of paper S and electricity removal electrode  62  is equal to or greater than a potential at a prescribed value. A potential produced by the charges accumulated in paper S is dependent on an electrostatic distance between paper S and electricity removal electrode  62 . 
     Charges different in polarity are present on the front surface and the rear surface of paper S as described above. Therefore, when the electrostatic distance between charges different in polarity is short, there will be no large potential difference. When an electrostatic distance between charges in paper different in polarity is long, a potential difference will become greater. As an electrostatic distance between the front surface and the rear surface of the paper is longer, that is, a capacitance of the paper is lower, a discharged current is higher. 
       FIG. 6  is a diagram showing relation between a discharged current which flows to the electricity removal electrode and an electrical resistance of paper according to the embodiment. Relation between a discharged current which flows to the electricity removal electrode and an electrical resistance of paper according to the embodiment will be described with reference to  FIG. 6 . 
       FIG. 6  shows a discharged current which flows from paper S to electricity removal electrode  62  and is sensed by second sensing unit  72  after paper S has passed through the secondary transfer apparatus, with electrical resistances of three types of paper S having thicknesses of 50 μm, 100 μm, and 150 μm, respectively, being varied as appropriate. 
     A process speed in passage through the secondary transfer apparatus is set, for example, to 100 mm/s, and a secondary transfer voltage applied to the secondary transfer apparatus (more specifically, across secondary transfer roller  33  and drive roller  39 ) is set, for example, to approximately 3000 V. Intermediate transfer belt  30  made of a polyimide film has a thickness, for example, of 130 μm and a volume resistance of approximately 1E3Ω. Ion conductive rubber used for a surface layer of secondary transfer roller  33  has a thickness, for example, of 3 mm and a volume resistance of approximately 1E5Ω. 
     Electricity removal electrode  62  has a sawtooth shape and is connected to a ground electrode (GND). A distance between electricity removal electrode  62  and paper S is set approximately to 0.1 mm. Electricity removal electrode  62  is made of SUS. 
     As shown in  FIG. 6 , when relation between the electrical resistance of paper S and the discharged current is shown with the electrical resistance of the paper being shown on the abscissa and the discharged current being shown on the ordinate, a distribution of electrical resistances of the paper is expressed with a projecting shape having a peak. 
     In a region where the electrical resistance of the paper is not higher than 1E8Ω, the discharged current abruptly decreases with lowering in electrical resistance of the paper and attains substantially to zero. In the region where the electrical resistance of the paper is not higher than 1E8Ω, even though an amount of current which flows to paper S in the secondary transfer nip portion is large, charges are removed on the front surface and the rear surface of the paper owing to the low electrical resistance of paper S. Therefore, it becomes difficult to hold the charges until the charges reach electricity removal electrode  62  and discharged current decreases. 
     In a region where the electrical resistance of the paper is from 2E8 to 3E8Ω, the discharged current has a peak, and in a region equal to or higher than that, the discharged current decreases with increase in electrical resistance of the paper. When the electrical resistance of the paper is high, a current which flows to paper S in the secondary transfer nip portion is lowered and hence discharged current decreases. 
     In general, as the paper has a greater thickness, that is, a lower capacitance, a higher discharged current flows. This is because of a difference in electrostatic distance between the front surface and the rear surface of paper S as described above. 
       FIG. 7  is a diagram showing one example of a thickness of paper and an electrical resistance of the paper which are specified from a secondary transfer current and a discharged current. 
     As shown in  FIG. 7 , when values for the discharged current sensed by second sensing unit  72  are different even though the secondary transfer current sensed by first sensing unit  71  is set to approximately 145 μA, a thickness and an electrical resistance of the paper are also different. 
     Thus, in the present embodiment, physical properties (a capacitance and an electrical resistance) of paper which have not been specified simply by sensing a secondary transfer current alone can accurately be estimated by sensing the secondary transfer current and the discharged current. 
     Strictly speaking, a value for the discharged current has a peak with respect to an electrical resistance of paper. Therefore, unless on which side of the peak a value for the discharged current is located can be specified, it may be difficult to uniquely determine physical properties of the paper. 
     In this case, paper S is transported again and a different secondary transfer voltage is applied across secondary transfer roller  33  and drive roller  30 . Then, first sensing unit  71  senses the secondary transfer current and second sensing unit  72  senses the discharged current which flows from paper S to electricity removal electrode  62  after the different secondary transfer voltage is applied. 
     Influence by a secondary transfer voltage is less in a region where the discharged current lowers due to a low electrical resistance of paper (on the left side of (before) the peak). The discharged current is raised by raising the secondary transfer voltage in the region where the discharged current lowers due to a high electrical resistance of the paper (on the right side of (after) the peak). 
     Therefore, on which side of the peak a value for the first measured discharged current is located can be distinguished by varying the secondary transfer voltage and sensing how the discharged current varies. 
       FIG. 8  is a diagram showing one example of a table used in calculation of physical properties of paper in the image formation apparatus according to the embodiment. One example of a table used in calculation of physical properties of paper will be described with reference to  FIG. 8 . 
     As shown in  FIG. 8 , a table to be used in calculation of physical properties of paper is stored in a storage (not shown) of control unit  101 . 
     In the table, a secondary transfer voltage used at the time of sensing, a thickness of paper, a capacitance of the paper, an electrical resistance of the paper, a secondary transfer current, and a discharged current are brought in correspondence with one another. In the table, (V1, T1, Q1, R1, TI1, RI1) to (Vn, Tn, Qn, Rn, TIn, RIn,) are stored as combination of a secondary transfer voltage used at the time of sensing, a thickness of paper, a capacitance of the paper, an electrical resistance of the paper, a secondary transfer current, and a discharged current, where n represents a natural number. 
     Control unit  101  estimates, by using the table, a capacitance of paper S and an electrical resistance of paper S as physical properties of paper S based on the secondary transfer voltage to be used at the time of sensing, a secondary transfer current sensed by first sensing unit  71 , and a discharged current sensed by second sensing unit  72 . 
     Control unit  101  sets a transfer condition for transferring a toner image to paper S based on the estimated capacitance of paper S and electrical resistance of paper S. 
     The table is prepared by conducting experiments in advance with various conditions being varied. When characteristics of secondary transfer roller  33  and intermediate transfer belt  30  significantly vary owing to an environment such as a temperature and a humidity, a plurality of tables corresponding to respective environments are preferably prepared. The table may be corrected with characteristics between a secondary transfer voltage and a secondary transfer current in the secondary transfer apparatus while paper S is not passed. 
     Control unit  101  sets a cooling condition of cooling apparatus  80  by using the secondary transfer current sensed by first sensing unit  71  and the discharged current sensed by second sensing unit  72 . 
       FIG. 9  is a schematic diagram showing a first state of the cooling apparatus according to the embodiment.  FIG. 10  is a schematic diagram showing a second state of the cooling apparatus according to the embodiment. Cooling apparatus  80  according to the embodiment will be described with reference to  FIGS. 9 and 10 . 
     As shown in  FIGS. 9 and 10 , cooling apparatus  80  is arranged downstream from fixation apparatus  50  in the direction of transportation of paper S. Cooling apparatus  80  cools paper S after a toner image transferred to paper S is fixed. 
     Cooling apparatus  80  has a cooling portion  81  and a pressing mechanism  82 . Cooling portion  81  is cylindrical and air sent from a cooling fan (not shown) passes through the inside of cooling portion  81 . Pressing mechanism  82  presses cooling portion  81  against transported paper S. 
     As shown in  FIG. 9 , in a first state, cooling portion  81  of cooling apparatus  80  is arranged at a distance from the transportation path for paper S. As shown in  FIG. 10 , in a second state, cooling portion  81  of cooling apparatus  80  is pressed against paper S located on the transportation path. 
     Paper S can be cooled by sending air into the inside of cooling portion  81  with the cooling fan while cooling portion  81  is pressed against paper S. 
     Control unit  101  adjusts an amount of heat absorption from paper S by adjusting a cooling condition such as the number of rotations of the cooling fan and strength of pressing. 
     Evaporation of moisture from paper S can be suppressed by cooling paper S after fixation. Thus, lowering in transferability to a second surface can be suppressed particularly when paper high in electrical resistance is used. When paper low in electrical resistance is used, paper S does not have to be cooled and cooling apparatus  80  does not have to be used. 
     Therefore, control unit  101  can reduce waste of energy used for driving cooling apparatus  80  by determining the cooling condition based on the secondary transfer current sensed by first sensing unit  71  and the discharged current sensed by second sensing unit  72 . 
     More specifically, control unit  101  estimates an electrical resistance of paper based on the sensed secondary transfer current and discharged current and increases an amount of heat absorption from paper S by cooling apparatus  80  when the estimated electrical resistance of a recording medium is high. 
     Cooling apparatus  80  may be implemented by a cooling fan. In this case, control unit  101  changes an amount of heat absorption from paper S by adjusting the number of rotations of the cooling fan. Cooling apparatus  80  may be implemented by a solid metal roller. In this case, an amount of heat absorption from paper S can be varied by adjusting strength of pressing by the metal roller. 
       FIG. 11  is a diagram showing one example of a secondary transfer current and a discharged current measured with the first sensing unit and the second sensing unit, physical properties of paper calculated from the secondary transfer current and the discharged current, and a transfer condition and a cooling condition determined based on the physical properties of the paper according to the embodiment. 
     One example of a secondary transfer current and a discharged current, physical properties of paper calculated from the secondary transfer current and the discharged current, and a transfer condition and a cooling condition determined based on the physical properties of the paper is as shown in  FIG. 11 . 
     For example, control unit  101  determines a transfer condition and a cooling condition based on the sensed secondary transfer current and discharged current by using a table in which a secondary transfer current and a discharged current, physical properties of paper calculated based on the secondary transfer current and the discharged current, and a transfer condition and a cooling condition determined based on the physical properties of paper are brought in correspondence with one another. 
     In paper high in electrical resistance, formation of electric field by application of charges to the paper is dominant. It has been known that, when charges are non-uniformly applied, creepage discharging occurs at a paper surface after the paper passes through a transfer nip and image noise is produced. This phenomenon can be suppressed by making charges applied to paper uniform by raising a secondary transfer voltage. 
     Therefore, in the present embodiment, when an electrical resistance of paper is as high as 6.00E+06, control unit  101  sets a secondary transfer voltage as high as approximately 3300 V to 4000 V. Since paper high in electrical resistance is highly resistant to a relatively high voltage, no problem arises even when the secondary transfer voltage is raised. 
     When a relatively high voltage is set for paper having an electrical resistance as low as 1.50E+06, noise due to discharging in the paper tends to be produced. Therefore, a secondary transfer voltage is preferably set to a value as low as approximately 3000 V to 3300 V. 
     In transfer to paper high in electrical resistance, electric field is formed in consideration of magnitude of a capacitance of paper to some extent. A capacitance of normal paper is affected by moisture contained in the paper. In general, moisture is lost due to a high temperature of the paper after fixation in a fixation process by making use of heat. Therefore, when printing on a second surface is performed with a capacitance being lowered, defective transfer tends to occur due to insufficient transfer electric field. 
     Therefore, in the present embodiment, by cooling paper with cooling apparatus  80  after fixation, decrease in moisture and resulting lowering in capacitance can be suppressed. In particular, lowering in transferability to a second surface can effectively be suppressed by using cooling apparatus  80  when paper having an electrical resistance as high as 6.00E+06 is used. 
     Even when paper high in electrical resistance is used, an effect of cooling of paper cannot be expected for paper having a capacitance as low as 1.2E-07. Therefore, such paper does not have to be cooled. In this case, a process speed is preferably lowered. For paper having an electrical resistance as low as 1.50E+06, paper S does not have to be cooled and cooling apparatus  80  does not have to be used. 
     By thus adjusting whether or not to use cooling apparatus  80  and an amount of heat absorption in accordance with physical properties of paper, waste of energy used for driving cooling apparatus  80  can be reduced. 
     When electric field is formed by applying charges to paper, charges which can be applied simply through a normal transfer process may be restricted by an upper limit of output from a high-voltage power supply. In this case, paper high in electrical resistance may be charged in advance before the paper is subjected to the normal transfer process. By charging paper in advance before the transfer process, transfer quality can be enhanced. 
     When paper has an intermediate electrical resistance as high as 1.80E+06 and a low capacitance, magnitude of transfer electric field can be ensured by raising a secondary transfer voltage. In this case, when the upper limit of output of the secondary transfer voltage is restricted, it is effective to suppress a process speed, for example, to approximately half. By suppressing the process speed, a time period required for paper to pass through the secondary transfer nip portion is longer and an amount of movement of charges in the paper increases. Consequently, intensity of transfer electric field can be higher. 
     Since it is not so effective to lower a process speed for paper high in electrical resistance, the paper is preferably charged in advance before the transfer process as described above. 
       FIG. 12  is a diagram showing a first example of a flow in which a transfer condition is determined in the image formation apparatus according to the embodiment. The first example of the flow for determining a transfer condition in image formation apparatus  100  according to the embodiment will be described with reference to  FIG. 12 . 
     The transfer condition is determined in printing of an image on a first sheet of paper or a first sheet of paper after change in type of paper. Information on physical properties of paper (a thickness of the paper and a capacitance of the paper) is stored for each cassette. 
     The first example shows a flow, for example, for determining a transfer condition in printing on a first sheet of paper when the paper accommodated in the cassette has an A4 size and in printing on a first sheet of paper after the size of the paper is changed. 
     As shown in  FIG. 12 , in determining a transfer condition, control unit  101  starts detection of physical properties of the paper in step S 10 . Then, control unit  101  determines in step S 20  whether or not there is sensing information resulting from sensing by first sensing unit  71  and second sensing unit  72 . Specifically, control unit  101  determines whether or not a secondary transfer current and a discharged current have been sensed. When it is determined that neither of the secondary transfer current and the discharged current has been sensed (step S 20 : NO), control unit  101  performs step S 30 . When it is determined that the secondary transfer current and the discharged current have been sensed (step S 20 : YES), control unit  101  performs step S 110 . 
     In step S 30 , control unit  101  obtains paper information (a width and a thickness of the paper) and humidity information. Control unit  101  obtains information on the paper from information on a cassette that is used and contents set through an operation panel. Control unit  101  obtains the humidity information from a hygrometer. 
     In step S 40 , control unit  101  provisionally determines a secondary transfer voltage which is generally considered as proper, based on the paper information and the humidity information obtained in step S 30 . Control unit  101  uses a table in which relation of the paper information and the humidity information with the secondary transfer voltage is set in advance. 
     In succession, in step S 50 , control unit  101  issues an image output instruction. Thus, a toner image corresponding to an image to be output by the image formation portion is formed. 
     In step S 60 , control unit  101  has paper S passed from the cassette toward tray  48  along the transportation path. 
     In succession, a secondary transfer current is sensed in step S 70 . Specifically, first sensing unit  71  senses a secondary transfer current which flows to the secondary transfer apparatus when paper S passes through the secondary transfer nip portion. The sensed secondary transfer current is input to control unit  101 . 
     A discharged current is sensed in step S 80 . Specifically, second sensing unit  72  senses a discharged current which flows from paper S charged during passage through the secondary transfer nip portion to electricity removal electrode  62 . The sensed discharged current is input to control unit  101 . 
     In succession, in step S 90 , control unit  101  estimates physical properties of the paper (more specifically, a capacitance of the paper and an electrical resistance of the paper). Control unit  101  estimates the physical properties of the paper by referring to the table in which a secondary transfer voltage used at the time of sensing, a thickness of paper, a capacitance of the paper, an electrical resistance of the paper, a secondary transfer current, and a discharged current are brought in correspondence with one another as described above. 
     When no table can be referred to, interpolation or extrapolation from a table in the vicinity may be performed as a supplement. 
     In step S 100 , control unit  101  determines a secondary transfer voltage as the transfer condition. Control unit  101  determines the secondary transfer voltage based on the estimated capacitance of the paper and electrical resistance of the paper. Control unit  101  may use the table above, or may use an operational expression set in advance to be able to determine a secondary transfer voltage based on the capacitance of the paper and the electrical resistance of the paper. 
     Though an example in which step S 100  is performed next to step S 90  has been exemplified and described, limitation thereto is not intended, and step S 90  and step S 100  may simultaneously be performed. 
     After step S 100  ends, the process returns to step S 10 . When printing on a second sheet or a subsequent sheet is performed, it is determined in step S 20  that there is sensing information (step S 20 : YES). In this case, step S 110  is performed. 
     Control unit  101  determines in step S 110  whether or not the cassette has been opened and closed. When it is determined that the cassette has not been opened and closed (step S 110 : NO), it is determined that the type of paper has not been changed. In this case, step S 170  is performed and the secondary transfer voltage determined in step S 100  is maintained. 
     When it is determined that the cassette has been opened and closed (step S 110 : YES), it is determined that the type of paper has been changed. In this case, step S 120  is performed. 
     In step S 120 , control unit  101  obtains information on paper determined to have been changed. In succession, control unit  101  determines in step S 130  whether or not paper has a prescribed size based on the obtained information on the paper. In the present flow, whether or not the obtained size of the paper is A4 is determined. As above, information on the paper is obtained from information on a cassette that is used and contents set through the operation panel. 
     When the size of the paper is determined as the prescribed size (step S 130 : YES), it is determined that the type of paper was not changed although the cassette was opened and closed, and step S 170  is performed. In step S 170 , the secondary transfer voltage determined in step S 100  is maintained as described above. 
     When the size of the paper is not determined as the prescribed size (step S 130 : NO), it is determined that the type of the paper has been changed and step S 131  is performed. 
     In step S 131 , control unit  101  issues an image output instruction. A toner image corresponding to an image to be output by the image formation portion is thus formed. 
     In step S 140 , control unit  101  obtains sensing information (a secondary transfer current and a discharged current). Specifically, first sensing unit  71  senses a secondary transfer current which flows to the secondary transfer apparatus when the paper different is size is passed from the cassette toward tray  48  and passes through the secondary transfer nip portion as in step S 60 , and second sensing unit  72  senses a discharged current which flows from paper S charged during passage through the secondary transfer nip portion to electricity removal electrode  62 . The sensed secondary transfer current and discharged current are input to control unit  101 . 
     In succession, in step S 150 , control unit  101  estimates physical properties of the paper (more specifically, a capacitance of the paper and an electrical resistance of the paper). When a width of the paper does not correspond to A4, an amount of inflow of a current is different for each width of paper in secondary transfer. Therefore, the sensed secondary transfer current is converted to a width corresponding to a lateral width of A4 by using a conversion table for each paper size set in advance. Since a discharged current is in proportion to a width of paper, the sensed discharged current is converted to a value corresponding to a lateral width of A4. 
     Control unit  101  estimates physical properties of the paper based on the converted values for the secondary transfer current and the discharged current, by referring to the table in which a secondary transfer voltage used at the time of sensing, a thickness of paper, a capacitance of the paper, an electrical resistance of the paper, a secondary transfer current, and a discharged current are brought in correspondence with one another as described above. 
     In step S 160 , control unit  101  determines a secondary transfer voltage as the transfer condition. Control unit  101  determines a secondary transfer voltage based on the estimated capacitance of the paper and electrical resistance of the paper. Control unit  101  may use the table above, or may use an operational expression set in advance to be able to determine a secondary transfer voltage based on the capacitance of the paper and the electrical resistance of the paper. 
     Though an example in which step S 160  is performed next to step S 150  has been exemplified and described, limitation thereto is not intended, and step S 150  and step S 160  may simultaneously be performed. After step S 160  or step  170  is performed, the process returns to step S 10 . 
       FIG. 13  is a diagram showing a second example of the flow in which a transfer condition is determined in the image formation apparatus according to the embodiment. The second example of the flow for determining a transfer condition in image formation apparatus  100  according to the embodiment will be described with reference to  FIG. 13 . 
     Though physical properties of paper are estimated at the time of printing of an image in the first example described above, in the second example of the flow for determining a transfer condition, the transfer condition is determined during transportation of a first sheet of paper or a first sheet of paper after change in type of the paper without forming an image. 
     When coverage of toner formed on the paper is large, a secondary transfer current and a discharged current are affected by charging of toner. Therefore, physical properties of the paper may not accurately be estimated depending on coverage and a humidity. 
     Physical properties of the paper can accurately be estimated by sensing a secondary transfer current and a discharged current while no image is formed as in the second example. 
     The second example includes a flow for specifying on which side of the peak in a distribution of electrical resistances of the paper a sensed discharged current is located, with the abscissa representing an electrical resistance of the paper and the ordinate representing a discharged current. 
     As shown in  FIG. 13 , the second example of the flow for determining a transfer condition is different from the first example in that steps S 41  to S 43  are performed between step S 40  of provisionally determining a secondary transfer voltage and step S 60  of passing the paper instead of step S 50  of issuing an image output instruction and steps S 91  to S 96  are performed between step S 90  of estimating physical properties of the paper and step S 100  of determining a secondary transfer voltage. 
     In determining a transfer condition, steps S 10  to S 40  are performed as in the first example. 
     In step S 41 , control unit  101  obtains printing information (image information). Control unit  101  determines whether or not to perform double-sided printing or single-sided printing by obtaining printing information (image information). A method of passage of paper is determined in step S 60  which will be described later, based on the printing information indicating either double-sided printing or single-sided printing. 
     In succession, control unit  101  determines in step S 42  whether or not a non-image formation mode has been set. When it is determined that the non-image formation mode has been set (step S 42 : YES), step S 60  is performed. When it is determined that the non-image formation mode has not been set (step S 42 : NO), step S 43  is performed. 
     In step S 43 , the non-image formation mode is set. For example, a user selects the non-image formation mode through the operation panel. 
     In step S 60 , control unit  101  has paper S passed from the cassette toward tray  48  along the transportation path. When it is determined in step S 41  that double-sided printing is to be performed as described above, steps S 70  to S 90  are performed and thereafter the paper is passed through a path for double-sided printing such that an image can be formed. The paper which has been passed for determining a transfer condition is passed such that the paper can be used again in formation of an image. 
     When it is determined in step S 41  that single-sided printing is to be performed, the paper is passed to be ejected onto tray  48 . The paper which has been passed for determining a transfer condition is ejected to tray  48 . In this case, an alarm for returning the paper ejected onto tray  48  to the cassette is preferably shown on a display portion of the operation panel. 
     In succession, steps S 70  to S 90  are performed as in the first example. Step S 91  is performed while or after physical properties of the paper are estimated in step S 90 . 
     In step S 91 , control unit  101  determines whether or not sensing information is insufficient. 
     When a discharged current sensed by second sensing unit  72  has a value in the vicinity of the peak in the distribution of electrical resistances of the paper, the value is substantially the same on a low resistance side and a high resistance side with respect to the peak, and whether the value of the discharged current is located on the low resistance side or the high resistance side should be determined. 
     In this case, it is difficult to accurately estimate physical properties of the paper simply based on the secondary transfer current sensed by first sensing unit  71  and control unit  101  determines that the sensing information is insufficient. Specifically, for example, when the sensed discharged current is within a range not lower than 80% of the peak value, control unit  101  determines that the sensing information is insufficient. When the sensed discharged current has a value distant from the peak value, for example, when the sensed discharged current is lower than 80% of the peak value, it is determined that the sensing information is sufficient. 
     When it is determined that the sensing information is sufficient (step S 91 : NO), step S 100  is performed. 
     When it is determined that the sensing information is insufficient (step S 91 : YES), step S 92  is performed. In step S 92 , control unit  101  changes a secondary transfer voltage. For example, control unit  101  raises the secondary transfer voltage by several hundred V. 
     In step S 93 , control unit  101  has paper S passed from the cassette toward tray  48  along the transportation path. When it is determined that double-sided printing is to be performed as above, the paper which has been passed in step S 60  is again passed. When it is determined that single-sided printing is to be performed as above, the paper which has been ejected to tray  48  and returned to the cassette is passed. When the paper ejected to tray  48  has not been returned to the cassette, a next sheet of paper may be passed. 
     In succession, a secondary transfer current is sensed in step S 94 . Specifically, first sensing unit  71  senses a secondary transfer current which flows to the secondary transfer apparatus when paper S passes through the secondary transfer nip portion. The sensed secondary transfer current is input to control unit  101 . 
     A discharged current is sensed in step S 95 . Specifically, second sensing unit  72  senses a discharged current which flows from paper S charged during passage through the secondary transfer nip portion to electricity removal electrode  62 . The sensed discharged current is input to control unit  101 . 
     In succession, in step S 96 , control unit  101  estimates physical properties of paper (more specifically, a capacitance of the paper and an electrical resistance of the paper). 
     When the secondary transfer voltage has been changed, in a region where a discharged current lowers (on the left side of (before) the peak) due to a low electrical resistance of the paper, the discharged current is less likely to be affected by the secondary transfer voltage, whereas the discharged current is raised if the secondary transfer voltage is raised in a region (on the right of (after) the peak) where the discharged current lowers due to a high electrical resistance of the paper. 
     Therefore, by sensing how the discharged current varies in response to variation in secondary transfer voltage, on which side of the peak a value for the first measured discharged current is located can be distinguished. 
     Control unit  101  makes distinction above and estimates physical properties of the paper by referring to the table in which a secondary transfer voltage used at the time of sensing, a thickness of paper, a capacitance of the paper, an electrical resistance of the paper, a secondary transfer current, and a discharged current are brought in correspondence with one another. 
     Control unit  101  determines in step S 100  a secondary transfer voltage as the transfer condition. 
     Control unit  101  determines a secondary transfer voltage based on the estimated capacitance of the paper and electrical resistance of the paper. Control unit  101  may use the table above, or may use an operational expression set in advance to be able to determine a secondary transfer voltage based on the capacitance of the paper and the electrical resistance of the paper. 
     Though the second example in which when it is determined that sensing information is sufficient in step S 91 , only first sensing information (a secondary transfer current and a discharged current sensed first) is used to determine a secondary transfer voltage has been exemplified and described, limitation thereto is not intended. On which side of the peak a value for a first measured discharged current is located may be distinguished by measuring a secondary transfer current and a discharged current in forming an image by passing paper after determination of a secondary transfer voltage and sensing how the discharged current varies. After such distinction is made, physical properties of the paper may be estimated and a secondary transfer voltage may be determined as the transfer condition in next or subsequent printing. 
       FIG. 14  is a diagram showing a third example of the flow in which a transfer condition and a cooling condition are determined in the image formation apparatus according to the embodiment. The third example of the flow for determining a transfer condition and a cooling condition in image formation apparatus  100  according to the embodiment will be described with reference to  FIG. 14 . 
     Though a transfer condition is determined by sensing a secondary transfer current and a discharged current for determining a transfer condition in the non-image formation mode in the second example described above, in the third example of the flow for determining a transfer condition and a cooling condition, a transfer condition and a cooling condition are determined by sensing a secondary transfer current and a discharged current for determining a transfer condition in an image formation mode. 
     As shown in  FIG. 14 , the third example is different from the second example in that steps S 42 A and S 50  are performed instead of steps S 42  and S 43  and step S 180  is further performed after step S 100 . 
     In the third example, in determining a transfer condition and a cooling condition, steps S 10  to S 41  are performed as in the second example. 
     Control unit  101  determines in step S 42 A whether or not to perform printing from a first sheet. Control unit  101  determines whether or not an image formation mode in which an image is formed on paper S has been set. 
     When it is determined that printing is to be performed from a first sheet (step S 42 A: YES), step S 50  is performed. In step S 50 , control unit  101  issues an image output instruction. A toner image corresponding to an image to be output by the image formation portion is thus formed. In succession, step S 60  and subsequent steps are performed as in the second example. 
     When it is determined that printing is not to be performed from a first sheet (step S 42 A: NO), it is determined that a transfer condition and a cooling condition are determined in the non-image formation mode. In this case, step S 60  and subsequent steps are performed as in the second example. 
     After step S 100  of determining a secondary transfer voltage, step S 180  is performed. Step S 180  may be performed simultaneously with step S 100 . 
     Control unit  101  determines in step S 180  the number of rotations of a cooling fan as the cooling condition. Specifically, control unit  101  estimates an electrical resistance of the paper based on the sensed secondary transfer current and discharged current, and when the estimated electrical resistance of the paper is high, the control unit increases an amount of heat absorption from paper S by cooling apparatus  80 . 
     Evaporation of moisture from paper S can be suppressed by cooling paper S after fixation. Lowering in transferability to a second surface particularly when paper high in electrical resistance is used can thus be suppressed. 
     When an electrical resistance of the paper is low, control unit  101  determines, for example, that cooling of paper S is not necessary, and the control unit lowers the number of rotations of the cooling fan or turns off the cooling fan. Power consumption can thus be reduced. 
       FIG. 15  is a diagram showing one example of a table that is used in determining physical properties of paper, a transfer condition, and a cooling condition based on a secondary transfer current and a discharged current sensed by the first sensing unit and the second sensing unit according to the embodiment. 
     As described above, when the number of rotations of the cooling fan is determined in step S 180  as the cooling condition, for example, a table as shown in  FIG. 15  in which sensed secondary transfer current and discharged current, a capacitance of paper, an electrical resistance of the paper, a secondary transfer voltage as the transfer condition, and the number of rotations of the cooling fan as the cooling condition are brought in correspondence with one another is used. 
     Control unit  101  sets as appropriate the number of rotations of the cooling fan in accordance with the sensed discharged current even though the sensed secondary transfer current has a prescribed value (the same value). Control unit  101  sets as appropriate the number of rotations of the cooling fan in accordance with the sensed secondary transfer current even though the sensed discharged current has a prescribed value (the same value). A cooling condition can thus appropriately be set. 
       FIG. 16  is a diagram showing a fourth example of the flow in which a transfer condition and a cooling condition are determined in the image formation apparatus according to the embodiment. The fourth example of the flow for determining a transfer condition and a cooling condition in image formation apparatus  100  according to the embodiment will be described with reference to  FIG. 16 . 
     The fourth example of the flow for determining a transfer condition and a cooling condition is mainly different from the third example in that a discharged current sensed in step S 95  after change in secondary transfer voltage in step S 92  and a discharged current sensed in step S 80  before change in secondary transfer voltage are compared with each other, and when an abnormal condition is sensed, notification of the abnormal condition is given. 
     A secondary transfer current can automatically be corrected in accordance with an environment and a resistance of a roller by applying a bias when no paper is passed. Therefore, when a secondary transfer current is abnormal, such an abnormal condition can be sensed at the time of correction. 
     An abnormal condition of a discharged current, however, cannot be sensed while no paper is passed. Unless physical properties of paper are known, it is difficult to sense an abnormal condition only with a measured discharged current even though paper is passed. Since a discharged current is less likely to flow when an electrical resistance of paper is low, one may not be able to distinguish whether such a situation is caused by an abnormal condition or a low resistance of the paper. Examples of the abnormal condition in discharged current include poor conduction due to paper dust and a foreign matter. 
     Since physical properties of paper are estimated based on sensed values of both of a secondary transfer current and a discharged current in the present embodiment, when an abnormal condition of a discharged current occurs, physical properties of paper may erroneously be estimated. In the fourth example, the flow allowing sensing of an abnormal condition of a discharged current as described above is provided. 
     Specifically, in the fourth example, when a discharged current sensed by second sensing unit  72  at the time of application of a higher voltage of secondary transfer voltages different from each other applied before and after change is equal to or lower than a discharged current sensed by second sensing unit  72  at the time of application of a lower voltage of the secondary transfer voltages different from each other, a notification portion provided in the image formation apparatus gives a notification of the abnormal condition. 
     In the fourth example, when the notification portion gives a notification about the abnormal condition, image formation processing is stopped or a secondary transfer voltage is set only based on a secondary transfer current sensed by first sensing unit  71 . 
     As shown in  FIG. 16 , the fourth example is different from the third example in that steps S 191  to S 193  are performed. 
     In the fourth example, in determining a transfer condition and a cooling condition, steps S 10  to S 96  are performed as in the third example. 
     When step S 96  is performed, step S 190  is performed. Control unit  101  determines in step S 190  whether or not an abnormal condition of a discharged current has occurred. Specifically, control unit  101  determines whether or not a discharged current sensed by second sensing unit  72  at the time of application of a higher voltage of secondary transfer voltages different from each other applied across secondary transfer roller  33  and drive roller  39  is higher than a discharged current sensed by second sensing unit  72  at the time of application of a lower voltage of the secondary transfer voltages different from each other. 
     When the discharged current sensed at the time of application of a higher voltage of the secondary transfer voltages different from each other is higher than the discharged current sensed at the time of application of a lower voltage of the secondary transfer voltages different from each other, it is determined that no abnormal condition has occurred. 
     When the discharged current sensed at the time of application of a higher voltage of the secondary transfer voltages different from each other is not higher than the discharged current sensed at the time of application of a lower voltage of the secondary transfer voltages different from each other, it is determined that an abnormal condition has occurred. 
     When it is determined that an abnormal condition of a discharged current has not occurred (step S 190 : NO), step S 100  is performed. 
     When it is determined that an abnormal condition of a discharged current has occurred (step S 190 : YES), step S 191  is performed. 
     In step S 191 , the notification portion gives a notification of an abnormal condition of the discharged current. Specifically, an indication of an abnormal condition is provided on a display panel or a notification of an abnormal condition is given to a maintenance service provider through wired or wireless communication. 
     In step S 192 , a secondary transfer voltage is provisionally determined only based on a value for a secondary transfer current sensed by first sensing unit  71  without using a sensed value for the discharged current. In succession, in step S 193 , the number of rotations of the cooling fan is provisionally determined. 
     Steps S 192  and S 193  may simultaneously be performed and steps S 191 , S 192 , and S 193  may also simultaneously be performed. 
     As set forth above, by setting a transfer condition for transferring a toner image to a recording medium based on a secondary transfer current sensed by first sensing unit  71  and a discharged current sensed by second sensing unit  72 , the image formation apparatus according to the present embodiment can set a transfer condition more accurately than in an example in which a transfer condition is set by sensing any one of a secondary transfer current and a discharged current. 
     In setting a transfer condition, by estimating an electrical resistance and a capacitance of paper based on the sensed secondary transfer current and discharged current and setting a transfer condition based on the estimated electrical resistance and capacitance of the paper, a transfer condition which is difficult to uniquely be determined based on only one of the electrical resistance and the capacitance of paper can accurately be set. 
     Though an example in which a first electrode portion including a contact electrode in contact with transported paper and a counter electrode arranged to be opposed to the contact electrode such that the transported paper lies between the contact electrode and the counter electrode is implemented by the secondary transfer apparatus has been exemplified and described in the present embodiment, limitation thereto is not intended. 
     So long as a voltage can be applied across the contact electrode and the counter electrode while the paper lies between the contact electrode and the counter electrode, the contact electrode and the counter electrode may be in a form of a plate or in a form of a roller. In this case, the contact electrode and the counter electrode may be arranged upstream or downstream from the secondary transfer apparatus in the direction of transportation of paper. 
     When the first electrode portion is implemented by the secondary transfer apparatus as described above, the number of components can be reduced. 
     Though an example in which the second electrode portion arranged as not being in contact with paper such that charges applied to paper are movable is implemented by an electricity removal electrode has been exemplified and described, limitation thereto is not intended and modification as appropriate within the scope not departing from the gist of the present invention can be made. 
     The image formation apparatus based on the present invention described above includes a first electrode portion including a contact electrode and a counter electrode, the contact electrode being in contact with a transported recording medium, the counter electrode being arranged to be opposed to the contact electrode such that the transported recording medium lies between the contact electrode and the counter electrode, a second electrode portion arranged as not being in contact with the recording medium such that charges applied to the recording medium are movable, a first sensing unit configured to sense a first current which flows to the first electrode portion as a result of application of a voltage across the contact electrode and the counter electrode while the recording medium lies between the contact electrode and the counter electrode, a second sensing unit configured to sense a second current which flows from the charged recording medium to the second electrode portion, and a control unit configured to receive input of results of sensing by the first sensing unit and the second sensing unit. The control unit is configured to set a transfer condition for transferring a toner image to the recording medium based on the first current sensed by the first sensing unit and the second current sensed by the second sensing unit. 
     In the image formation apparatus based on the present invention, the control unit preferably estimates an electrical resistance and a capacitance of the recording medium based on the sensed first current and the sensed second current and preferably sets the transfer condition based on the estimated electrical resistance and capacitance of the recording medium. 
     In the image formation apparatus based on the present invention, preferably, the first electrode portion is implemented by a transfer apparatus configured to transfer the toner image carried on a toner image carrier to the recording medium. 
     In the image formation apparatus based on the present invention, preferably, the second electrode portion is implemented by an electricity removal electrode which removes charges applied to the recording medium. 
     In the image formation apparatus based on the present invention, preferably, the second electrode portion is arranged downstream from the first electrode portion in a direction of transportation of the recording medium. 
     In the image formation apparatus based on the present invention, when relation between an electrical resistance of the recording medium and the second current is shown with the electrical resistance of the recording medium being shown on an abscissa and the second current being shown on an ordinate, a distribution of electrical resistances of the recording medium may be expressed with a projecting shape having a peak. In this case, preferably, when the sensed second current has a value in a region around the peak, the control unit sets the transfer condition based on the first current sensed by the first sensing unit as a result of application of different voltages across the contact electrode and the counter electrode and the second current which flows from the recording medium to the second electrode portion after application of the different voltages and is sensed by the second sensing unit. 
     The image formation apparatus based on the present invention may further include a notification portion which gives a notification about an abnormal condition when the second current sensed by the second sensing unit at the time of application of a higher voltage of the different voltages applied across the contact electrode and the counter electrode is not higher than the second current sensed by the second sensing unit at the time of application of a lower voltage of the different voltages applied to the first electrode portion. 
     In the image formation apparatus based on the present invention, preferably, when the notification portion gives the notification about the abnormal condition, the control unit stops image formation processing or sets the transfer condition only based on the first current sensed by the first sensing unit. 
     In the image formation apparatus based on the present invention, preferably, the control unit sets the transfer condition based on the first current sensed by the first sensing unit and the second current sensed by the second sensing unit when printing is performed on a first sheet of the recording medium or a first sheet of the recording medium after change in type of the recording medium. 
     In the image formation apparatus based on the present invention, the control unit may set the transfer condition based on the first current sensed by the first sensing unit and the second current sensed by the second sensing unit in transporting a first sheet of the recording medium or a first sheet of the recording medium after change in type of the recording medium without forming an image. 
     In the image formation apparatus based on the present invention, preferably, the control unit estimates an electrical resistance and a capacitance of the recording medium based on the sensed first current and the sensed second current. In this case, the control unit preferably raises a transfer voltage to be applied to the transfer apparatus in transfer of the toner image from a toner image carrier to the recording medium when the estimated electrical resistance of the recording medium is high, and lowers the transfer voltage when the capacitance of the recording medium is high. 
     The image formation apparatus based on the present invention may further include a cooling apparatus configured to cool the recording medium after the toner image transferred to the recording medium is fixed. In this case, preferably, the control unit sets a cooling condition of the cooling apparatus based on the first current sensed by the first sensing unit and the second current sensed by the second sensing unit. 
     In the image formation apparatus based on the present invention, preferably, the control unit estimates an electrical resistance of the recording medium based on the sensed first current and the sensed second current. In this case, the control unit preferably increases an amount of heat absorption from the recording medium by the cooling apparatus when the estimated electrical resistance of the recording medium is high. 
     Although embodiments of the present invention have been described and illustrated in detail, the disclosed embodiments are made for the purposes of illustration and example only and not limitation. The scope of the present invention should be interpreted by terms of the appended claims.