Abstract:
Provided is an electrode for stably generating a glow corona discharge. The tip of an electrode applied in a discharge testing device either forms a continuous blade constituting a closed curve within a plane orthogonal to the length direction of the electrode, with serrated teeth being formed in the direction in which the blade extends, or, forms a continuous blade constituting a closed curve within a plane orthogonal to the length direction of the electrode, with a base portion for supplying a voltage to the electrode being connected to the tip through a resistor having a predetermined resistance value.

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
       [0001]    This application is a continuation application of international application No. PCT/JP2014/078784 filed Oct. 29, 2014, which claims priority to Japanese Patent Application No. 2014-94461 filed May 1, 2014, each of which is hereby incorporated by reference in their entity. 
     
    
     BACKGROUND 
       [0002]    1. Technical Field 
         [0003]    Embodiments described herein relates to a discharge electrode suitable for generation of glow corona discharge and a testing device including the discharge electrode. 
         [0004]    2. Description of Related Art 
         [0005]    As a conventionally-known phenomenon during operation of airplanes, particles in the air, including water vapor, rain, and snow, are charged by friction therebetween, and when such particles collide with airplane bodies or engines, the bodies and engines are charged with static electricity. Components provided with no grounding path to the atmosphere generate glow corona discharge to the atmosphere due to the charged static electricity. Such a discharge of static electricity sometimes causes failure in radios and flight equipment as noise or the like. This phenomenon is called precipitation static. 
         [0006]    Recently, airplanes use many nearly insulating composite materials, and there is a demand for proper evaluation of precipitation static in order to prevent precipitation static even in the airplanes using such composite materials. The evaluation needs to generate stable glow corona discharge so as to replicate the precipitation static. In the process of finally discharging the charged static electricity out of airplanes, it is necessary to provide a proper conducting path for even a metallic component as a conductor to lead the static electricity. Gap in the middle of the conducting path causes unintended discharge. 
       PATENT DOCUMENT 
     Patent Document 1: JP S60-132666 A 
     SUMMARY 
       [0007]    There is no method established which can stably generate glow corona discharge. Moreover, the glow corona discharge is known to become unstable depending on the conditions, including the electrode shape and the range of voltage, and produce Trichel pulses. 
         [0008]    The present embodiments are proposed in the light of the aforementioned circumstances, and an object thereof is to provide a discharge electrode capable of stably generating glow corona discharge and a testing device using the discharge electrode to evaluate a component. 
         [0009]    To solve the aforementioned problems, a discharge electrode according to the application is a discharge electrode generating glow corona discharge, including: a tip section including a continuous blade forming a closed curve in a plane vertical to the longitudinal direction of the discharge electrode, the blade including a serrated edge formed in the direction that the blade extends. 
         [0010]    The tip section is a cylindrical shape, and the blade is formed by a part of the inner surface of the cylindrical shape and a part of the outer surface thereof inclined inward. The serrated edge may include a plurality of serrations having an identical shape. 
         [0011]    A discharge electrode according to the application is a discharge electrode generating glow corona discharge, including: a tip section including a continuous blade forming a closed curve in a plane vertical to the longitudinal direction of the discharge electrode; and a base section supplying voltage to the discharge electrode. The tip section is connected to the base section through a resistor having a predetermined resistance value. 
         [0012]    The tip section may have a cylindrical shape, and the blade is formed by a part of the inner surface of the cylindrical shape and a part of the outer surface thereof inclined inward. The tip section and resistor may be joined coaxially by screwing or conducting adhesive. 
         [0013]    A testing device according to the application evaluates a component by using any one of the discharge electrodes to supply glow corona discharge to the component through the discharge electrode. 
         [0014]    The discharge electrode according to the application is capable of stably generating glow corona discharge with reduced Trichel pulses independently of the conditions including the electrode shape and voltage range. 
         [0015]    The discharge electrode according to the application includes the electrode stably generating glow corona discharge and thereby enabling to effectively pursue the test of components over the precipitation static. 
     
    
     
       BRIEF DESCRIPTION OF DRAWINGS 
         [0016]      FIGS. 1A and 1B  are block diagrams illustrating a schematic configuration of a discharge testing device. 
           [0017]      FIGS. 2A and 2B  are perspective views illustrating appearances of an electrode section and an electrode section supporting member. 
           [0018]      FIG. 3  is a perspective view illustrating the appearance of a blade-shaped/serrated electrode. 
           [0019]      FIGS. 4A and 4B  are diagrams illustrating the dimensions of the blade-shaped/serrated electrode. 
           [0020]      FIG. 5  is a partially-enlarged side view illustrating the dimensions of the blade-shaped/serrated electrode. 
           [0021]      FIG. 6  is a partially-enlarged side view illustrating the dimensions of the blade-shaped/serrated electrode. 
           [0022]      FIGS. 7A and 7B  are diagrams illustrating the potential distribution around the blade-shaped/serrated electrode. 
           [0023]      FIGS. 8A and 8B  are diagrams illustrating the potential distribution around the blade-shaped/serrated electrode when each serration has a smaller angle than that of  FIGS. 7A and 7B . 
           [0024]      FIGS. 9A and 9B  are diagrams illustrating the potential distribution around the blade-shaped/serrated electrode when each serration has a larger angle than that of  FIGS. 7A and 7B . 
           [0025]      FIG. 10  is a graph illustrating measurements of applied voltage and discharge current. 
           [0026]      FIG. 11  is a perspective view illustrating the appearance of a pair of the blade-shaped electrode and a resistor. 
           [0027]      FIGS. 12A and 12B  are side and perspective views illustrating the pair of the blade-shaped electrode and resistor, respectively. 
           [0028]      FIG. 13  is a cross-sectional diagram illustrating the potential distribution around the joint between the pair of the blade-shaped electrode and resistor. 
           [0029]      FIGS. 14A and 14B  are cross-sectional diagrams illustrating the potential distribution around the joint of the pair of the blade-shaped electrode and resistor under the same conditions as those in  FIG. 13  except that the resistor has a smaller diameter than the electrode. 
           [0030]      FIG. 15  is a cross-sectional diagram illustrating the electric field distribution around the pair of the blade-shaped electrode and resistor. 
           [0031]      FIG. 16  is a cross-sectional diagram illustrating the electric field distribution around the joint of the pair of the blade-shaped electrode and resistor, which is obtained by reducing the resistance of the resistor in  FIG. 15 . 
           [0032]      FIG. 17  is a perspective view illustrating the appearance of the electrode section including a comb-shaped electrode, an electrode section supporting member, and a test piece. 
           [0033]      FIG. 18  is a graph illustrating measurements of applied voltage and discharge current for the comb-shaped electrode. 
           [0034]      FIG. 19  is a perspective view illustrating the appearance of the blade-shaped electrode. 
           [0035]      FIG. 20  is a perspective view illustrating the appearances of the electrode section including a linear electrode, an electrode section supporting member, and a test piece. 
       
    
    
     DETAILED DESCRIPTION OF EMBODIMENTS 
       [0036]    Hereinafter, a description is given of embodiments of a discharge electrode and a testing device in detail with reference to the drawings.  FIGS. 1A and 1B  are block diagrams illustrating a schematic configuration of a discharge testing device including a discharge electrode. In  FIG. 1A , a test piece  100  to be tested is placed. 
         [0037]    The discharge testing device includes an electrode section  10  and an electrode section supporting member  20 . On the lower surface of the electrode section supporting member  20 , at least one electrode section  10  can be provided. The discharge testing device further includes a grounding plate  30  and an insulator  31 . The grounding plate  30  is situated under the electrode section  10  and faces the electrode section  10 . The insulator  31  is situated on the grounding plate  30  and has a predetermined thickness. The test piece  100  to be subjected to a discharge test is placed on the grounding plate  30  with the insulator  31  interposed therebetween, and there is a distance D 0  between the test piece  100  and electrode section  10 . 
         [0038]    The grounding plate  30  provides a potential reference to the discharge testing device. The grounding plate  30  can be made of an aluminum or copper plate and have a rectangular or circular shape. The distance D 0  between the test piece  100  and electrode section can be 1 inch, or 25.4 mm, for example. 
         [0039]    To the electrode section supporting member  20 , a predetermined voltage is applied from a power supply  40 . The power supply  40  includes a high-voltage DC power supply  41  and a switch  43  and can supply a voltage at a predetermined timing. The high-voltage DC power supply  41  can change the supply voltage in a range from −30 kV to 0 V and can supply 10 μA current at −30 kV. 
         [0040]    The test piece  100  is subjected to a test that evaluates performances related to discharge by the discharge testing device. The test piece  100  is an airplane component, for example. The test piece  100  is grounded through a shunt resistance  50 . The shunt resistance  50  detects discharge current flown to the test piece  100  from the electrode section  10  as a voltage. The shunt resistance  50  can be 5 kΩ, for example. In this case, discharge current of 10 μA generates a voltage of 50 mV. 
         [0041]    The voltage generated across the shunt resistance  50  is measured by an oscilloscope  60  and a personal computer (PC)  70  connected to the oscilloscope  60 , The shunt resistance  50  and oscilloscope  60  are connected to each other through a 5 m-long 50 Ω coaxial cable, for example. The shunt resistance  50  is equipped with an overvoltage protection circuit and a filter circuit. The overvoltage protection circuit includes a Zener circuit or the like. The overvoltage protection circuit prevents the oscilloscope  60  from undergoing overvoltage failure due to excess discharge including Trichel pulses. The filter circuit is configured so that the measurement frequency band does not change depending on the length of and bend in the coaxial cable. 
         [0042]    The oscilloscope  60  measures the voltage generated across the shunt resistance  50  and converts the measured voltage into discharge current flowing through the test piece  100  and discharge current flowing without passing through the test piece  100 . The PC  70  analyses the measurements obtained by the oscilloscope  60  and provides a voltage-current characteristic (VI characteristic) graph, for example. 
         [0043]      FIG. 1B  illustrates the installation for correcting only the performance of the electrode section  10  in the discharge testing device. Herein, the grounding plate  30  is situated at the distance D 0  from the electrode section  10 . The installation for correction is the same as that with the test piece  100  placed in the discharge testing device as illustrated in  FIG. 1A  except that the test piece  100  and insulator  31  are removed and the grounding plate  30  is brought close to the position corresponding to the upper end of the test piece  100 , that is, the position at the distance D 0  from the electrode section  10 . Hereinafter, the installation for correction illustrated in  FIG. 1B  is used to evaluate the performances of various types of electrodes. 
         [0044]      FIGS. 2A and 2B  are diagrams illustrating the configurations of the electrode section  10  and electrode section supporting member  20 .  FIG. 2A  is a perspective view illustrating an appearance of the electrode section  10  and electrode section supporting member  20  as seen from the side of the electrode section supporting member  20  with the electrode section  10  attached.  FIG. 2B  is a perspective view illustrating the appearance of the electrode section  10  and electrode section supporting member  20  which are situated on the grounding plate  30 . 
         [0045]    The electrode section  10  includes an electrode  10   a  rotationally axisymmetric in the direction of an axis and a shaft  10   b  having the same diameter as that of the electrode  10   a.  The shaft  10   b  is attached to the electrode  10   a  so as to extend on the same axis. The base of the electrode  10   a,  which is opposite to the tip thereof, is screwed to an end of the shaft  10   b.  The electrode  10   a  can be fixed by a conductive adhesive instead of screwing. The same goes for the following description. 
         [0046]    The electrode section supporting member  20  is made of a circular metallic plate with a predetermined thickness and includes holes  20   b  at predetermined intervals so as to penetrate the metallic plate. The electrode section  10  is attached to the electrode section supporting member  20  with the other end of the shaft  10   b  screwed to one of the holes  20   b  so that the longitudinal direction of the electrode section  10  is vertical to one surface of the electrode section supporting member  20 . 
         [0047]    The shaft  10   b  is substantially longer than the electrode  10   a  in the longitudinal direction so that the electrode  10   a  is situated at a predetermined distance from the surface of the electrode section supporting member  20 . The electrode  10   a  and shaft  10   b  of the electrode section  10  and the electrode section supporting member  20  can be made of proper metal such as brass or aluminum, for example. 
         [0048]      FIGS. 2A and 2B  illustrate an example in which nine electrode sections  10  are attached at regular intervals in a radial direction of the electrode section supporting member  20 . The installation is not limited to this example. In actual testing of the test piece  100  where the electrode sections  10  are provided so as to face the test piece  100 , a number of electrode sections  10  can be arranged at positions in accordance with the size of the test piece  100  by attaching the electrode sections  10  to the respective holes  20   b  of the electrode section supporting member  20 . 
         [0049]    As illustrated in  FIG. 2B , the electrode sections  10  and electrode section supporting member  20  are arranged with the electrode sections  10  facing down so as to face the grounding plate  30 . On the surface of the electrode section supporting member  20  with the electrode sections  10  attached, plural spacers  20   a  having a predetermined length are attached. 
         [0050]    These spacers  20   a  ensure the predetermined distance between the electrode section supporting member  20  and grounding plate  30  so as to maintain the distance D 0  between the electrode section  10  and grounding plate  30 . Each of the spacers  20   a  is made of an insulator so as to electrically isolate the electrode section supporting member  20  from the grounding plate  30 . 
       First Embodiment 
       [0051]    As a first embodiment, a description is given of an electrode  10   a  having a blade-shaped and serrated tip (hereinafter, just referred to as a blade-shaped/serrated type). The dimensions thereof are just examples of electrodes used in testing, and the actual profile is not limited to the embodiment. 
         [0052]      FIG. 3  is a perspective view illustrating the appearance of the electrode  10   a . The electrode has a cylindrical or columnar profile rotationally axisymmetric in the direction of the axis and includes a blade section at the tip. The blade section has an outer surface inclined inward. At the tip of the electrode  10   a,  the blade section includes  54  identical serrations to form a serrated section. 
         [0053]    Considering the necessity to equally and continuously arrange the serrations on the circumference and the workability thereof, the number of the serrations may be even but may be odd. At processing of the serration part including an even number of serrations by using a discharge wire, two serrations facing each other across the axis can be simultaneously formed, thus shortening the processing time. 
         [0054]      FIG. 4A  is a side view of the electrode  10   a,  The electrode  10   a  has dimensions of: height H 1 =15 mm and diameter D 1 =10 mm.  FIG. 4B  is a cross-sectional view of the electrode  10   a.  The electrode  10   a  has dimensions of: height H 2  of the inner surface from the base to the tip=10 mm and inner diameter D 2 =9 mm. 
         [0055]      FIG. 5  is a partially-enlarged view of the side surface of a part of the electrode  10  including the tip. As illustrated in  FIG. 5 , the dimensions of the tip are: height E of each serration=1 mm, the distance L between apexes of adjacent serrations or the base length=0.52 mm, and apex angle F 1  of each serration in the circumferential direction=29°. 
         [0056]      FIG. 6  is a partially-enlarged cross-sectional view of the part including the tip of the electrode  10   a.  As illustrated in  FIG. 6 , the outer surface of the tip has a surface inclined at a predetermined angle toward the inner surface. Herein, base thickness T of each serration=0.5 mm and apex angle F 2  of each serration in the thickness direction=30°. 
         [0057]      FIGS. 7A and 7B  are diagrams illustrating the potential distribution around the electrode  10   a.  As described above, at the tip of the electrode  10   a,  the apex angles F 1  and F 2  of each serration in the circumferential direction and in the thickness direction are 29° and 30°, respectively. 
         [0058]      FIG. 7A  is a cross-sectional view illustrating the potential distribution around the electrode  10   a  and shaft lob, and  FIG. 7B  is a partially-enlarged cross-sectional view illustrating the potential distribution around the tip of the electrode  10   a.  These potential distributions are calculated by simulation. The same goes for the following graphs. 
         [0059]    As apparent from  FIGS. 7A and 7B , equipotential surfaces are densely distributed in the axial direction from the tip of the electrode  10   a,  and the electric field is large in the axial direction. In contrast, equipotential surfaces are sparsely distributed in the circumferential direction from the tip of the electrode  10   a,  and the electric field is small in the circumferential direction. 
         [0060]      FIGS. 8A and 8B  are diagrams illustrating the potential distribution around the electrode  10   a  in a comparative example in which the apex angles F 1  and F 2  of each serration in the circumferential direction and in the thickness direction are set to 5°.  FIG. 8A  is a cross-sectional view illustrating the potential distribution around the electrode  10   a  and shaft  10   b,  and  FIG. 8B  is a partially-enlarged cross-sectional view illustrating the potential distribution around the tip of the electrode  10   a.    
         [0061]    Compared with the first embodiment illustrated in  FIGS. 7A and 7B , in the comparative example of  FIGS. 8A and 8B , in which the apex angles F 1  and F 2  of each serration are reduced, the equipotential surfaces are distributed more densely in the axial and circumferential directions of the electrode  10   a,  and the electric field is increased. Accordingly, discharge is more likely to occur in the environment of the comparative example than in the environment of the first embodiment. However, the comparative example of  FIGS. 8A and 8B  has concerns about erosion due to heat of discharge current and reduction in processing accuracy. 
         [0062]      FIGS. 9A and 9B  are diagrams illustrating the potential distribution around the electrode  10   a  in a comparative example in which the apex angles F 1  and F 2  of the serrations in the circumferential direction and in the thickness direction are set to 55°.  FIG. 8A  is a cross-sectional view illustrating the potential distribution around the electrode  10   a  and shaft  10   b,  and  FIG. 8B  is a partially-enlarged cross-sectional view illustrating the potential distribution around the tip of the electrode  10   a.    
         [0063]    In the comparative example of  FIGS. 9A . and  9 B, in which the apex angles F 1  and F 2  of each serration are increased, compared with the first embodiment illustrated in  FIGS. 7A and 7B , the equipotential surfaces are distributed more sparsely both in the axial and circumferential directions of the electrode  10   a,  and the electric field is reduced. Accordingly, discharge is less likely to occur in the environment of the comparative example than in the environment of the first embodiment. 
         [0064]    In the first embodiment illustrated in  FIGS. 7A and 7B , the apex angles F 1  and F 2  of each serration in the circumferential and thickness directions are 29° and 30°, respectively. The apex angles F 1  and F 2  of each serration are not limited thereto. Comparison with the comparative examples of  FIGS. 8A and 8B and 9A and 9B  reveals that the apex angles F 1  and F 2  of each serration need to be 5°&lt;F 1 &lt;55° and 5°&lt;F 2 &lt;55°. The apex angles F 1  and F 2  of each serration are preferably 20°&lt;F 1 &lt;40° and 20°&lt;F 2 &lt;40° and more preferably 25°&lt;F 1 &lt;35° and 25°&lt;F 2 &lt;35°. 
         [0065]    By setting the apex angles F 1  and F 2  of each serration in the aforementioned ranges, it is possible to produce an electric field of a predetermined magnitude and ensure the strength and heat radiation performance of the serrations. Accordingly, discharge can be easily generated without causing erosion due to heat or reducing the processing workability. 
         [0066]      FIG. 10  is a diagram illustrating the relationship between voltage applied to the electrode section  10  and discharge current which are measured in the discharge measurement device. The scales thereof are dimensionless. In the measurement, a predetermined voltage supplied from the high-voltage DC power supply  41  of the power supply  40  is applied across the electrode section  10  and grounding plate  30  for a predetermined period of time through the switch  43 , and the current flowing therebetween due to discharge is measured through the shunt resistance  50  using the oscilloscope  60  and PC  70 . 
         [0067]    A line a in  FIG. 10  illustrates the case where two blade-shaped/serrated electrodes are extended by 15 mm from the original position toward the grounding plate  30 . Lines g to l in  FIG. 10  corresponds to comparative examples later described. The slope of the change in current with respect to the change in voltage is smaller than that of the comparative examples. In other words, the embodiment has such robust properties that current is stable under variations in the applied voltage. 
         [0068]    In the first embodiment, it is thought that production of the above-described unequal electric field concentrating at the tip of each serration generates stable discharge as a whole. The discharge voltage is therefore reduced, and there is no Torricelli pulse. As the results of measurement, discharge occurs in a range from 10 to 28 kV and is glow corona discharge stably maintained without producing Trichel pulses. Moreover, as described above, the slope of the change in discharge current with respect to the change in applied voltage is small, and the robust properties are ensured. 
       Second Embodiment 
       [0069]    In a second embodiment, the electrode  10   a  has a blade-shaped tip and is combined and paired with a 100 MΩ resistor having the same diameter as that of the electrode  10   a .  FIG. 11  is a perspective view illustrating the appearance of the electrode section  10  of the second embodiment.  FIG. 12A  is a side view of the electrode section  10  of the second embodiment, and  FIG. 12B  is a cross-sectional view. 
         [0070]    In the second embodiment, the electrode section  10  includes a pair of the blade-shaped electrode  10   a  and 100 MΩ resistor  10   c  which are combined and joined with a screw  10   d.  The blade-shaped electrode  10   a  is different from the blade-shaped/serrated electrode  10   a  of the first embodiment only in including no serrations. The dimensions of the blade-shaped electrode  10   a  are the same as those of the blade-shaped/serrated electrode  10   a  of the first embodiment. The resistor  10   c  has the same dimensions as the shaft  10   b  of the first embodiment and gives a resistance of 100 MΩ in the longitudinal direction. 
         [0071]    The resistor  10   c  includes a coating which is formed by uniformly applying a resistor to the circumferential surface thereof. The resistor  10   c  can be a resistor having a cylindrical structure and including a resistive coating on the surface of the composite or glass cylinder, such as a metal glaze resistor or a ceramic resistor. 
         [0072]      FIG. 13  is a cross-sectional view illustrating the potential distribution around the joint between the paired electrode  10   a  and resistor  10   c.  In  FIG. 13 , in order to reveal the potential distribution around the joint between the electrode  10   a  and resistor  10   c,  the resistance of the resistor  10   c  is set to 0 for convenience. The same goes for  FIGS. 14A and 14B  illustrating a comparative example. 
         [0073]    The resistor  10   c  includes the resistor coating uniformly formed on the outer circumferential surface and can be considered as metal having a resistivity. Accordingly, the equipotential surfaces are continuous smoothly near the surface across the joint between the electrode  10   a  and resistor  10   c.  The electric field is continuous smoothly from the electrode  10   a  in the longitudinal direction of the resistor  10   c.    
         [0074]      FIGS. 14A and 14B  are diagrams illustrating the potential distribution in the comparative example in which the diameter of the resistor  10   c  is smaller than that of the electrode  10   a.    FIG. 14A  is a cross-sectional view illustrating the potential distribution around the pair of the electrode  10   a  and resistor  10   c,  and  FIG. 14B  is a partially-enlarged cross-sectional view illustrating the potential distribution around the joint between the electrode  10   a  and resistor  10   c.    
         [0075]    In this case, the equipotential surfaces are discontinuous at the joint between the electrode  10   a  and resistor  10   c.  In this region, the electric field is significantly disturbed from the electrode  10   a  in the longitudinal direction of the resistor  10   c.    
         [0076]      FIG. 15  is a cross-sectional view illustrating the potential distribution around the electrode  10   a  and resistor  10   c.  Since the resistance of the resistor  10   c  is as high as 100 MΩ, the potential at the tip of the electrode  10   a  is raised, and the electric field is enhanced. The electric field is thereby uniformly distributed along the electrode  10   a  and resistor  10   c  in the longitudinal direction thereof and is remarkably stable. In the case where plural electrodes are used together, the aforementioned effects eliminate the need to control the intervals between the electrodes so as to distribute the electric field uniformly. 
         [0077]      FIG. 16  is a cross-sectional view illustrating the potential distribution in a comparative example with the resistance of the resistor  10   c  set to 0. In this comparative example, the potential at the tip of the electrode  10   a  is not raised, and the direction of the electric field along the electrode  10   a  and resistor  10   c  is inhomogeneous in the longitude direction thereof. Accordingly, in the case where plural electrodes are used together, the electric field could be disturbed due to the interference between adjacent electrodes. 
         [0078]    In  FIG. 10  illustrating the relationship between the voltage applied to the electrode section  10  and discharge current, the second embodiment corresponds to the lines b to e. The line b corresponds to the case where two pairs of the electrode  10   a  and resistor  10   c  are provided on the electrode section supporting member  20 . Similarly, the lines c, d, and e correspond to five pairs, seven pairs, and nine pairs of the electrode  10   a  and resistor  10   c,  respectively. 
         [0079]    Comparing the lines b to e of the second embodiment with the lines g to l of the later-described comparative examples, the slope of the change in current with respect to the change in voltage applied to the electrode section  10  is smaller in the second embodiment than in the comparative examples. This shows the robust properties that current is stable under variations in the applied voltage. 
         [0080]    In the second embodiment, it is thought that addition of the 100 MΩ resistor  10   c  increases the intensity of the electric field at the tip of the blade-shaped electrode  10   a  and therefore reduces the discharge voltage. As the result of measurement, discharge occurs in a range from 10 to 28 kV, and the discharge is glow corona discharge stably maintained without producing Trichel pulses. 
       Third Embodiment 
       [0081]    In a third embodiment, the electrode  10   a  has a blade-shaped/serrated tip and is combined and paired with a 100 MΩ resistor  10   c  having the same diameter as that of the electrode  10   a.  The third embodiment is the same as the second embodiment except that the blade-shaped electrode  10   a  in the second embodiment illustrated in  FIGS. 11 and 12  is replaced with the blade-shaped/serrated electrode  10   a  illustrated in  FIGS. 3 to 6 . 
         [0082]    In  FIG. 10  illustrating the relationship between the voltage applied to the electrode section  10  and discharge current, the line f illustrates the case where two pairs of the blade-shaped/serrated electrode  10   a  and 100 MΩ resistor  10   e  are provided on the electrode section supporting member  20 . 
         [0083]    As for the line f of the third embodiment, the slope of the change in current with respect to the change in voltage is smaller than the slope of not only lines g to l of the comparative examples described later but also the slope of the line a of the first embodiment and the lines b to e of the second embodiment. This shows that the third embodiment provides better robust properties that current is more stable under variations in the applied voltage. 
         [0084]    In the third embodiment, it is thought that the serrated tip generates a non-uniform electric field while the addition of the 100 MΩ resistor  10   c  increases the intensity of the electric field at the tip of the electrode  10   a.  The discharge voltage is therefore further reduced. The discharge is glow corona discharge stably maintained without producing Trichel pulses. Moreover, in the third embodiment, the robust properties are excellent as described above. 
       Fourth Embodiment 
       [0085]    The fourth embodiment is a comb-shaped electrode in which the electrode section  10  includes metallic pins arranged on the surface of the electrode section supporting member  20  in a comb fashion. 
         [0086]      FIG. 17  is a perspective view illustrating the appearance of the electrode section  10 , electrode supporting member  20 , and grounding plate  30  situated facing the electrode section  10  in the fourth embodiment. In the fourth embodiment, the electrode section supporting member  20  is rectangular, unlike the circular shape illustrated in  FIGS. 1A and 1B . 
         [0087]    The electrode section  10  includes plural metallic pointed pins on the surface of the electrode section supporting member  20 . The plural pins are arranged at predetermined intervals in a tetragonal arrangement so that the longitudinal direction of each pin extends vertically to the surface of the electrode section supporting member  20 . The electrode section  10  can be formed by sticking the pins into the electrode section supporting member  20 , which is made of a thin metallic plate, from the back side, for example. 
         [0088]      FIG. 18  is a diagram illustrating the result of measuring the relationship between the voltage applied to the electrode section  10  and discharge current in the fourth embodiment using the discharge measurement device. A line a in  FIG. 18  illustrates the result when the intervals of the pins are 10 mm and a distance D 0  between the electrode section  10  and grounding plate  30  is 25.4 mm. Lines b, c, and d in  FIG. 18  illustrate the results when the distance D 0  between the electrode section  10  and grounding plate  30  is changed to 34, 52, and 64 mm with the intervals of the pins maintained at 10 mm. 
         [0089]    A line e illustrates the result when the area of the electrode section  10  or electrode section supporting member  20  facing the grounding plate  30  is reduced to one third while the distance D 0  between the electrode section  10  and grounding plate  30  is maintained at 25.4 mm and the intervals of the pins are maintained at 10 mm. A line f illustrates the result when the distance D 0  is maintained at 25.4 mm and the intervals of the pins are increased to 20 mm. 
         [0090]    As apparent from the results, as the distance D 0  between the electrode section  10  and grounding plate  30  is increased to 25.4, 34, 52, and 64 mm with the intervals of the pins fixed to 110 mm, as illustrated by the lines a to d, the discharge current lowers, and the discharge inception voltage increases. As illustrated by the line e, the discharge current is reduced when the areas of the electrode section  10  and electrode section supporting member  20  are reduced to one third. 
         [0091]    On the other hand, as illustrated by the line f, the discharge current increases when the intervals of the pins are increased from 10 mm to 20 mm. This is considered to be because uniform electric fields at the tips of the pins are more likely to be cancelled each other when the intervals of the pins are smaller as 10 mm. 
         [0092]    According to the fourth embodiment, stable glow corona discharge is obtained without including Trichel pulses. The discharge occurs in a range from 7 to 11 kV when the pitch is 10 mm and occurs in a range from 5 to 10 kV when the pitch is 20 mm. The robust properties are ensured. Moreover, the discharge occurs uniformly across the electrodes, 
       Comparative Example 1 
       [0093]    In Comparative example 1, the electrode  10   a  has a blade-shaped tip.  FIG. 19  is a perspective view illustrating the appearance of the electrode  10   a  of Comparative example 1. The electrode  10   a  of Comparative example 1 has the same dimensions as those of the electrodes  10   a  of the first embodiment illustrated in  FIGS. 4A and 4B and 6  except that the tip includes the blade shape but is not serrated. 
         [0094]    In  FIG. 10  illustrating the relationship between the voltage applied to the electrode section  10  and discharge current, Comparative example 1 corresponds to the lines g to l. The line g illustrates the case where the pair of the electrode  10   a  and shaft  10   b  is provided on the electrode section supporting member  20 . Similarly, the lines j, k, and l correspond to two pairs, three pairs, five pairs, seven pairs, and nine pairs of the electrode  10   a  and resistor  10   c,  respectively. 
         [0095]    As apparent from the lines g to l, as the number of electrodes  10   a  is increased from one to nine, the discharge current increases and becomes more stable under variations in voltage. However, in every case of Comparative example 1, discharge includes Trichel pulses and is unstable. 
       Comparative Example 2 
       [0096]    Comparative example 2 includes linear electrodes of metallic wires with a predetermined diameter. The metallic wires are linearly extended on the front side of the electrode section supporting member  20  and are arranged at a predetermined distance from the surface of the electrode section supporting member  20  with predetermined intervals. 
         [0097]      FIG. 20  is a perspective view illustrating the appearance of the electrode section  10 , electrode supporting member  20 , and grounding plate  30  situated facing the electrode section  10  in Comparative example 2. In Comparative example 2, the electrode section supporting member  20  is rectangular similarly to the fourth embodiment, unlike the circular shape illustrated in  FIGS. 1A and 1B . 
         [0098]    In Comparative example 2, the metallic wires as the electrodes can be 0.2 diameter wires for use in discharge processing, for example. In Comparative example 2, Trichel pulses sometimes occur, and spark discharge sometimes appears. To generate a non-uniform electric field, the diameter of each metallic wire is preferably smaller. It can be thought that discharge is stabilized with thinner wires. 
         [0099]    The embodiments show the electrodes  10   a  having a tip rotationally axisymmetric by way of example. However, the present embodiments are not limited thereto. The tip of the electrode does not need to be rotationally symmetric and needs to be continuous in a plane. For example, the tip may be a continuous blade forming a closed curve in a plane vertical to the longitudinal direction of the discharge electrode. For example, the electrode may have a tip formed by diagonally cutting the top or may have a polygonal cross section. 
         [0100]    In the embodiments, the dimensions of the electrodes and various specifications of the testing device are described by way of example, but the present embodiments are not limited thereto. It can be understood by those skilled in the art that the present embodiments can be also implemented by properly changing the numerical values of the dimensions and specifications. 
         [0101]    While embodiments have been exemplified with the help of the drawings, many modifications and changes are apparent to those skilled in the art.