Patent Publication Number: US-2009236988-A1

Title: Electrode for discharge lamp and discharge lamp

Description:
TECHNICAL FIELD 
     The present invention relates to an electrode for a discharge lamp, and a discharge lamp. The present invention relates particularly to an electrode for a discharge lamp which is packed with electron emitting material for emitting electrons and a discharge lamp having the electrode. 
     BACKGROUND ART 
     In recent years, demands for power saving and long life discharge lamps have been increasing. One possible method for achieving power saving is increasing the luminous efficiency by making an arc tube thinner. Making an arc tube thinner decreases electric current loss and electrode loss at the time of discharge. Therefore, luminous efficiency increases. 
     On the other hand, as one possible method for realizing an extension of lamp life is increasing the amount of electron emitting material with which a filament coil of an electrode is packed.  FIG. 9  is a photograph showing a triple coil pertaining to a conventional example. For example, the so-called triple coil as shown in  FIG. 9  is used for a conventional electrode for a discharge lamp in order to increase the packing amount of electron emitting material (Patent document 1). 
       FIGS. 10A ,  10 B and  10 C describe coiling processes of the triple coil pertaining to the conventional example. Next, a description of the triple coil continues, referring to  FIGS. 10A ,  10 B and  10   c . The triple coil is manufactured as follows. Firstly, as shown in  FIG. 10A , a first winding of a filament  201  is performed around a first core  202  to make a single coil  203 . Next, as shown in  FIG. 10B , a secondary winding of the single coil  203  is performed around a second core  204  to make a double coil  205 . Furthermore, as shown in  FIG. 10C , a tertiary winding of the double coil  205  is performed around a third core  206  to make the triple coil. Note that each of the cores  202 ,  204  and  206  is melted so as to be removed after the coiling process is complete. 
     According to a triple coil  207  that is manufactured in such way, not only a hollow space  202 ′ surrounded by the first winding but also a hollow space  204 ′ surrounded by the secondary winding can be packed with electron emitting material. Here, the hollow space  202 ′ surrounded by the first winding is where the first core  202  existed, and the hollow space  204 ′ surrounded by the secondary winding is where the second core  204  existed. Therefore, the triple coil  207  has a larger packing capacity for electron emitting material compared to a single coil or the like. Also, when a coil size of a triple coil becomes too large, an electrode will not be fit in an arc tube. Therefore, the number of windings in the tertiary winding which is the number of times that the double coil  205  is wound around the third core  206  is usually limited to about one turn. Therefore, as shown in  FIG. 10C , the size of a hollow space  206 ′ surrounded by the tertiary winding is short in a winding axis direction. Here, the hollow space  206 ′ surrounded by the tertiary winding is where the third core  206  existed. Also, the electron emitting material is not stably kept in the hollow space  206 ′ surrounded by the tertiary winding. Therefore, it is difficult to pack the hollow space  206 ′ surrounded by the tertiary winding with the electron emitting material. 
     Patent Document 1: Japanese Laid-open publication No. 2004-356060 
     DISCLOSURE OF THE INVENTION 
     Problems to be Solved 
     When an attempt is made to make an arc tube thinner for saving power, a compact electrode that fits in the arc tube is needed. That is, it is possible to enhance the luminous efficiency of a lamp by making an arc tube thin and long, which makes it possible to save power. In other words, it is possible to reduce lamp power with the same brightness maintained by making an arc tube thin and long. As a result, it is possible to achieve power saving. However, since a coil size needs to be small in order to downsize an electrode, an amount of electron emitting material with which a triple coil can be packed decreases. This shortens the life of a discharge lamp. 
     The present invention, in view of the above problem, has a main objective to provide an electrode for a discharge lamp that is compact and has a large packing capacity for electron emitting material. The present invention has another objective to provide a power saving and long life discharge lamp having such electrode for a discharge lamp. 
     Means to Solve the Problems 
     In order to solve the above problems, the electrode for a discharge lamp pertaining to the present invention includes a quadruple coil which is made by performing a first winding of a filament to make a single coil, performing a secondary winding of the single coil to make a double coil, performing a tertiary winding of the double coil to make a triple coil and performing a quaternary winding of the triple coil, wherein, in the quadruple coil, at least a hollow space surrounded by the tertiary winding is packed with an electron emitting material. 
     Also, another electrode for a discharge lamp pertaining to the present invention includes a bent triple coil which is made by performing a first winding of a filament to make a single coil, performing a secondary winding of the single coil to make a double coil, performing a tertiary winding of the double coil to make a triple coil and bending the triple coil, wherein, in the quadruple coil, at least a hollow space surrounded by the tertiary winding is packed with an electron emitting material. 
     The discharge lamp pertaining to the present invention has the above electrode for a discharge lamp. 
     EFFECT OF THE INVENTION 
     The present invention has an electrode for a discharge lamp, the electrode including a quadruple coil which is made by performing a first winding of a filament to make a single coil, performing a secondary winding of the single coil to make a double coil, performing a tertiary winding of the double coil to make a triple coil and performing a quaternary winding of the triple coil, wherein, in the quadruple coil, at least a hollow space surrounded by the tertiary winding is packed with an electron emitting material. According to the stated structure, the triple coil is quaternarily wound, and increases in the coil size are suppressed even if the number of windings in the tertiary winding increases. If the number of windings in the tertiary winding increases, the length of the hollow space surrounded by the tertiary winding in the winding axis direction increases, and the function of keeping the electron emitting material is enhanced. Therefore, the hollow space surrounded by the tertiary winding can be packed with the electron emitting material. The electrode for a discharge lamp pertaining to the present invention has a quadruple coil having a small coil size and a large packing capacity for electron emitting material as mentioned above. Since the hollow space surrounded by the tertiary winding in the quadruple coil can be packed with the electron emitting material, the electrode for a discharge lamp pertaining to the present invention is compact and has a large packing capacity for electron emitting material. Specifically, the packing capacity for electron emitting material is 1.5 to 2 times larger compared to a conventional electrode. 
     Also, with a structure in which the tertiary winding of the quadruple coil has a mandrel diameter MD 3  of 0.15 mm to 0.45 mm, it is possible to evenly heat the entire electron emitting material at the time of discharge while ensuring the adequate packing capacity for electron emitting material. Therefore, it is possible to extend lamp life more effectively. That is, if the mandrel radius MD 3  becomes larger than 0.45 mm, the hollow space surrounded by the tertiary winding becomes so wide that it is not possible to evenly heat, in a filament, the electron emitting material with which the hollow space surrounded by the tertiary winding is packed. That is, the heat of the filament is easily conducted to apart close to the filament, which often causes overheat. On the other hand, the heat of the filament is hardly conducted to a part far from the filament, which often causes insufficient heat. As a result, the generation of free barium from the electron emitting material is interfered with. Also, the effect of extending lamp life is not achieved despite the fact that the packing amount of electron emitting material has been increased. On the other hand, when the mandrel diameter MD 3  becomes smaller than 0.15 mm, the packing capacity for electron emitting material becomes smaller because the hollow space surrounded by the tertiary winding is narrow. Therefore, with the mandrel diameter MD 3  smaller than 0.15 mm, the effect of the present invention that the packing capacity is larger than the packing capacity for the conventional triple coil is not adequately achieved. 
     Also, with a structure in which a coil pitch P 3  is larger than the mandrel diameter MD 3  by 1.2 to 2.4 times in the tertiary winding of the quadruple coil, it is possible to extend lamp life more effectively. That is, when the coil pitch P 3  becomes less than 1.2 times larger than the mandrel diameter MD 3 , a distance between filaments that are adjacent to each other becomes too short, which causes an electrical short, causing the quadruple coil to generate insufficient heat. As a result, the generation of free barium from the electron emitting material is interfered with. Also, the effect of extending lamp life is not achieved despite the fact that the packing amount of electron emitting material has been increased. On the other hand, when the coil pitch P 3  becomes more than 2.4 times larger than the mandrel diameter MD 3 , the distance between filaments that are adjacent to each other becomes too long, which might cause the electron emitting material to fall off from the hollow space surrounded by the tertiary winding due to impact and vibration caused in transporting a lamp, making the packing amount of electron emitting material insufficient. 
     Also, with a structure in which a second filament which is provided in addition to the filament is arranged so as to pass through at least one of a hollow space surrounded by a first winding, a hollow space surrounded by the secondary winding and the hollow space surrounded by the tertiary winding in the quadruple coil, it is possible to stably maintain the shape of the quadruple coil. Accordingly, it is possible to obtain an electrode which is made such that the electron emitting material hardly falls off and an electrical short hardly occurs. 
     Also, with a structure in which a diameter Da of the second filament and a diameter Db of the filament of the quadruple coil satisfy a relation of Db&lt;Da&lt;1.5 Db, a current appropriately splits and flows through both of the filaments since the difference is small between the diameter of the filament that composes the quadruple coil and the diameter of the second filament. Therefore, even if the filament that composes the quadruple coil is long, the total resistance value of the quadruple coil does not become very large. Thus, a discharge does not occur between electrode lead lines that support the quadruple coil even if the number of windings in the tertiary winding increases. 
     Also, with a structure in which a number of windings in the tertiary winding of the quadruple coil is 20 turns or more, the hollow space surrounded by the tertiary winding can be packed with an adequate amount of electron emitting material. 
     Also, an electrode for a discharge lamp includes a bent triple coil which is made by performing a first winding of a filament to make a single coil, performing a secondary winding of the single coil to make a double coil, performing a tertiary winding of the double coil to make a triple coil and bending the triple coil, wherein, in the quadruple coil, at least a hollow space surrounded by the tertiary winding is packed with an electron emitting material. With such structure, the same effect as in the case of using the electrode having the above-described quadruple coil can be achieved. 
     The discharge lamp pertaining to the present invention has the above-described electrode for a discharge lamp. Therefore, it is possible to manufacture a discharge lamp that has a small inner diameter of an arc tube and has a large packing amount of electron emitting material. Also, it is possible to obtain a power saving and long life discharge lamp. Specifically, although the rated life of a conventional discharge lamp had been 6,000 hours, the rated life of the discharge lamp pertaining to the present invention is extended to more than 10,000 hours. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a sectional view that shows a discharge lamp pertaining to a first embodiment; 
         FIG. 2  is a photograph that shows a quadruple coil pertaining to the first embodiment; 
         FIGS. 3A and 3B  show an electrode for a discharge lamp pertaining to the first embodiment with  FIG. 3A  showing a front elevational view and  FIG. 3B  showing a side view; 
         FIGS. 4A ,  4 B,  4 C and  4 D show coiling processes of the quadruple coil pertaining to the first embodiment with  FIG. 4A ,  FIG. 4B ,  FIG. 4C  and  FIG. 4D  showing a first winding step, a secondary winding step, a third winding step and a quaternary winding step, respectively; 
         FIG. 5  is a graph showing a relation between a coil pitch P 3 /a mandrel diameter MD 3  and a fall-off rate of electron emitting material; 
         FIG. 6  shows a comparison between specifications of a quadruple coil pertaining to the present invention and specifications of a conventional triple coil; 
         FIG. 7  is a partially broken view showing a structure of a lamp having electrodes for a discharge lamp pertaining to a second embodiment; 
         FIG. 8  shows an electrode for a discharge lamp pertaining to a modification; 
         FIG. 9  is a photograph showing a triple coil pertaining to a conventional example (comparative example); and 
         FIGS. 10A and 10B  and  10 C describe coiling steps of the triple coil pertaining to the conventional example (comparative example), and  FIG. 10A ,  FIG. 10B  and  FIG. 10C  show a first winding step, a secondary winding step and a third winding step, respectively. 
     
    
    
     DESCRIPTION OF NUMERAL REFERENCES 
     
         
           1 ,  100  discharge lamps 
           14 ,  110  electron emitting material 
           15 ,  16 ,  102 ,  103 ,  150  electrodes for discharge lamps 
           41  filament 
           42  second filament 
           43 ′ hollow space surrounded by a first winding 
           44  single coil 
           45 ′ hollow space surrounded by a secondary winding 
           46  double coil 
           47 ′ hollow space surrounded by a tertiary winding 
           48  triple coil 
           50 ,  105  quadruple coil 
           151  bent triple coil 
       
    
     BEST MODE FOR CARRYING OUT THE INVENTION 
     Electrodes for discharge lamps pertaining to embodiments of the present invention are described based on the drawings. 
     First Embodiment 
     Hereinafter, a description is given of an electrode for a discharge lamp and a discharge lamp that pertain to the first embodiment using  FIG. 1  to  FIG. 6 . 
       FIG. 1  is a sectional view showing the discharge lamp pertaining to the first embodiment. The discharge lamp pertaining to the first embodiment (hereinafter, shown as a “lamp”) is a bulb-type fluorescent lamp (12 W) alternative to a general incandescent lamp (60 W), and the basic structure is pursuant to a conventional lamp. 
     As shown in  FIG. 1 , a lamp  1  has an arc tube  10 , a holding resin member  30  that holds the arc tube  10 , an eggplant-shaped glass outer tube bulb  31  including therein the arc tube  10 , an electronic ballast  32  of a so-called series inverter system for lighting that is integrally fixed to the holding resin member  30 , a resin casing  33  that covers the electronic ballast  32 , and a base  34  that is provided at an end portion of the resin casing  33 . 
     The arc tube  10  is composed of a bent glass tube  11  whose container is formed and processed into a double helical form. A swollen part  22  is formed in a vicinity of the center of the bent glass tube  11  in the arc tube  10 . Also, a convex part  23  is further formed on the swollen part  22 . The convex part  23  is connected to a tip portion  31   t  of the outer tube bulb  31  by a heat conductive medium  35  made of silicon resin, and the inner surface of the end portion of the convex part  23  is designed to be the coldest point. Also, the inner surface of the outer tube bulb  31  is coated with a calcium carbonate-based diffusion membrane  36 . 
     Electrodes  15  and  16  are disposed at both of tube end portions  12  and  13  of the arc tube  10 . The electrodes  15  and  16  include quadruple coils  50  and  51  each of which is composed of a tungsten filament formed into a quaternarily wound coil, and pairs of electrode lead lines  17   a  and  17   b  and  18   a  and  18   b  that support those coils  50  and  51  using a bead mounting method. Each of the pairs of the electrode lead lines  17   a  and  17   b  and  18   a  and  18   b  is sealed airtight through both of the tube end portions  12  and  13  of the arc tube  10 . Also, an exhaust tube  19  is sealed at the tube end  12  (tip portions are sealed after exhausting gas from an arc tube). Note that the detail of the electrodes  15  and  16  is described later. 
     A phosphor layer  20  that converts ultraviolet rays emitted by mercury into visible light is formed on the main inner surface of the arc tube  10 . The phosphor layer  20  is, for example, composed of a rare earth phosphor that is a mixture of a red phosphor (Y 2 O 3 : Eu), a green phosphor (LaPO 4 : Ce, Tb) and a blue phosphor (BaMg 2 Al 16 O 27 : Eu, Mn). 
     Inside the arc tube  10 , for example, single mercury (Hg)  21  of 3 mg and mixed gas (not shown) (80% argon (Ar) and 20% krypton (Kr)) at 400 Pa as buffer gas are enclosed. Note that the buffer gas is not limited to the above mixed gas. The buffer gas can be, for example, single gas such as argon, neon (Ne), krypton or the like, or mixed gas which is a mixture of the stated gases. 
     The dimensions in a typical structure of the lamp  1  are as follows. As to the arc tube  10 , the inner tube diameter of a main body is 6.4 mm, an outer tube diameter is 8.0 mm and the distance between electrodes is 480 mm. The height of the convex part  23  of the swollen part  22  is 2 mm. As to the double helical-shaped bent glass tube  11 , a gap between adjacent winds of the tube is 1.0 mm, the number of windings is about 5.25 turns, an outer diameter φao is 36.5 mm and a total length La is 63 mm. As to the form of an outer periphery of the lamp  1 , an outer diameter Do of the outer tube bulb  31  is 55 mm and a total lamp length Lo is 110 mm. 
     As with the conventional lamp, the lamp  1  has an outer diameter Do of 55 mm and a total lamp length Lo of 110 mm. However, since an outer tube diameter of the arc tube  10  becomes thinner from 9.0 mm (conventional outer diameter) to 8.0 mm, the distance between electrodes is 480 mm which is 1.2 times longer than a conventional distance between electrodes. Thus, the lamp  1  has a luminous flux of 810 lm although the power consumption is 10 W. 
     Next, the structures of the electrodes  15  and  16  are described in detail. Also, since the electrodes  15  and  16  have the same structure, only the structure of the electrode  15  is described. 
       FIG. 2  shows the quadruple coil pertaining to the first embodiment.  FIGS. 3A and 3B  show an electrode for a discharge lamp pertaining to the first embodiment with  FIG. 3A  showing a front elevational view and  FIG. 3B  showing a side view.  FIGS. 4A ,  4 B,  4 C and  4 D show coiling processes of the quadruple coil pertaining to the first embodiment with  FIG. 4A ,  FIG. 4B ,  FIG. 4C  and  FIG. 4D  showing a first winding step, a secondary winding step, a third winding step and a quaternary winding step, respectively. 
     The electrode  15  includes a quadruple coil  50  as shown in  FIG. 2 . The quadruple coil  50  has the packing capacity for electron emitting material larger than the packing capacity for the triple coil although the quadruple coil  50  has the same size (coil length CL) as the conventional triple coil. Accordingly, the rated life of the lamp  1  is 10,000 hours, which is longer than the rated life of the conventional lamp (6,000 hours). 
     As shown in  FIGS. 3A and 3B , the quadruple coil  50  is packed with electron emitting material  14 . Firstly, the electrodes  15  and  16  are applied with and packed with the electron emitting material  14  in the form of complex carbonate of alkaline earth metals Ba—Sr—Ca including zirconium oxide. Next, the complex carbonate is converted to composite oxide by a so-called decomposition process. 
     As shown in  FIG. 3A , a vicinity of caulking parts of the electrode lead lines  17   a  and  17   b  of the quadruple coil  50  is not packed with the electron emitting material  14 . This is because an adequate temperature rise of the electron emitting material  14  cannot be expected in an electrode decomposition process at the time of manufacturing a lamp even if the vicinity of the caulking parts are packed with the electron emitting material  14 . 
     As shown in  FIG. 3B , in the quadruple coil  50 , a hollow space  49 ′ surrounded by a quaternary winding is packed with little electron emitting material  14 . As described below, since the number of windings in a quaternary winding is one turn, the length of the hollow space  49 ′ surrounded by the quaternary winding in a winding axis direction is insufficient. Therefore, even if the hollow space  49 ′ surrounded by the quaternary winding is packed with the electron emitting material  14 , the electron emitting material  14  might fall off due to impact and vibration caused in transporting a lamp. 
     Next, a description is given of a method for manufacturing the quadruple coil  50 . 
     Firstly, a coiling process of manufacturing the quadruple coil  50  by coiling a filament is described. The quadruple coil  50  is made by performing a first winding of a filament to make a single coil, performing a secondary winding of the single coil to make a double coil, performing a tertiary winding of the double coil to make a triple coil and performing a quaternary winding of the triple coil. 
     The coiling process has the following four steps. Firstly, as shown in  FIG. 4A , a secondary wire  41  (filament) made of tungsten is wound around a main line  42  (second filament) made of tungsten and a first core  43  made of molybdenum to make a single coil  44 . Next, as shown in  FIG. 4B , the single coil  44  is wound around a second core  45  made of molybdenum to make a double coil  46 . Next, as shown in  FIG. 4C , the double coil  46  is wound around a third core  47  made of molybdenum to make a triple coil  48 . Next, as shown in  FIG. 4D , the triple coil  48  is wound one turn around a fourth core  49  made of molybdenum to make a quadruple coil  50 . 
     Next, cores  43 ,  45 ,  47  and  49  made of molybdenum are melted so as to be removed in a melting process. Specifically, the quadruple coil  50  is immersed in mixed acid solution with the quadruple coil  50  wound around the cores  43 ,  45 ,  47  and  49 . Then only the cores  43   45 ,  47  and  49  are melted so as to be removed in the mixed acid solution. 
     In the quadruple coil  50  after the melting process is complete, space where the first core  43  existed, space where the main line  42  exists and the like are collectively called a hollow space  43 ′ surrounded by a first winding. Space where the second core  45  existed is called a hollow space  45 ′ surrounded by a secondary winding. Space where the third core  47  existed is called a hollow space  47 ′ surrounded by a tertiary winding. Space where the fourth core  49  existed is called a hollow space  49 ′ surrounded by a quaternary winding. 
     A mandrel diameter MD 1  of the first winding is substantially the same as a sum of a diameter of the first core  43  and a diameter Da of the main line  42 . A mandrel diameter MD 2  of the secondary winding is substantially the same as a diameter of the second core  45 . A mandrel diameter MD 3  of the tertiary winding is substantially the same as a diameter of the third core  47 . A mandrel diameter MD 4  of the quaternary winding is substantially the same as a diameter of the fourth core  49 . 
     Being made of tungsten, the main line  42  does not melt in the mixed acid solution. Therefore, the main line  42  remains, while passing through the hollow space  43 ′ surrounded by the first winding. That is, the secondary wire  41  that composes the single coil  44  is wound around the main line  42  as a basket line. Also, although the secondary wire  41  that composes the single coil  44  works as a basket line in the above, it is possible to have a structure in which the secondary wire  41  that composes the double coil  46  and the triple coil  48  works as a basket line. Such structure also makes it possible to stably maintain the shape of the quadruple coil  50 . 
     Next, the quadruple coil  50  is packed with the electron emitting material  14  after fixing the quadruple coil  50  to the electrode lead lines  17   a  and  17   b  by so-called caulking. Specifically, the quadruple coil  50  is packed with the electron emitting material  14  by applying suspension of the electron emitting material  14  to the quadruple coil  50  and then drying the suspension. Thus, each of the hollow space  43 ′ surrounded by the first winding, the hollow space  45 ′ surrounded by the secondary winding and the hollow space  47 ′ surrounded by the tertiary winding is packed with the electron emitting material  14 . Also, the electron emitting material  14  is attached on the surfaces of the secondary wire  41  and the main line  42 . 
     Note that at least the hollow space  47 ′ surrounded by the tertiary winding simply needs to be packed with the electron emitting material  14 . In some cases, the hollow space  43 ′ surrounded by the first winding and the hollow space  45 ′ surrounded by the secondary winding do not have to be packed with the electron emitting material  14 . This is because it is possible to ensure a larger packing capacity than the packing capacity of the conventional triple coil as long as the hollow space  47 ′ surrounded by the tertiary winding has the largest packing capacity, and is packed with the electron emitting material  14 . Also, the hollow space  47 ′ surrounded by the tertiary winding as a whole does not have to be packed with the electron emitting material  14 , and a part of the hollow space  47 ′ surrounded by the tertiary winding may be packed with the electron emitting material  14 . 
     Hereinafter, the feature of the quadruple coil pertaining to the present invention is described. 
     As to a conventional triple coil (comparative example) as shown in  FIG. 9 , the number of windings in a tertiary winding is limited to about 1 turn in order to downsize an electrode. Also, in order to avoid contact between filaments due to a flexure, there is a limit to how big the mandrel diameter can be. Therefore, it is difficult to further increase the packing capacity for electron emitting material of the conventional triple coil. 
     In order to solve this problem, although the quadruple coil pertaining to the present invention has substantially the same size as the conventional triple coil, the number of windings in the tertiary winding is 20 turns or more (for example, the number of windings in the tertiary winding of the quadruple coil  50  in the first embodiment is 27 turns). That is, since the length of the hollow space surrounded by the tertiary winding in the winding axis direction is long, the hollow space surrounded by the tertiary winding can also be packed with the electron emitting material, too. Therefore, the packing capacity for electron emitting material is notably large compared to the conventional triple coil in which only the hollow space surrounded by the first winding and the hollow space surrounded by the secondary winding can be packed with the electron emitting material. Specifically, the packing amount is 1.5 to 2.0 times larger compared to the conventional triple coil. As a result, the rated life of a lamp is extended from 6,000 hours (a conventional rated life of a lamp) to more than 10,000 hours. 
     In order to achieve an adequate effect of extending lamp life which is realized by increasing the packing capacity for electron emitting material, the following structures are needed: a structure in which the electron emitting material does not easily falls off from the quadruple coil, and a structure in which the electron emitting material as a whole is heated evenly by the heat of a filament. This is because it is not possible to expect an extension of life of a steady lamp even if the packing amount of electron emitting material increases if the electron emitting material easily falls off from the quadruple coil due to impact and vibration caused in transporting a lamp. Also, in order to decrease a work function φe that is for an electrode to emit electrons, free barium needs to be appropriately generated from the electron emitting material. However, in order for that, it is necessary to evenly heat the electron emitting material as a whole at an appropriate temperature. Also, the electron emitting material does not contribute to the extension of lamp life efficiently if excessively heated or insufficiently heated parts exist. 
     The above problems are an obstacle to achieving extended lamp life that is the object of the present invention. In order to solve the above two problems and to achieve an adequate effect of extending lamp life by increasing the amount of electron emitting material, the present inventors studied the specific optimum range of the size of an electrode. 
     As a result, it was found that in particular conditions of the tertiary winding are most important. Specifically, it was found that it is possible to deal with the above problems if a mandrel diameter MD 3  of the tertiary winding is in the range of 0.15 to 0.45 mm and a coil pitch P 3  is 1.2 to 2.4 times larger than the mandrel diameter MD 3 . 
     Note that if the mandrel diameter MD 3  is larger than 0.45 mm, the heat of a filament does not sufficiently conduct to electron emitting material far from the filament, making it difficult to generate free barium from the electron emitting material. As a result, the effect of extending lamp life is reduced. On the other hand, if the mandrel diameter MD 3  is smaller than 0.15 mm, a hollow space surrounded by the tertiary winding becomes too narrow and the packing capacity for electron emitting material is little different from the conventional triple coil. 
     Therefore, it is not possible to achieve an adequate effect of extending lamp life when the mandrel diameter MD 3  is too large or too small. Thus, it is preferable that the mandrel diameter MD 3  is in the range of 0.15 to 0.45 mm. 
     Next, if the coil pitch P 3  of the tertiary winding is less than 1.2 times larger than the mandrel diameter MD 3 , the distance between filaments that are adjacent to each other becomes too short. Therefore, an electrical short easily occurs between those filaments. Thus an adequate amount of free barium might not be generated in manufacturing. As a result, problems such as shortening of lamp life and the like might arise. 
     Also, if the coil pitch P 3  is more than 2.4 times larger than the mandrel diameter MD 3 , the distance between filaments that are adjacent to each other becomes too long. Therefore, the electron emitting material easily falls off. As a result, problems such as the shortening of lamp life and the like might arise since the electron emitting material easily falls off from a coil due to impact and vibration caused in transporting a lamp. Therefore, it is preferable that the coil pitch P 3  is 1.2 to 2.4 times larger than the mandrel diameter MD 3 . 
       FIG. 5  is a graph showing a relationship between P 3 /MD 3  and a fall-off rate of the electron emitting material. As shown in  FIG. 5 , four types of coils are manufactured with a ratio between the coil pitch P 3  and the mandrel diameter MD 3  being a parameter. The graph also shows how easy electron emitting material falls off. A horizontal axis shows the ratio between the coil pitch P 3  and the mandrel diameter MD 3  (i.e. P 3 /MD 3 ). On the other hand, the vertical axis shows the fall-off rate of the electron emitting material. 
     The fall-off rate is obtained as follows. Firstly, a lamp is manufactured using a coil to be measured. Next, the lamp is destroyed in a way that the electron emitting material does not fall off due to impact in destroying. Then the coil is taken out. After that, the weight of the coil is measured (the weight of the coil before a test: W1). Furthermore, the weight of the coil is measured again after performing a drop impact test using the coil whose weight has been measured (the weight of the coil after the test: W2). Also, all the attached electron emitting material is removed from the coil using acid, and the weight of the coil after the removal is measured (the weight of the coil after removing the electron emitting material: W3). Then, the fall-off rate is calculated by the following formula. 
       (fall-off rate)=( W 1− W 2)/( W 1− W 3) 
       FIG. 5  shows the plotted result of the fall-off rate obtained experimentally in such way. It is known from experience that if the fall-off rate exceeds 30%, the electron emitting material easily falls off, which affects lamp life. Therefore, it can be judged from the graph in  FIG. 5  that it is possible to keep the fall-off rate to 30% or less if P 3 /MD 3  is 2.4 or less, and as a result, it is possible to prevent the electron emitting material from falling off due to impact and vibration caused in transporting a lamp. 
     Next, as the number of windings in the tertiary winding increases, it is possible to pack a lot of electron emitting material. However, an overall resistance value becomes too large since a coil length CL becomes longer. As a result, a difference in potential between electrode lead lines becomes large when applying a desired amount of current, which causes a discharge. In order to solve this problem, a structure is made such that the main line passes through the hollow space surrounded by the first winding of the secondary wire as a basket line, and the relation of Db&lt;Da&lt;1.5 Db is satisfied when a diameter of the main line is expressed as Da and a diameter of the secondary wire is expressed as Db. 
     If such relation is satisfied, a current appropriately splits and flows through the main line and the secondary wire. Therefore, even if the coil length CL becomes longer, the total resistance value does not increase a lot. Therefore, a discharge does not occur between electrode lead lines even if the number of windings in the tertiary winding is 20 turns or more. Also, as to the quadruple coil  50  pertaining to the first embodiment, a diameter of the main line  42  Da is 0.028 mm and a diameter of the secondary wire  41  Db is 0.020 mm. 
     The lamp  1  including the electrodes  15  and  16  pertaining to the first embodiment was manufactured, and life tests and measuring of characteristics was carried out.  FIG. 6  shows a comparison between specifications of the quadruple coil pertaining to the present invention and specifications of the conventional triple coil. 
     As shown in  FIG. 6 , the filing amount of the electron emitting material  14  of the quadruple coil  50  is 2.8 mg, which means the packing amount has increased by 70% compared to the packing amount of the conventional triple coil which is 1.6 mg. Thus, the rated life of the lamp  1  is extended from 6,000 hours (the conventional rated life of a lamp) to more than 10,000 hours. 
     Also, although the lamp  1  is substantially the same size as a general incandescent lamp (60 W), the efficiency is 81 lm/W (an input wattage of 10 W and a luminous flux of 810 ml), which achieves marked power saving compared to 60 W lamp (810/60=13.5 lm/W) and a conventional bulb-type fluorescent lamp (810/12=67.5 lm/W). 
     Second Embodiment 
       FIG. 7  is a partially broken view showing a structure of a lamp having electrodes for a discharge lamp pertaining to a second embodiment. 
     As shown in  FIG. 7 , a discharge lamp  100  (hereinafter, “lamp  100 ”) is a low-pressure mercury discharge lamp and includes a glass tube  101  and hot cathode type electrodes  102  and  103  sealed at both ends of the glass tube  101 . 
     The glass bulb  101  has, for example, an external diameter of 18 mm, a wall thickness of 0.8 mm and a length of 1010 mm. In addition to mercury (for example, 4 mg to 10 mg) as light emitting material, mixed gas of argon and krypton (50%:50%) as buffer gas at a gas pressure of 600 Pa, for example, is enclosed in the glass bulb  101 . 
     A phosphor layer  104  that converts ultraviolet rays emitted by mercury into visible light is formed in the inner surface of glass bulb  101 . The phosphor layer  104  is, for example, composed of a rare earth phosphor formed from a mixture of a red phosphor (Y 2 O 3 : Eu), a green phosphor (LaPO 4 : Ce, Tb) and a blue phosphor (BaMg 2 Al 16 O 27 : Eu, Mn). 
     Since an electrode  102  and an electrode  103  have the same structure, only the structure of the electrode  102  is described. A so-called bead glass mounting method is adopted for the electrode  102 . Also, the electrode  102  includes a quadruple coil  105  made of tungsten, a pair of lead lines  106  and  107  that support the quadruple coil  105 , and a bead glass  108  that integrally fixes this pair of lead lines  106  and  107 . 
     The electrode  102  is sealed to the glass tube  101  at a part of each of the lead lines  106  and  107  (specifically, a portion extending from the bead glass  108  in the direction opposite to the quadruple coil  105 ). Also, the electrode  102  is sealed to the glass tube  101  by pinch sealing, for example. 
     Note that an exhaust tube  109  is provided together with the electrode  102  at one end of the glass tube  101  (here, an end on the side at which the electrode  102  the glass tube  101  is provided). The exhaust tube  109  is used in exhausting gas from the glass tube  101 , and enclosing the above buffer gas or the like after sealing the electrode  106 , the electrode  107  and the like. On completion of enclosing the buffer gas or the like in the glass tube  101 , tip-off sealing, for example, is performed at a part of the exhaust tube  109  positioned outside the glass tube  101 . 
     Next, the quadruple coil  105  is described in detail. The quadruple coil  105  pertaining to the second embodiment basically has the same structure as the quadruple coil  50  pertaining to the first embodiment. Accordingly, the description focuses on the parts of the structure that differ, with a description of the common parts of the structure being kept brief or omitted. 
     The quadruple coil  105  is made by performing a first winding of a filament to make a single coil, performing a secondary winding of the single coil to make a double coil, performing a tertiary winding of the double coil to make a triple coil and performing a quaternary winding of the triple coil. Also, a main line is provided in a hollow space surrounded by the first winding so as to pass though the hollow space surrounded by the first winding. 
     As to a measurement of each part of the quadruple coil, a diameter of the main line Da is 70 μm, a diameter of a secondary wire Db is 50 μm, a first mandrel diameter MD 1  is 90 μm, a first pitch length P 1  is 89 μm, a second mandrel diameter MD 2  is 200 μm, a second pitch length P 2  is 381 μm, a third mandrel diameter MD 3  is 398 μm, a third pitch length P 3  is 710 μm, a fourth mandrel diameter MD 4  is 1500 μm and a fourth pitch length P 4  is 1800 μm. 
     Alternatively, a measurement of each part of the quadruple coil may be as follows. For example, as to a measurement of each part of the quadruple coil, a diameter of the main line Da is 90 μm, a diameter of a secondary line Db is 20 μm, a first mandrel diameter MD 1  is 90 μm, a first pitch length P 1  is 89 μm, a second mandrel diameter MD 2  is 200 μm, a second pitch length P 2  is 381 μm, a third mandrel diameter MD 3  is 398 μm, a third pitch length P 3  is 710 μm, a fourth mandrel diameter MD 4  is 1200 μm and a fourth pitch length P 4  is 1800 μm. 
     In the quadruple coil  105 , each of the hollow space surrounded by the first winding, a hollow space surrounded by the secondary winding and a hollow space surrounded by the tertiary winding is packed with an electron emitting material  110 . Also, the electron emitting material  14  is attached on each surface of the main line  41  and the secondary wire  42 . 
     As to the quadruple coil  105 , the packing amount of the electron emitting material  110  is 60 mg which is 12 times the packing amount of electron emitting material of a triple coil mounted on a conventional low-pressure mercury discharge lamp. Thus, the rated life of the lamp  100  is extended from 10,000 hours (a conventional rated life of a lamp) to more than 120,000 hours. 
     Also, the packing amount of the electron emitting material  110  of the quadruple coil  105  can be 15 mg to 60 mg in accordance with required lifetime. In this case, the rated life of the lamp  100  is 30000 to 120000 hours. 
     (Modification) 
     Hereinbefore, the electrode for a discharge lamp and the discharge lamp that pertain to the present invention are specifically described based on the embodiments. However, the present invention is not limited to the above embodiments. 
     In addition to the lamps pertaining to the above embodiments, the electrode pertaining to the present invention works effectively for, for example, a lamp having a comparatively thin arc tube whose inner tube diameter is 6 mm or less. Thus, it is possible to provide a power saving, long life and more compact bulb-type fluorescent lamp. 
     The quadruple coil pertaining to the present invention is not limited to a coil whose number of windings in the quaternary winding is 1 turn like the quadruple coil  50  pertaining to the first embodiment or a coil whose number of windings in the quaternary winding is 4 turns like the quadruple coil  105  pertaining to the second embodiment. Therefore, the number of windings in the quaternary winding does not matter as long as the size of an electrode allows the electrode to be structured to be fit in an arc tube. Also, the number of windings is not limited to a natural number as long as the number of windings is a decimal which is 0 or more. That is, a mixed decimal such as 2.5 turns or a pure decimal such as 0.5 turns are possible. 
     Furthermore, the electrode pertaining to the present invention is not limited to an electrode including a quadruple coil, and an electrode including a bent triple coil is possible. Here, the bent triple coil means a coil which is made by performing a winding of a filament to make a single coil, performing a secondary winding of the single coil to make a double coil, performing a tertiary winding of the double coil to make a triple coil, and further bending the triple coil. 
     A shape into which a triple coil is bent can be any shape, for example, a substantially Ω shape, a substantially M shape, a substantially inverted U shape, substantially inverted V shape, a spiral shape or the like as long as a coil length CL can be kept short and the number of windings in the tertiary winding can be increased. 
       FIG. 8  shows an electrode for a discharge lamp pertaining to a modification. For example, an electrode  150  shown in  FIG. 8  includes a bent triple coil  151  which is made by bending a triple coil into a substantially Ω shape. The bent triple coil  151  is made by making a tungsten filament into a triple coil in the same process as the coiling process pertaining to the first embodiment, and further bending the triple coil into a substantially Ω shape. The bent triple coil  151  basically has the same structure as the quadruple coil  50  of the first embodiment except that the triple coil is bent instead of performing the quaternarily winding of the triple coil. The bent triple coil  151  is supported by a pair of electrode lead lines  152  and  153  using a bead mounting method. 
     The bent triple coil  151  is made by further bending a triple coil. Therefore, although a distance between the electrode lead lines  152  and  153  is the same, the number of windings in the tertiary winding is large compared to the conventional triple coil which is not bent. Therefore, it is possible to increase the length of the hollow space surrounded by the tertiary winding in the winding axis direction without increasing the size of a coil (coil length CL). Also, the hollow space surrounded by the tertiary winding can be packed with more electron emitting material. 
     Also, as to the bent triple coil  151 , it is preferable that the mandrel diameter MD 3  of the tertiary winding is 0.15 to 0.45 mm like the quadruple coil  50  pertaining to the first embodiment. With this structure, it is possible to heat evenly the electron emitting material as a whole at the time of discharge with the adequate packing capacity for electron emitting material ensured. Thus is it possible to extend lamp life more effectively. 
     Also, according to the bent triple coil  151 , it is preferable that, in the tertiary winding, a coil pitch P 3  is larger than the mandrel diameter MD 3  by 1.2 to 2.4 times. With this structure, it is possible to extend lamp life more effectively. 
     Also, according to the bent triple coil  151 , a second filament which is provided in addition to the filament may be arranged so as to pass through at least one of a hollow space surrounded by a first winding, a hollow space surrounded by a second winding and the hollow space surrounded by a tertiary winding in the quadruple coil. With such structure, it is possible to stably maintain the shape of the quadruple coil. Therefore, it is possible to obtain an electrode which is made such that the electron emitting material hardly falls off and an electrical short does not occur much. 
     Also, as to the bent triple coil  151 , it is preferable that a diameter Da of the second filament and a diameter Db of the filament of the quadruple coil satisfy a relation of Db&lt;Da&lt;1.5 Db. With this structure, a current appropriately splits and flows through each of the filament that composes the quadruple coil and the second filament. Also, a discharge does not happen between electrode lead lines. Therefore, a discharge does not occur between the electrode lead lines. 
     Also, as to the bent triple coil  151 , it is preferable that a number of windings in the tertiary winding is 20 turns or more. With this structure, the hollow space surrounded by the tertiary winding can be packed with the adequate packing amount of electron emitting material. 
     INDUSTRIAL APPLICABILITY 
     The electrode for a discharge lamp pertaining to the present invention can be applied to a compact fluorescent lamp that has been popular in recent years as a power saving light source together with a bulb-type fluorescent lamp. Also, by increasing the number of windings in the tertiary winding, for example, the electrode for a discharge lamp pertaining to the present invention basically can be applied to various fluorescent lamps for general illumination and to other lamps including a compact hot-cathode type fluorescent lamp which is an alternative to a conventional compact cold-cathode fluorescent lamp as a power saving light source for a liquid crystal back light. That is, it is possible to markedly extend not only the life of a compact lamp but also the life of a large-size lamp.