Abstract:
An ozonizer and water purifier equipped with the ozonizer comprising an ozonizing discharge element; an electric circuit for applying a voltage to the ozonizing discharge element so as to produce an ozone-generating discharge; a housing having an opening formed therein for receiving the ozonizing discharge element; a cover which seals the ozonizing discharge element in the housing; and a device for turning off the voltage applied to the ozonizing discharge element when the cover is removed. In another embodiment, at least a part of the cover or housing is transparent so as to enable detection of the discharge state of the ozonizing discharge element. Also disclosed is an ozonizer and a water purifier comprising the ozonizer which includes a discharge element for generating ozone by discharge, wherein ammonium nitrate and other substances adhere to the discharge element upon discharge; and a heat generating element for heating the discharge element to a predetermined temperature which induces scattering of at least ammonium nitrate molecules among those substances adhering to the discharge element.

Description:
This is a divisional of application Ser. No. 09/085,102 filed May 28, 1998, now U.S. Pat. No. 6,039,816 the disclosure of which is incorporated herein by reference. 
    
    
     BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to an ozonizer for generating ozone from the oxygen contained in air, and more particularly, to an ozonizer well adapted for use in 24-hour working baths, circulating water purifiers such as a Jacuzzi, ozonized water generators, water purifiers and the like. Furthermore, the present invention relates to a water purifier equipped with an ozonizer for use with 24-hour working baths, Jacuzzis, ponds, water tanks and pools, and to a method of cleaning the ozonizer. 
     2. Description of the Related Art 
     Ozone has conventionally been used in industrial as well as household applications for purifying and deodorizing water and the like. A relatively small-sized apparatus for generating ozone for household use employs a creeping discharge element including a filamentary discharge electrode and a surface induction electrode disposed opposite each other and a dielectric layer interposed therebetween. A voltage is applied between the electrodes to thereby excite discharge on the filamentary discharge electrode. This type of creeping discharge element is disclosed, for example, in U.S. Pat. No. 4,652,318. 
     More particularly, such ozonizers include a creeping discharge element, a power circuit and a resin case for housing the creeping discharge element and power circuit. The creeping discharge element is typically composed of a dielectric layer formed from ceramic, a filamentary discharge electrode disposed on one surface of the dielectric layer, and a surface induction layer disposed on the other surface of the dielectric layer opposite the filamentary discharge electrode. The power circuit applies a voltage between the filamentary discharge electrode and surface induction electrode so as to excite a discharge from the filamentary discharge electrode. 
     In Japanese Patent Application Laid-Open (kokai) No. 8-171979, the present applicant proposed an ozonizer employing a creeping discharge element for use in the circulating water purifier of a 24-hour working bath. This ozonizer is described below with reference to FIGS. 8A-8D. FIG. 8B shows a plan view of the ozonizer  310 . FIG. 8A shows a plan view of a cover  330  that attaches to the ozonizer. FIG. 8C shows the ozonizer of FIG. 8B as viewed in the direction of arrow C of FIG.  8 B. FIG. 8D shows a sectional view along line  8 D— 8 D of FIG.  8 B. 
     As shown in FIG. 8D, a creeping discharge element, i.e. an ozonizing element, is formed as part of a high-voltage generating board  350  including a high-voltage-generating circuit element  352 . Specifically, the high-voltage generating board  350  is formed from a dielectric having a surface induction electrode  366  embedded in a portion thereof and a filamentary discharge electrode  368  disposed on the top surface thereof. The high-voltage generating board  350  is disposed within a housing  320  such that the filamentary discharge electrode  368  mounted on the high-voltage generating board  350  faces an opening  320   a  formed in the housing  320 . The cover shown in FIG. 8A is attached to the housing  320  so as to close the opening  320   a , to thereby prevent ozone leakage from the housing  320 . 
     Large-sized creeping discharge type ozonizers for industrial use employ pure oxygen or dry air as a starting material, whereas small-sized ozonizers for household use employing the above-described creeping discharge element use untreated air as a starting material. Accordingly, small-sized ozonizers are disadvantageous in that when the creeping discharge element is used continuously, the material of the creeping discharge element reacts with nitrogen or the like in air to form an ammonium salt on the element surface. The ammonium salt hinders creeping discharge with a resulting failure in the proper generation of ozone. Thus, for such small-sized creeping discharge type ozonizers, it is important to check whether ozone continues to be generated. Hitherto, this checking was difficult to conduct. 
     More particularly, because untreated air has a humidity higher than that of artificially-produced dry air, large amounts of nitrogen oxides are produced when ozone is generated by discharge. 
     The nitrogen oxides chemically react with ammonia present in the air to produce ammonium nitrate. The thus-produced ammonium nitrate covers the filamentary discharge electrode. 
     Accordingly, the density of the electric field generated by the filamentary discharge electrode is reduced. Also, ammonium nitrate covering the filamentary discharge electrode absorbs water present in the air and becomes electrically conductive, thus increasing the apparent area of the filamentary discharge electrode. As a result, the capacitance of the dielectric increases. 
     That is, in a conventional ozonizer, because ammonium nitrate covers the filamentary discharge electrode, the density of the electric field generated by the filamentary discharge electrode is reduced. The capacitance of the dielectric increases, resulting in reduced ozone generation. 
     Conventionally, therefore, the ozonizer is disassembled, and adhering ammonium nitrate is wiped off from the filamentary discharge electrode using water or a solvent. That is, a conventional ozonizer must be maintained through manual labor. 
     After cleaning, the creeping discharge element resumes discharging to thereby generate ozone. However, a high electric potential of several kilovolts is applied to the creeping discharge element even though the current flowing through the element is very small. Therefore, it is dangerous for an ordinary household user to clean the element. That is, even though designed for household use, conventional ozonizers are difficult to maintain. 
     SUMMARY OF THE INVENTION 
     It is therefore an object of the present invention to provide an ozonizer which is easy to maintain and a water purifier equipped with the ozonizer. 
     Yet another object of the present invention is to provide an ozonizer, a water purifier and a method of cleaning the ozonizer which allows for easy removal of at least ammonium nitrate among those substances adhering to a discharge element without the need for manual cleaning and which dispenses with the need for touching the discharge element. 
     The above objects have been achieved according to a first aspect of the present invention by providing an ozonizer which comprises an ozonizing discharge element, an electric circuit for applying a voltage to said ozonizing discharge element so as to produce an ozone-generating discharge; a housing having an opening formed therein for receiving said ozonizing discharge element, a cover which seals the ozonizing discharge element in said housing, and means for turning off the voltage applied to said ozonizing discharge element when the cover is removed. 
     In the ozonizer according to the above first aspect of the present invention, it is safe to clean the ozonizing discharge element because the voltage applied to the ozonizing discharge element is turned off when the cover is removed. 
     In the ozonizer, preferably at least part of the cover or housing is transparent so as to enable visual detection of the discharge state of the ozonizing discharge element. Instead of visual inspection, for example, a light sensor which detects a discharge light of the ozonizing discharge element through the transparent cover or housing may be placed outside the transparent cover or housing to confirm the discharge state of the ozonizing discharge element. Thus, the ozonizer is easy to maintain. 
     According to a second aspect, the present invention provides an ozonizer which comprises an ozonizing discharging element, an electric circuit for applying a voltage to said ozonizing discharge element so as to produce an ozone-generating discharge, a housing having an opening formed therein for receiving said ozonizing discharge element, and a cover which seals the ozonizing discharge element in said housing, wherein at least part of said cover or housing is transparent so as to enable visual detection of the discharge state of the ozonizing discharge element. 
     In the ozonizer according to the above second aspect of the present invention, the discharge state of the ozonizing discharge element can be visually observed or easily detected with a sensor. 
     Also, in the above-described ozonizers, an ozone discharge pipe is preferably provided on said housing separate from said cover. 
     Namely, because a piping portion is provided on the housing side, the piping portion, to which an ozone pipe is connected, remains stationary when the cover is removed. Accordingly, protection is provided against accidentally disconnecting the ozone pipe from the piping portion, to thereby prevent a gas leak which might otherwise result and assure safe operation. 
     In the above-described ozonizers, each of the housing and the cover preferably comprises engagement means for fixedly engaging one another. More preferably, one of the engagement means comprises a hook portion and the other comprises an engagement portion for engaging the hook portion. 
     In this case, because the housing and the cover are fixed together via the engagement means, the cover is easily detached from or attached to the housing by disengaging or engaging the engagement means. 
     According to a third aspect, the present invention provides a water purifier equipped with an ozonizer which comprises an ozonizing discharge element, an electric circuit for applying a voltage to said ozonizing discharge element so as to produce an ozone-generating discharge; a housing having an opening formed therein for receiving the ozonizing discharge element, and a cover which seals said ozonizing discharge element in said housing, wherein at least part of the cover or housing is transparent so as to enable visual detection of the discharge state of the ozonizing discharge element. 
     In the water purifier according to the above third aspect of the present invention, the discharge state of the ozonizing discharge element can be visually observed with ease because at least a part of the cover or housing is transparent. Thus, the water purifier is easy to maintain. 
     The water purifier preferably includes a window through which the transparent portion of the cover of the ozonizer can be visually observed from the outside. Thus, it is easy to visually observe the discharge state of the ozonizing discharge element. 
     According to a fourth aspect, the present invention provides a water purifier equipped with an ozonizer which comprises an ozonizing discharge element, a power unit for energizing and applying a voltage to said ozonizing discharge element so as to produce an ozone-generating discharge, a housing having an opening formed therein for receiving said ozonizing discharge element, a cover which seals said ozonizing discharge element in said housing, and means for turning off the voltage applied to the ozonizing discharge element when the cover is removed. 
     In the water purifier according to the above fourth aspect of the present invention, it is safe to clean the ozonizing discharge element because the voltage applied to the ozonizing discharge element is turned off when the cover is removed. 
     Furthermore, in the above first through fourth aspects of the present invention, the cover preferably hermetically seals the ozonizing discharge element in the housing. 
     According to a fifth aspect, the present invention provides an improved ozonizer having a discharge element for generating ozone by electric discharge. The ozonizer includes a heat generating element for generating heat upon input of current so as to heat the discharge element. The ozonizer also includes a heat generating circuit for supplying current to the heat generating element so as to heat the heat generating element and thereby heat the discharge element to a predetermined temperature. This induces scattering of at least ammonium nitrate molecules among those substances adhering to the discharge element. 
     In the ozonizer according to the above fifth aspect of the present invention, the discharge element preferably includes a dielectric formed from ceramic, a discharge electrode disposed on one surface of the dielectric, and an induction electrode disposed in the dielectric opposed to and separate from the discharge electrode. The heat generating element is preferably disposed on the other surface of the dielectric opposed to the induction electrode. 
     Because ammonium nitrate adhering to the discharge element can be evaporated by operating the heat generating circuit, the user does not have to touch or handle the discharge element to clean the same. In contrast, in a conventional cleaning practice, the user wipes off adhering ammonium nitrate from a discharge element using water or a solvent. 
     In the ozonizer according to the above fifth aspect of the present invention, the discharge element is heated preferably to a set temperature within a range of from 200° C. to 500° C., more preferably, within a range of from 250° C. to 350° C. A broad temperature range of from 200° C. to 500° C. is employed because ammonium nitrate adhering to the discharge element can be evaporated at a temperature within this range. Ammonium nitrate adhering to the discharge element begins to evaporate at a temperature slightly above 200° C. However, in order to reduce the evaporation time, the discharge element is preferably heated to a temperature of at least 250° C. Also, if the discharge element is heated to an excessively high temperature, the resin case which houses the discharge element may become deformed. Therefore, a temperature range of from 250° C. to 350° C. is more preferred. 
     In the ozonizer according to the above fifth aspect of the present invention, a heat generating time control means is preferably provided in order to control the period of time during which the heat generating element generates heat. 
     In this manner, the heating time for heating the discharge element can be controlled. That is, the discharge element can be maintained at the set temperature under control of the heat generating time control means. 
     The heat generating time control means preferably comprises a thermistor having a positive characteristic connected in series with the heat generating element. 
     Because the thermistor having a positive characteristic increases in resistance with an increase in temperature, the thermistor connected to the heat generating element shuts off current flow to the heat generating element after a predetermined time has elapsed, to thereby prevent overheating of the discharge element. 
     Also, the use of the thermistor reduces the cost of the ozonizer as compared with the case where a complicated timer circuit is employed. 
     The ozonizer according to the above fifth aspect of the present invention preferably comprises a discharge element housed in a resin case. The induction electrode is connected to a high-voltage supply, and the discharge electrode is connected to ground. A portion of the discharge electrode is covered with a protective film against wear caused by discharge, and the uncovered portion of the discharge electrode is exposed from one surface of the dielectric. 
     In this structure, the induction electrode is connected to a high-voltage supply, and the discharge electrode is connected to ground. Therefore, even when water enters the case and wets the discharge electrode, the electric potential between the electrodes is rendered identical to that of the water. Accordingly, one would not suffer electric shock by touching the ozonizer. 
     Furthermore, the discharge electrode excluding a certain portion thereof is covered with a protective film against wear caused by discharge, and the uncovered portion is exposed from one surface of the dielectric. Accordingly, even if the dielectric breaks with the resulting exposure of a high-voltage portion (for example, a portion of the induction electrode or heat generating element), current flows into the exposed portion of the discharge electrode such that electric shock is prevented. 
     The discharge element is preferably housed in a case with a heat resistant rubber gasket interposed therebetween. This prevents heat generated by the discharge element from being transmitted to the resin case which might otherwise cause the resin case to deteriorate or deform. 
     In the ozonizer according to the above fifth aspect of the present invention, a timer is preferably provided in order to control the period of time during which electrical power s supplied to the discharge element and the heat generating circuit. 
     According to a sixth aspect, the present invention provides a water purifier which includes the above described ozonizer, a filter for filtering water, and ozone discharging means for discharging ozone generated by the ozonizer into water filtered through the filter. 
     When a water purifier equipped with an ozonizer is disassembled and maintained, water entering into the ozonizer may cause electric shock. By contrast, in the case of a water purifier equipped with the ozonizer according to the present invention, the ozonizer can be maintained merely by operating the heat generating circuit with no need of disassembly. Thus, maintaining the ozonizer does not involve the risk of electric shock. 
     According to a seventh aspect, the present invention provides a method of cleaning an ozonizer having a discharge element for generating ozone by electric discharge. In this method, the discharge element is heated to a predetermined temperature using a heat generating element and a heat generating circuit for supplying current to the heat generating element so as to heat the heat generating element, to thereby evaporate at least ammonium nitrate among those substances adhering to the discharge element. 
     Because the cleaning method of the present invention allows a user to evaporate ammonium nitrate adhering to the discharge element by operating the heat generating circuit, the invention dispenses with the need for handling the discharge element in order to clean the same. In contrast, in a conventional cleaning practice, the user wipes off adhering ammonium nitrate from a discharge element using water or a solvent. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     Various other objects, features and attendant advantages of the present invention will be understood by reference to the following detailed description of the preferred embodiments when considered with the accompanying drawings, in which: 
     FIG. 1 is a schematic view showing the structure of a circulating water purifier according to a first embodiment of the present invention; 
     FIG. 2A is a perspective front-side view of an ozonizing element used in an ozonizer according to the first embodiment; 
     FIG. 2B is a perspective back-side view of the ozonizing element of FIG. 2A; 
     FIG. 2C is a side view of another type of ozonizing element according to another embodiment of the present invention; 
     FIG. 3A is a front view of the ozonizer according to the first embodiment; 
     FIG. 3B is a side view of the ozonizer of FIG. 3A; 
     FIG. 3C is a view showing the ozonizer of FIG. 3A with its cover separated therefrom; 
     FIG. 3D is a sectional view along line  3 D— 3 D of FIG. 3A; 
     FIG. 3E is a bottom view of the ozonizer of FIG. 3A; 
     FIGS. 3F and 3G show the ozonizer of FIG. 3A mounted on the circulating water purifier of FIG. 1; 
     FIGS. 4A and 4B are circuit diagrams of the high-voltage generating board of the ozonizer according to the first embodiment; 
     FIG. 4C is a circuit diagram of the high-voltage generating board of an ozonizer according to a second embodiment of the present invention; 
     FIG. 5A is a front view of the ozonizer according to the second embodiment; 
     FIG. 5B is a side view of the ozonizer of FIG. 5A; 
     FIG. 5C is a view showing the ozonizer of FIG. 5A with its cover separated therefrom; 
     FIG. 5D is a sectional view along line  5 D— 5 D of FIG. 5A; 
     FIG. 5E is a bottom view of the ozonizer of FIG. 5A; 
     FIG. 6 is a front view of an ozonizer according to a modification of the second embodiment; 
     FIG. 7A is a perspective view of an ozonizer according to a third embodiment of the present invention; 
     FIG. 7B is a side view of the cover of the ozonizer of FIG. 7A; 
     FIG. 7C is a side view of the housing of the ozonizer of FIG. 7A; 
     FIG. 7D is a sectional view along line  7 D— 7 D of FIG. 7A; 
     FIG. 7E is a sectional view of an ozonizer according to a modification of the third embodiment; 
     FIG. 8A is a plan view of a cover for mounting on a conventional ozonizer; 
     FIG. 8B is a plan view of a conventional ozonizer; 
     FIG. 8C is a view of the ozonizer of FIG. 8B in the direction of arrow C of FIG. 8B; 
     FIG. 8D is a sectional view along line  8 D— 8 D of FIG.  8 B. 
     FIG. 9 is an exploded view of an ozonizer according to an embodiment of the present invention; 
     FIG. 10A is an exploded view of a discharge element employed in the ozonizer of FIG. 9; 
     FIG. 10B is a perspective bottom view of the discharge element of FIG. 10A; and 
     FIG. 11 is a circuit diagram of an electric circuit used in the ozonizer of FIG.  9 . 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     The present invention will be described in greater detail below with reference to the drawings. 
     FIG. 1 shows the structure of a circulating water purifier  80  for use in a 24-hour working-type Jacuzzi (whirlpool bath) according to a first embodiment of the present invention. 
     Hot water in a bathtub  98  is drawn in through a water intake unit  82 , and debris such as hair is filtered from the hot water by a filter  84  disposed within the water intake unit  82 . Bucket  86  purifies the filtered hot water drawn in through the water intake unit  82 . The bucket  86  contains activated carbon  86 B and porous natural stone  86 A containing silicon dioxide (SiO 2 ) as a main component, and a temperature sensor  88  is disposed at the bottom of the bucket  86 . Microorganisms adhering to the natural stone  86 A and activated carbon  86 B act as a biofilter to decompose impurities contained in the hot water. The temperature of the hot water leaving the bucket  86  is monitored by the temperature sensor  88 , and the hot water is heated to an appropriate bathing temperature of 42° C. to 44° C. by a heater  90  equipped with a ceramic heater (not shown). Hot water heated by the heater  90  is pumped by a circulation pump  92  and discharged into the bathtub  98  from a jet nozzle  96  via a water flow sensor  94 . The water flow sensor  94  monitors water flow from the circulation pump  92  and turns off the circulation pump  92  when needed to protect its built-in motor. This occurs, for example, when the filter  84  is clogged and hot water in the bathtub  98  is not being pumped to the circulation pump  92 . 
     The circulating water purifier  80  contains an ozonizer  10  for generating ozone from oxygen contained in air. A first solenoid valve  16 A is mounted on a first air intake pipe  12   a  used for drawing air into the ozonizer  10 . A pipe  18   a  open to the atmosphere at the tip end thereof is connected to the first solenoid valve  16 A. A second air intake pipe  12   b  is connected to a discharge pipe  14  used for discharging ozone generated in the ozonizer  10  into the jet nozzle  96 . A second solenoid valve  16 B is mounted at the tip end of the second air intake pipe  12   b . A pipe  18   b  open to the atmosphere at the tip end thereof is connected to the second solenoid valve  16 B. 
     Under control of a controller (not shown), the ozonizer  10  is operated intermittently (for example, a 10-minute operation followed by a 50-minute pause). While the ozonizer  10  is operating, the first solenoid valve  16 A is opened, and the second solenoid valve  16 B is closed, so that air is taken into the ozonizer  10  through the first solenoid valve  16 A to thereby generate ozone. The ozone thus generated is drawn into the jet nozzle  96  via the discharge pipe  14  and discharged into the hot water contained in the bathtub  98  in the form of bubbles. Thus, the ozone is introduced into the hot water. On the other hand, while operation of the ozonizer  10  is suspended, the first solenoid valve  16 A is closed, and the second solenoid valve  16 B is opened. As a result, air is taken in through the second solenoid valve  16 B and drawn into the discharge pipe  14  via the second air intake pipe  12   b . Then, air is discharged from the jet nozzle  96  into hot water contained in the bathtub  98  in the form of bubbles. 
     Next, an ozonizing element accommodated in the ozonizer  10  is described below with reference to FIGS. 2A-2C. 
     As shown in FIG. 2A, a creeping discharge type ozonizing element  60  includes a first dielectric layer  62  and a second dielectric layer  64 , both formed from ceramic. A surface induction electrode  66  is interposed between the first dielectric layer  62  and the second dielectric layer  64 . A filamentary discharge electrode  68  is disposed on the upper surface of the first dielectric layer  62 . The surface of the filamentary discharge electrode  68  is covered with a glaze layer or ceramic layer (not shown) to prevent wear due to discharge. FIG. 2B shows the ozonizing element  60  of FIG. 2A viewed from underneath (back side). A terminal  66   a  connected to the surface induction electrode  66  and a terminal  68   a  connected to the filamentary discharge electrode  68  are exposed on the surface of the second induction layer  64 . Also, heaters H are mounted on the surface of the second dielectric layer  64  to prevent dew condensation on the ozonizing element  60  which is described below. Power from a high-voltage generating board, which is also described below, is supplied to the electrodes  66  and  68  via the terminals  66   a  and  68   a.    
     FIG. 2C shows another type of ozonizing element  160  according to another embodiment of the present invention. In the creeping discharge type ozonization element  160 , a filamentary discharge electrode  168  is disposed on the upper surface of a dielectric layer  164 , and electrodes  167   a  and  167   b  for connection to a power supply are disposed on the lower surface of the dielectric layer  164 . 
     Next, the structure of the ozonizer  10  shown in FIG. 1 is described below with reference to FIGS. 3A-3G. FIG. 3A shows a front view of the ozonizer  10 ; FIG. 3B shows a side view of the ozonizer  10 ; and FIG. 3C shows the ozonizer  10  with a cover  30  separated therefrom. FIG. 3D shows a sectional view along line  3 D— 3 D of FIG. 3A; FIG. 3E shows a bottom view of the ozonizer  10 ; FIGS. 3F and 3G show the ozonizer  10  mounted on the circulating water purifier  80 . 
     As shown in FIG. 3C, the ozonizer  10  includes the ozonizing element  60 , a box-like housing  20  which accommodates a high-voltage generating board  50 , described below, for driving the ozonizing element  60 , and a cover  30  for hermetically closing a first opening  20   a  formed in the housing  20 . In the present embodiment, the housing  20  comprises a rectangular box-shape, but may assume various kinds of shapes such as a cylindrical shape. 
     The housing  20  is integrally formed from a material resistant to ozone-induced oxidation such as vinyl chloride, stainless steel, Teflon, or the like. A flange portion  20   b  having a second opening  20   c  formed therein is provided inside the housing  20 . The ozonizing element  60  is mounted on the flange portion  20   b  via a packing  24  formed from an ozone-resistant fluorine-containing rubber. The packing  24  prevents ozone generated by the ozonizer  10  from leaking into the high-voltage generating board  50  side through the second opening  20   c . A through-hole  20   d  is provided in a side wall of the housing  20 . A screwdriver can be inserted through the through-hole  20   d  to adjust a variable resistor, described below, provided on the high-voltage generating board  50 . On the bottom portion of the housing  20  are formed a socket flange  20   f  for accommodating sockets  22   a  and  22   b  and six screw flanges  20   e  through which corresponding screws  28  (see FIG. 3B) are inserted in order to fix the cover  30  on the housing  20 . As shown in FIG. 3D, the sockets  22   a  and  22   b  are connected to the high-voltage generating board  50  via lead wires  56   a  and  56   b.    
     The cover  30  is formed from a transparent vinyl chloride which is resistant to ozone. Here, the term “transparent” means a degree of transparency such that a user can determine whether or not there is a discharge at the inner ozonizing element  60 , and thus includes semitransparent materials. Therefore, in order to achieve the above objects of the present invention, the cover  30  is preferably located so as to face the filamentary electrode  68  side of the creeping discharge element (creeping discharge type ozonization element)  60 , namely, the side of the creeping discharge element  60  where corona discharge occurs. As shown in FIG. 3C, an upright wall  30   a  is formed on the cover  30 . The upright wall  30   a  is inserted into the first opening  20   a  of the housing  20  and abuts the flange portion  20   b  via the packing  24  to thereby prevent ozone from leaking out of the apparatus. 
     An air intake pipe  30   b  for taking in air and an ozone discharge pipe  30   c  for discharging ozone are provided on the cover  30 . The first air intake pipe  12   a  shown in FIG. 1 is connected to the air intake pipe  30   b , whereas a discharge pipe  14  shown in FIG. 1 is connected to the ozone discharge pipe  30   c . On the periphery of the cover  30 , six screw flanges  30   d  are provided into which the corresponding screws  28  are driven in order to fix the cover  30  on the housing  20  (see FIG.  3 B), and a terminal flange  30   e  is provided which supports terminals  32   a  and  32   b  for inserting into the sockets  22   a  and  22   b , respectively. In the terminal flange  30   e , external lead wires  54   a  and  54   b  are connected to the terminals  32   a  and  32   b , respectively. 
     Also, as shown in FIGS. 3B and 3C, a pair of mounting brackets  30   f  extend longitudinally outward from both ends of the cover  30 . As shown in FIG. 3F, the ozonizer  10  is fixedly mounted on the housing  81  of the circulating water purifier  80  by means of screws  34  which are inserted through the through-holes  30   g  formed in the mounting brackets  30   f.    
     As shown in FIG. 3E, the ozonizing element  60  can be visually observed because the cover  30  is transparent. As shown in FIG. 3F, the ozonizer  10  is mounted on a window  81   a  formed in the housing  81  of the circulating water purifier  80 . Accordingly, the discharge state of the ozonizer  10  can be monitored from outside the circulating water purifier  80 . In FIG. 3F, the window  81   a  is formed in the housing  31  in the form of an opening. However, as shown in FIG. 3G, a glass plate  83  may be fit into the window  81   a.    
     As described above, the ozonizer  10  allows a user to monitor the discharge state of the ozonizing element  60  from outside the circulating water purifier  80 . When the discharge is properly carried out, a purple corona discharge light shines around the filamentary discharge electrode  68  of the ozonizing element  60  shown in FIG.  3 E. The corona discharge light indicates that ozone is being generated. 
     In contrast, when the discharge is disabled due to accumulation of an ammonium salt on the ozonizing element  60  over long-term use, the above-described discharge light is not observed. In that case, the screws  28  (see FIG. 3F) are removed to thereby separate the cover  30  from the housing  20  as shown in FIG.  3 C. Then, the ozonizing element  60  equipped in the housing  20  is cleaned using water or a solvent, to thereby remove the accumulated ammonium salt. This restores the ozonizing element  60  which can once again generate ozone. When the cover  30  is separated from the housing  20 , the terminals  32   a  and  32   b  are disconnected from the sockets  22   a  and  22   b , respectively, whereby the power supply is shut off. Thus, voltage applied to the ozonizing element  60  is reliably turned off. In yet another embodiment, a push-button switch (on when depressed) connected in series with the power supply may be employed. In this embodiment, the push-button switch is mounted such that the cover  30  depresses and engages the switch when fixed to the housing  20 . When the cover  30  is removed, the circuit is broken such that the voltage applied to the ozonizing element  60  is reliably turned off. This enables a user to safely carry out the above-described cleaning work. 
     The circuit of the high-voltage generating board  50  is described below with reference to FIGS. 4A-4C. As shown in FIG. 4A, the high-voltage generating board  50  has an IC 1  which receives an external electric potential of 12 V sequentially via the lead wires  54   a  and  54   b , the terminals  32   a  and  32   b , the sockets  22   a  and  22   b , and the lead wires  56   a  and  56   b  (see FIG. 3D) and which provides a regulated voltage supply. The heater H for heating the ozonizing element  60  is connected to the IC 1 . Being located on the back surface side of the ozonizing element  60 , the heater H continues heating the ozonizing element  60  to a temperature of approximately 40° C. even when power to the ozonizing element  60  is shut off, to thereby prevent dew condensation on the ozonizing element  60 . In FIG. 4B, the oscillation of transistor TR 1  can be stopped by applying a voltage from a terminal  69 . This discontinues ozone generation while power is continuously supplied to the heater H. 
     As shown in FIG. 4B, the high-voltage generating board  50  includes a transformer T, the transistor TR 1 , a transistor TR 2 , an IC 2  and a variable resistor RV. The transistor TR 1  together with the transformer T oscillate to generate a high electric potential of 5 kV at 40 kHz. The thus-generated high electric potential of 5 kV is applied to the ozonizing element  60 . The transistor TR 2  is adapted to cause the transistor TR 1  to start or stop oscillating. The IC 2  is used to adjust the amount of ozone that is generated by the ozonizing element  60  by altering its duty ratio. In order to adjust the value of the variable resistor RV to thereby set the duty ratio of the IC 2 , a user may insert a screwdriver through the through-hole  20   d  formed in the housing  20  as shown in FIG.  3 A. The high-voltage generating board  50  can include a power source such as a battery. 
     Next, an ozonizer  110  according to a second embodiment of the present invention is described below with reference to FIGS. 5A-5E. As in the case of the first embodiment, the ozonizer  110  is also intended for a circulating water purifier for use in a 24-hour working bath. A circulating water purifier employing the ozonizer  110  is similar to that of the first embodiment described above. Thus, a description thereof is not repeated. Members of the ozonizer  110  similar to those of the ozonizer  10  are denoted by common reference numerals, and the description thereof is not repeated. 
     FIG. 5A shows a front view of the ozonizer  110 ; FIG. 5B shows a side view of the ozonizer  110 ; and FIG. 5C shows the ozonizer  110  with a cover  130  separated therefrom. FIG. 5D shows a sectional view along line  5 D— 5 D of FIG. 5A, and FIG. 5E is a bottom view of the ozonizer  110 . 
     As shown in FIG. 5C, the ozonizer  110  includes the ozonizing element  60  which has been described above with reference to FIGS. 2A-2C, a box-like housing  120  which accommodates a high-voltage generating board  150  (FIG.  5 D), and a cover  130  for hermetically closing a first opening  120   a  of the housing  120 . 
     The housing  120  is integrally formed from vinyl chloride. A flange portion  120   b  having a second opening  120   c  formed therein (see FIG. 5A) is provided inside the housing  120 . The ozonizing element  60  is mounted on the flange portion  120   b  via a packing  124  formed from ozone-resistant fluorine-containing rubber. On the bottom portion of the housing  120  are provided a socket flange  120   f  for accommodating sockets  122   a  and  122   b  and six screw flanges  120   e  through which corresponding screws  28  are inserted in order to fix the cover  130  on the housing  120 . A through-hole  120   d  is provided in a side wall of the housing  120  to allow for adjusting the variable resistor of the high-voltage generating board  150 . As shown in FIG. 5D, the socket  122   a  is connected to a lead wire  154   b , and the socket  122   b  is connected to the high-voltage generating board  150  via a lead wire  156   b . Furthermore, an external lead wire  154   a  is directly connected to the high-voltage generating board  150 . 
     In contrast to the ozonizer  10  of the first embodiment which has been described above with reference to FIGS. 3A-3G, in the ozonizer  110  of the second embodiment, an air intake pipe  120   h  and an ozone discharge pipe  120   g  are provided on the housing  120 . The air intake pipe  12   a  shown in FIG. 1 is connected to the air intake pipe  120   h , and the discharge pipe  14  shown in FIG. 1 is connected to the ozone discharge pipe  120   g . Furthermore, a pair of mounting brackets  120   j  extend longitudinally outward from both ends of the top portion of the housing  120 . After the ozonizer  110  is turned upside down from the state shown in FIG. 5A, the ozonizer  110  is fixedly mounted on the housing  81  of the circulating water purifier  80  by means of screws (not shown) which are inserted through through-holes  120   k  formed in the mounting brackets  120   j.    
     The cover  130  is formed from a transparent vinyl chloride which is resistant to ozone. As shown in FIG. 5C, an upright wall  130   a  is formed on the cover  130 . The upright wall  130   a  is inserted into the first opening  120   a  of the housing  120  and abuts the flange portion  120   b  via the packing  124  to thereby prevent ozone from leaking out of the apparatus as shown in FIG.  5 A. Through-holes  130   f  are formed in the upright wall  130   a  so as to communicate with the air intake pipe  120   h  and the ozone discharge pipe  120   g  provided on the housing  120 . A flange  130   g  extends outward from the cover  130  and abuts the bottom surface  120   n  of the housing  123  as shown in FIG. 5A. A packing  126  interposed between the flange  130   g  and the bottom surface  120   n  maintains a hermetic seal. That is, in the second embodiment, an ozone leak is prevented by using the packings  124  and  126 . 
     On the periphery of the cover  130  are provided six screw flanges  130   d  through which the corresponding screws  28  (see FIG. 5A) are inserted in order to fix the cover  130  on the housing  120 , and a terminal flange  130   e  which supports a U-shaped jumper  132  for inserting into the sockets  122   a  and  122   b . Via the jumper  132 , the external lead wire  154   b  and the lead wire  156   b  connected to the high-voltage generating board  150  are connected as described above with reference to FIG.  5 D. 
     The circuit of the high-voltage generating board  50  in the second embodiment is described below with reference to FIGS. 4A-4C. 
     As shown in FIG. 4C, the high-voltage generating board  50  has the voltage regulating IC 1  which receives an external electric potential of 12 V sequentially via the lead wire  154   b , the jumper  132  and the lead wire  156   b , and via the lead wire  154   a . The circuit diagram of the high-voltage generating section of the high-voltage generating board  150  shown in FIG. 4B is similar to that of the first embodiment, and thus a description thereof is not repeated. 
     As shown in FIG. 5E, the ozonizing element  60  can be visually observed because the cover  130  is transparent. When ozone is not properly generated due to accumulation of ammonium salt on the ozonizing element  60 , the cover  130  is removed and the ozonizing element  60  is cleaned. When the cover  130  is removed, the jumper  132  is disconnected from the sockets  122   a  and  122   b  as shown in FIG.  5 D. As a result, the lead wire  154   b  is disconnected from the lead wire  156   b  such the electric potential is no longer applied to the ozonizing element  60 . Accordingly, it is then safe to clean the ozonizing element  60 . 
     Also, in the ozonizer  110 , an air intake pipe  120   h  and an ozone discharge pipe  120   g  are provided on the housing  120 . Accordingly, when the cover  130  is removed, the ozone discharge pipe  120   g  to which the discharge pipe  14  (see FIG. 1) is connected remains stationary. This prevents the discharge pipe  14  from accidentally being disconnected from the ozone discharge pipe  120   g  with a resultant ozone leak. Thus, safety is assured. 
     Next, an ozonizer according to a modification of the second embodiment is described below with reference to FIG.  6 . 
     In this modification, a check valve is unitarily provided in an ozone discharge pipe  120   v . A slit  120   r  is formed in the interior of the cylindrical portion  120   s  of the ozone discharge pipe  120   v , and a valve disk  128  moves along the slit  120   r . When ozone flows back toward the ozonizer  110 , the valve disk  128  abuts the inner wall  120   q  (a right-hand inner wall in FIG. 6) of the cylindrical portion  120   s , to thereby prevent ozone from entering the ozonizer  110 . This modification of the second embodiment does not involve installation of an external check valve, thereby avoiding an ozone leak which could otherwise occur at the connection between the check valve and a pipe used for connecting the check valve to the ozonizer  110 . 
     Next, an ozonizer according to a third embodiment of the present invention is described below with reference to FIGS. 7A-7E. 
     An ozonizer  210  according to the third embodiment has a structure substantially similar to that of the second embodiment as described above with reference to FIGS. 5A-5E. In the second embodiment, the cover  130  is fixed onto the housing  120  with screws, whereas in the third embodiment, a cover  230  is removably attached to a housing  220  by means of hook-like engagement portions. 
     FIG. 7A shows a perspective view of the ozonizer  210  according to the third embodiment. FIG. 7B shows a side view of the cover  230 . FIG. 7C shows a side view of the housing  220 . FIG. 7D shows a sectional view along the line  7 D— 7 D of FIG.  7 A. As shown in FIG. 7B, the cover  230  has engagement portions  230   b  serving as the engagement means of the present invention. The engagement portion  230   b  includes a flexible support piece  230   c  extending sideward from the cover  230 , a hook  230   e  formed at the tip end of the support piece  230   c , and a projection  230   d  formed substantially at the center of the support piece  230   c  and projecting upward. Engagement hole portions  220   b  serving as the engagement means of the present invention are formed in the housing  220  so as to engage the engagement portions  230   b  of the cover  230 . The engagement hole portion  220   b  includes a stepped engagement portion  220   c  for engaging the hook  230   e  and a through-hole  220   d  for receiving the projection  230   d.    
     In the ozonizer  210 , the cover  230  is press-fitted into the housing  220 , whereby the hooks  230   e  of the engagement portions  230   b  of the cover  230  engage the stepped engagement portions  220   c  of the engagement hole portions  220   b  of the housing  220 . Thus, the cover  230  is fixed on the housing  220 . When the cover  230  is to be removed from the housing  220 , the projections  230   d  of the engagement portions  230   b  are pressed down to thereby disengage the hooks  230   e  from the stepped engagement portions  220   c  of the engagement hole portions  220   b . In FIG. 7B,  230   a  is a peripheral projecting portion for holding a packing inside and providing an air-tight seal. 
     In the third embodiment, the ozonizing element can be readily cleaned because the cover  230  is removably attached to the housing  220  without using screws. In FIGS. 7A-7E, a jumper used for shutting off power to the high-voltage generating board is omitted for convenience of illustration. 
     FIG. 7E shows an ozonizer  210  according to a modification of the third embodiment. In this modification, the housing  220  has an engagement portion  220   e , and the cover  230  has an engagement hole  230   f  formed therein. 
     In the above-described first, second, and third embodiments, the entire cover  30 ,  130 , or  230  is transparent. However, only a portion of the cover  30 ,  130  or  230  or housing need be transparent so long as the ozonizing element  60  is visible. The transparent part of the cover or housing is preferably made of an inorganic transparent material such as glass as opposed to a transparent plastic (organic) material. This is because the transparent plastic loses its transparency faster than glass over an extended period of use. 
     In the above-described embodiments, a low electric potential supplied to the high-voltage generating board is disconnected when the cover is removed. Alternatively, a high electric potential applied to the ozonizing element  60  is disconnected when the cover is removed. Also, in the above-described embodiments, the high-voltage generating board is accommodated within the housing. Alternatively, the ozonizing element  60  alone may be accommodated within the housing, and a high electric potential may be applied to the ozonizing element  60  from a high-voltage generating board disposed outside the housing. 
     Next, the main structure of the ozonizer  10  in accordance with the fifth through seventh aspects of the present invention is described below with reference to FIG.  9 . 
     The ozonizer  10  includes a box-shaped resin case  11 , which houses a circuit board  12  on which an electric circuit shown in FIG. 11 is formed. A board  13  is mounted on the top portion of the case  11 . The board  13  has four sockets  14 ,  15 ,  16 , and  17 , which are electrically connected to the electric circuit formed on the circuit board  12 . A frame-shaped packing  18  formed from a heat resistant rubber is disposed on the peripheral edge of the top of the case  11 . An ozone generating element  21  is fitted into the space surrounded by the packing  18 . Four connection pins  21   a ,  21   b ,  21   c , and  21   d  project from the back surface of the ozone generating element  21  and are inserted into the sockets  14  through  17 , respectively. 
     A frame-shaped packing  40  formed from a heat resistant rubber is disposed on the peripheral edge of the upper surface of the ozone generating element  21  fitted into the packing  18 . A cover  41  is placed on the upper surface of the case  11  with the packing  40  interposed therebetween. 
     That is, the ozone generating element  21  is not in direct contact with the case  11 . This prevents heat generated from the ozone generating element  21  from being transmitted to the case  11  which might otherwise deteriorate or deform the case  11 . 
     An opening  42  is formed in the lower surface of the cover  41 . The air intake valve  43  for drawing in the air and the discharge pipe  44  for discharging ozone are provided on opposing end surfaces of the cover  41 , respectively. The air intake pipe  43  and the discharge pipe  44  communicate with the opening  42 . A mounting bracket  19  for mounting the ozonizer  10  inside the housing  81  of the water purifier  80  is provided at each end surface of the case  11  at a lower position thereof. A screw hole  19   a  is provided through the mounting bracket  19 . 
     In this embodiment, a fluorine-containing rubber is used as the heat resistant rubber. 
     Next, the structure of the ozone generating element  21  is described below with reference to FIGS. 10A and 10B. 
     As shown in FIG. 10A, the ozone generating element  21  includes a discharge element  22 , which in turn includes a sheet-like first dielectric layer  25  and second dielectric layer  26 , and a third dielectric layer  27  in the form of a laminate. A filamentary discharge electrode  25   a  is provided on the surface of the first dielectric layer  25 . Most of the surface of the filamentary discharge electrode  25   a  is covered with a protective film  25   b  to protect against wear caused by the discharge. A portion of the filamentary discharge electrode  25   a  that is not covered with the protective film  25   b  is exposed to the atmosphere and forms an exposed portion  25   d.    
     Even if the ozone generating element  21  breaks with a resulting exposure of a surface of the induction electrode  26   a  or heater electrode  27   a , current flows into the exposed portion  25   d . Thus, a user is protected from electric shock. 
     The surface induction electrode  26   a  is provided on the front surface of the second dielectric layer  26  such that its position corresponds to that of the filamentary discharge electrode  25   a . The heater electrode  27   a  serving as the heat generating element of the present invention is provided on the front surface of the third dielectric layer  27  such that its position corresponds to that of the filamentary discharge electrode  25   a.    
     In this embodiment, the heater electrode  27   a  is preferably located within 5 mm from the filamentary discharge electrode  25   a  for better heating efficiency. 
     One end of the filamentary discharge electrode  25   a  is electrically connected to a terminal  25   c  formed on the back surface of the third dielectric layer  27 . The terminal  25   c  is electrically connected to the ground side of the electric circuit via the connection pin  21   a  (see FIG.  9 ). One end of the surface induction electrode  26   a  is electrically connected to a terminal  26   c . The terminal  26   c  is electrically connected to the high-voltage side of the electric circuit via the connection pin  21   c . Both ends of the heater electrode  27   a  are connected to terminals  27   c . The terminals  27   c  are electrically connected to a heat generating circuit formed in the electric circuit via the connection pins  21   b  and  21   d.    
     In this embodiment, the filamentary discharge electrode  25   a  and the surface induction electrode  26   a  are preferably formed from tungsten, and the protective film  25   b  is preferably formed from glaze or a ceramic. A material for the heater electrode  27   a  is selected such that the temperature of the discharge element  22  reaches 200° C. to 500° C. approximately 10 seconds after power is applied to the discharge element  22  in the case of using a 110V AC power source. 
     This is because ammonium nitrate adhering to the discharge element  22  can be evaporated at a temperature of 200° C. to 500° C. 
     The discharge element  22  preferably reaches a temperature of from 250° C. to 350° C. 
     That is, ammonium nitrate adhering to the discharge element  22  begins to vaporize at a temperature slightly above 200° C. However, in order to reduce evaporation time, the discharge element  22  is preferably heated to a temperature of at least 250° C. Also, if the discharge element  22  is heated to an excessively high temperature, the case  11  may deteriorate or deform. 
     Thus, in view of the above, the heater electrode  27   a  having a resistance of 50 Ω at room temperature and a power consumption of 50 W is preferably formed from a mixed material of tungsten and ceramic so that the temperature of the discharge element  22  reaches 250° C. to 350° C. in 10 seconds. 
     Next, the electric circuit formed on the circuit board  12  is described with reference to FIG.  11 . 
     A heat generating circuit  53  and a power circuit  65  are provided on the circuit board  12 . The heat generating circuit  53  supplies current to the heater electrode  27   a  so as to generate heat from the heater electrode  27   a . The power circuit  65  supplies power to the ozone generating element  21  and the heat generating circuit  53 . 
     The heat generating circuit  53  includes a thermistor  51  having a positive characteristic and a diode  52 . The thermistor  51  is connected in series with the heater electrode  27   a  and functions as the heat generating time control means of the present invention. The diode  52  is connected in series between the thermistor  51  and the heater electrode  27   a . The power circuit  64  includes a half-wave diode bridge  61 , a transistor  62 , and a transformer  63 . The diode bridge  61  rectifies alternating current supplied from an AC power source  71 . The thus half-wave rectified current causes the transistor  62  to perform a switching operation. Switching of the transistor  62  causes the transformer  63  to apply a voltage between the filamentary discharge electrode  25   a  and the surface induction electrode  26   a.    
     Also, the filamentary discharge electrode  25   a  of the ozone generating element  21  is connected to a ground wire  64 . 
     Accordingly, even when water enters the case  11  and wets the filamentary discharge electrode  25   a , there is no potential difference between the filamentary discharge electrode  25   a  and the water. Thus, a user does not suffer from electric shock. 
     Next, the operation of the water purifier  80  and ozonizer  10  is described below. 
     In this embodiment, the voltage applied between both electrodes is 5 kV at 40 kHz. The resistance of the thermistor  51  is 15 Ω at room temperature. The maximum voltage of the AC power source  71  is approximately 140 V. 
     When the timer  70  turns ON at a predetermined time, power from the AC power source  71  is supplied to a pump-driving circuit  72 . As a result, the circulation pump  92  is driven to thereby pump hot water from the bathtub  98  through the water intake  82 . Hot water is then filtered by the bucket  86  and heated by the heater  90 . The thus-heated hot water is discharged from the jet nozzle  96 . The first solenoid valve  16 A is opened, and the second solenoid valve  16 B is closed, such that air is drawn into the ozonizer  10  through the air intake pipe  12   a.    
     When the timer  70  is turned ON, alternating current is supplied from the AC power source  71  to the circuit board  12 . The thus-supplied alternating current undergoes half-wave rectification by the diode bridge  61 . An electrolytic capacitor C 1  is charged with the thus half-wave rectified current. When the electrolytic capacitor C 1  is charged, base current flows to the base of the transistor  62  via a resistor R 1 ; consequently, the transistor  62  turns ON. As a result, current flows to the secondary of the transformer  63 , and an electric potential is established between the filamentary discharge electrode  25   a  and surface induction electrode  26   a  of the ozone generating element  21  sufficient to generate a discharge. The discharge converts oxygen contained in the air, which has been drawn into the opening  42  through the air intake pipe  12   a  (see FIG.  1 ), into ozone. The ozone thus generated is transferred through the discharge pipe  14  and discharged from the jet nozzle  96  into hot water contained in the bathtub  98  in the form of bubbles. 
     The above-described alternating current supplied from the AC power source  71  to the circuit board  12  also flows through the thermistor  51  and then to the diode  52 . The diode  52  performs half-wave rectification on the alternating current to thereby produce a DC voltage of approximately 70 V. Thus, direct current flows through the heater electrode  27   a  to thereby heat the heater electrode  27   a . The magnitude of current I flowing to the heater electrode  27   a  is approximately  1 A (I=70 V/(15 Ω+50 Ω)≅ 1 A). Accordingly, the power consumption P of the heater electrode  27   a  is approximately 50 W (P=1 2 ×50) 
     Subsequently, as current flows continuously, the temperature of the discharge element  22  reaches 250° C. to 350° C. in approximately 10 seconds. This elevated temperature induces scattering of ammonium nitrate molecules adhering to the filamentary discharge electrode  25   a . Meanwhile, the resistance of the thermistor  51  increases to 2.5 Ω due to temperature rise, such that current stops flowing through the thermistor  51 . Consequently, the heater electrode  27   a  stops generating heat. 
     In this embodiment, the timer  70  goes ON at 50-minute intervals and goes OFF 10 minutes after it goes ON. The ozone generating element  21  discharges continuously to generate ozone until the timer  70  goes OFF. 
     As described above, according to this embodiment, the ozone generating element  21  is heated by the heater electrode  27   a  to thereby induce scattering of ammonium nitrate molecules adhering to the filamentary discharge electrode  25   a . This, in turn, removes the adhering ammonium nitrate. 
     Accordingly, this aspect of the present invention dispenses with the need for conventional manual maintenance which involved disassembling an ozonizer and wiping the discharge element using water or a solvent. 
     Furthermore, because measures for preventing electric shock are employed, maintenance can be readily performed. 
     Particularly, when an ozonizer used in a water purifier is maintained, there is a high possibility of electric shock due to the entry of water. However, the ozonizer of the present invention provides an electric shock-free environment. 
     The ozonizer of the present invention can be used in various ozonized water-producing apparatuses without particular limitation. Namely, the water purifier of the present invention is applicable to water purification systems for ponds, water tanks, pools and the like. 
     It should further be apparent to those skilled in the art that various changes in form and detail of the invention as shown and described above may be made. It is intended that such changes be included within the spirit and scope of the claims appended hereto.