Patent Publication Number: US-8525045-B2

Title: Faraday cage and device having same

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     The present application is a national stage application under 35 U.S.C. §371 of PCT Application No. PCT/JP2009/061514, filed on Jun. 24, 2009, which claims priority from Japanese Patent Application No. 2008-167129 filed on Jun. 26, 2008, Japanese Patent Application No. 2008-184562 filed on Jul. 16, 2008, Japanese Patent Application No. 2008-263542 filed on Oct. 10, 2008, Japanese Patent Application No. 2009-042969 filed on Feb. 25, 2009, and Japanese Patent Application No. 2009-056179 filed on Mar. 10, 2009, the contents of which are incorporated herein by reference. 
     TECHNICAL FIELD 
     The present invention relates to a Faraday cage and a device having the same, each of which is for sucking in a measurement sample which is charged fine particles such as a toner for use in an electrophotographic technology and a charged powder coating or the like for the electrostatic powder coating technology, and measuring an electric charge of the measurement sample. 
     BACKGROUND ART 
     As an example of a known Faraday cage, Patent Document 1 describes a Faraday cage including an insulated container, and a suction nozzle (conductive container) disposed inside the insulation container, which has an intake vent part and an exhaust vent part sandwiching therebetween a filter for collecting toner, whereby the electric charge of the toner inside the suction nozzle serving as a conductive container is measured. In such a Faraday cage is measured the electric charge of the toner sucked into the suction nozzle in the insulation container. The lid of the insulation container is removed to take out the suction nozzle from the insulation container, and the weight of the suction nozzle containing the toner is measured. The per-unit weight electrostatic charge of the toner is then calculated by dividing the measured electric charge by the difference between the weight of the suction nozzle alone, which is measured beforehand, and the measured weight of the suction nozzle containing the toner. 
     PRIOR ART DOCUMENT 
     Patent Document 
     
         
         [Patent Document 1] Publication of Japanese Patent No. 3567463 (FIG. 5) 
       
    
     DISCLOSURE OF THE INVENTION 
     The Faraday cage described in Patent Document 1 necessitates removal of the lid of the insulation container, when measuring the weight of the suction nozzle. Patent Document 1 however is silent as to the specific structure of attaching the lid to the insulation container. If the lid is firmly fixed to the insulation container by using a plurality of screws, troublesome work is required every time the suction nozzle is taken out from or placed in the insulation container. Further, if there is another measurement, the suction nozzle has to be dismembered for cleaning up the toner adhered to the suction nozzle. This further necessitates work such as replacement of the filter in the suction nozzle, which consequently leads to a problem such as one that too much time is taken for preparing for the measurement. 
     In view of the above described problems, it is an object of the present invention to provide a Faraday cage and a device having the same, in which a conductive container detachably accommodates a filter cartridge and enables the filter cartridge to be easily placed in or taken out from the conductive container, thereby simplifying preparation work for a measurement after another. 
     A Faraday cage of the present invention includes: a casing structured by a first housing having a first outer cover made of a conductive material, and a first inner cover made of a conductive material, which is accommodated in the first outer cover and is electrically insulated from the first outer cover, and a second housing having a second outer cover made of a conductive material, which fits the first outer cover, and a second inner cover made of a conductive material, which is accommodated in the second outer cover and is electrically insulated from the second outer cover, the first and second housings being separable from each other; and a filter cartridge disposed inside the casing configured to be separable into two pieces, which accommodates therein a first filter for collecting fine particles sucked in from the outside the casing. 
     With the casing structured to be separable into two pieces, the filter cartridge is easily placed in or taken out from the casing. Further, preparation for the subsequent measurement only requires replacement of the filter cartridge with a new one. There is no longer a need for disassembling the suction nozzle or the like and clean the same. Therefore, the workability of the measurement is improved. 
     Further, a Faraday cage of the present invention includes: a casing structured by a first housing having a first outer cover made of a conductive material, and a first inner cover made of a conductive material, which is accommodated in the first outer cover and is electrically insulated from the first outer cover, and a second housing having a second outer cover made of a conductive material, which fits the first outer cover, and a second inner cover made of a conductive material, which is accommodated in the second outer cover and is electrically insulated from the second outer cover; and a filter cartridge disposed inside the casing, which accommodates therein a first filter for collecting fine particles sucked in from the outside the casing. The first and second housings have a lock mechanism which, by fitting the first and second outer covers together, enables the both housings to be engaged with each other, while keeping the respective end surfaces of the first and second inner covers pressed against each other relative to a fitting direction. 
     With this, simply fitting the first and second outer covers together causes the lock mechanism to work, and the both housings are engaged with each other, with the filter cartridge mounted therein. With the lock mechanism for engaging the both housings with each other, the filter cartridge is easily placed in or taken out from the casing separable into two pieces. Further, preparation for the subsequent measurement only requires replacement of the filter cartridge with a new one. There is no longer a need for disassembling the suction nozzle or the like and clean the same. Therefore, the workability of the measurement is improved. Further, when the both housings are engaged with each other, the electric contact between the first and second outer covers and the electric contact between the first and second inner covers are firmly maintained. It is therefore possible to accurately measure the electric charge in the space closed by the first and second inner covers. At the same time, with the electrically contacted first and second outer covers, the influence of an external electric field is effectively eliminated, when measuring the electric charge in the space closed by the first and second inner covers. 
     In the present invention, the lock mechanism includes a projection projecting in the fitting direction, from a portion of an inner circumferential surface of one of the first and second outer covers facing another one of the first and second outer covers, and an annular projection projecting from a portion of an outer circumferential surface of the other one of the first and second outer covers facing the one of the first and second outer covers, the annular projection having a notch which corresponds to the projection. It is preferable that the projection engage with the annular projection by rotating one of the housings less than once in a circumferential direction, after the first and second outer covers are fit together in such a manner that the projection passes the notch. This way, the lock mechanism is made simple. 
     In the present invention, it is preferable to provide a first biasing member disposed between the first outer cover and the first inner cover, which biases the first inner cover away from the first outer cover along the fitting direction. This increases the pressure for pressing the end surface of the second inner cover against the end surface of the first inner cover when the both housings are engaged with each other. Therefore, the further reliable electric contact between these members is maintained. 
     In the present invention, it is preferable to provide a second biasing member disposed between the first inner cover and the filter cartridge, which biases the filter cartridge away from the first inner cover in the fitting direction. With this, the second biasing member absorbs variation of a certain level in the size of the filter cartridge relative to the fitting direction. 
     Further, in the present invention, it is preferable that the first and second outer covers have a hard coating on their respective fitting areas; and that the hard coating be harder than a base material of the covers. Since the hard coatings are formed on the fitting areas of the first and second outer covers, respectively, it is possible to restrain the chipping off, galling, or the like, which is attributed to the friction of the first and second outer covers in the fitting area at the time of coupling the first and second outer covers. The first and second outer covers can be repetitively coupled with or separated from each other. 
     Further, in the present invention, it is preferable that the hard coating be insulative; and that the hard coating be not formed on respective contact areas of the first and second outer covers, the contact areas being respective portions of the fitting areas, which contact each other when the first and second outer covers are coupled with each other. With the structure, even if the hard coating is insulative, the first and second outer covers are electrically connectable to each other, when the first and second outer covers are coupled with each other. Thus, with the first and second outer covers, the influence of an external electric field is effectively eliminated, when measuring the electric charge in the space closed by the first and second inner covers. 
     Further, in the present invention, it is preferable that the hard coating be formed on the entire surfaces of the first and second outer covers, except for the contact areas. Since the first and second outer covers are coated by the hard coating, continuity is prevented between the first outer cover and the first inner cover, and between the second outer cover and the second inner cover, even a conductive foreign matter or a water droplet enters between the first outer cover and the first inner cover, or between the second outer cover and the second inner cover. Therefore, the electric charge in the first and second inner covers is accurately measured. 
     Further, in the present invention, it is preferable that the first and second outer covers be made of aluminum or an alloy containing aluminum; and that the hard coating be formed by anodizing. With this, the respective weights of the first and second outer covers are made relatively light, and therefore the entire weight of the Faraday cage is reduced. 
     Further, in the present invention, it is preferable that the hard coating be conductive. Thus, when the first and second outer covers are coupled with each other, the first and second outer covers are electrically connectable. Thus, with the first and second outer covers, the influence of an external electric field is effectively eliminated, when measuring the electric charge in the space closed by the first and second inner covers. 
     Further, in the present invention, the filter cartridge is made of a synthetic resin, and includes a first cylindrical part extended in a direction of sucking in fine particles, an increased-diameter part accommodating therein the first filter, whose diameter is larger than that of the first cylindrical part, and a second cylindrical part extended in the suction direction, which is disposed in such a manner that the increased-diameter part is interposed between the second cylindrical part and the first cylindrical part in the suction direction. It is further preferable that a light-transmissive area be formed at least one of the first cylindrical part, the increased-diameter part, and the second cylindrical part. With this, it is possible to easily confirm, through the light-transmissive area, whether the filter cartridge is a new one or one which is already used and have collected the fine particles. This prevents inadvertent usage of an already-used filter cartridge. 
     Further, in the present invention, it is preferable that the light-transmissive area be formed upstream of the first filter of the increased-diameter part, relative to the suction direction. This structure enables confirmation of the fine particles collected by the first filter through the light-transmissive area. Therefore, it is possible to reliably prevent a usage of an already-used filter cartridge. 
     Further, in the present invention, it is preferable that the filter cartridge be made of a transparent or semi-transparent synthetic resin, and that the light-transmissive area be formed on the entire filter cartridge. With this, whether or not the filter cartridge is used one is easily confirmed. Further, it is also possible to confirm the status of the first filter accommodated in the filter cartridge, if the filter cartridge is transparent. 
     Further, in the present invention, the first cylindrical part is disposed upstream of the increased-diameter part relative to the suction direction, and has a length which is longer than the diameter of the increased-diameter part and longer than the second cylindrical part. It is preferable that the first cylindrical part have a degression area in which the inner diameter of the first cylindrical part gradually decreases in the suction direction, and a progressive area formed at the downstream of the degression area, in which the inner diameter gradually increases in the suction direction. This structure enables an easier operation of sucking in the charged fine particles from the outside, and separate dies can be adopted for manufacturing the lengthy first cylindrical part. This contributes to reduction of the manufacturing cost of the filter cartridge. 
     Further, in the present invention, it is preferable that the second cylindrical part have a progressive area in which the inner diameter of the second cylindrical part gradually increases in the suction direction; and that an outlet port at the most downstream of the second cylindrical part have a larger diameter than that of a suction port at the most upstream of the first cylindrical part. This strengthens the suction force from the suction port. 
     Further, in the present invention, it is preferable that the second cylindrical part have, at its downstream end portion relative to the suction direction, an annular projection projecting from the outer circumferential surface of the second cylindrical part. This makes it easier to hold the filter cartridge when the filter cartridge is placed in or taken out from the casing. 
     Further, in the present invention, it is preferable that an outer circumferential side surface of the increased-diameter part be chamfered to form a polygonal shape. This way, the filter cartridge removed from the casing is restrained from rolling. Therefore, the fine particles are less likely spilled from the filter cartridge, and the weight of the filter cartridge having collected the fine particles is stably measured. 
     Further, in the present invention, the filter cartridge includes a container made of a synthetic resin. It is preferable that the container be made conductive. In the structure, the container is made conductive. Therefore, it is possible to highly accurately measure the weight of the filter cartridge including the container, by using a weight gauge. With the conductive filter cartridge, equal quantities of opposite charges occur in the container due to electrostatic induction caused by the charge of the fine particles collected by the first filter, the charge on the interior surface of the container and the charge occurred on the fine particles which are caused by triboelectric charging when the fine particles are being sucked in, respectively. The charges caused by the electrostatic induction closes the electric lines of forces from the charge of the fine particles, the charge on the interior surface of the filter cartridge and the charge occurred on the fine particles which are caused by triboelectric charging when the fine particles are being sucked in, respectively. Thus, when measuring the weight, the influence of the charges to the outside of the container is restrained or prevented. 
     Further, in the present invention, it is preferable that the synthetic resin contains a conductive material. This enables highly accurate measurement of the filter cartridge by using a weight gauge. 
     Further, in the present invention, it is preferable that the container have a conductive film which is formed on at least a part of its exterior surface. This enables highly accurate measurement of the filter cartridge which is the container by using a weight gauge. This is because, for a charge in the filter cartridge, an equal quantity of opposite charge occurs on the conductive film, due to electrostatic induction, and it is possible to restrain or prevent the influence of these charges to the outside of the container. 
     Further, in the present invention, it is preferable that the container have a conductive film throughout its entire exterior surface. This enables highly accurate measurement of the filter cartridge by using a weight gauge. 
     Further, in the present invention, the outer circumferential surface of the filter cartridge has thereon a plurality of projections which are disposed apart from one another along the circumferential direction. It is preferable that the plurality of projections each have a leading end which engages with the first inner cover, when the filter cartridge is inserted into the first inner cover. This prevents the filter cartridge from falling off from the casing, when the filter cartridge is taken out from the casing. Thus, the fine particles are less likely spilled from the filter cartridge. 
     Further, in the present invention, it is preferable that the upstream end of the first inner cover and the upstream end of the filter cartridge be disposed at substantially the same position relative to the suction direction of sucking in fine particles. This way, all the fine particles sucked into the first and second inner covers are entirely collected in the filter cartridge. 
     Further, in the present invention, it is preferable to further provide a filter unit which is provided in a midway portion of a route downstream from the filter cartridge, and which includes a second filter whose filtration accuracy is equal to or higher than that of the first filter. In the above structure, the filter unit is provided in the midway portion of the route. Thus, even when the first filter is damaged, or when the first filter is not in the filter cartridge, the fine particles sucked in with the air is collected by the filter unit. It is therefore possible to reliably prevent the risk of scattering to the outside the fine particles having been sucked in. 
     Further, in the present invention, it is preferable that the filter unit be provided outside the casing. This enables downsizing of the casing. Further, whether or not the filter unit is provided is confirmed at one glance. 
     Further, in the present invention, the filter unit has a resin case for accommodating the second filter. It is preferable that the resin case have a light-transmissive area in its portion to face the second filter. With the structure, it is possible to know at one glance whether or not the filter unit has collected the fine particles which are essentially supposed to be collected by the first filter. This way, a damage to the first filter or an absence of the first filter is surely confirmed. 
     Further, in the present invention, it is preferable that the filtration accuracy of the second filter be higher than that of the first filter. Thus, it is possible to reliably collect fine particles having passed the filter cartridge, which have a particle diameter too small for the filtration accuracy of the first filter. 
     A device of the present invention includes: a Faraday cage; an electric potential meter connected to wiring which is connected to one of the first and second outer covers and one of the first and second inner covers; and a weight gauge capable of measuring the weight of the filter cartridge. The Faraday cage includes: a casing structured by a first housing having a first outer cover made of a conductive material, and a first inner cover made of a conductive material, which is accommodated in the first outer cover and is electrically insulated from the first outer cover, and a second housing having a second outer cover made of a conductive material, which fits the first outer cover, and a second inner cover made of a conductive material, which is accommodated in the second outer cover and is electrically insulated from the second outer cover, the first and second housings being separable from each other; and a filter cartridge disposed inside the casing configured to be separable into two pieces, which accommodates therein a first filter for collecting fine particles sucked in from the outside the casing. 
     With this, efficient measurement of the electric charge and the weight of the fine particles is possible, by measuring the electric charge of the fine particles sucked into the filter cartridge, and measuring the weight of the fine particles in the filter cartridge thereafter. After the measurement of the electric charge, the fine particles are easily and reliably removed from the casing simply by taking out the filter cartridge from the casing. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  shows a schematic diagram of a device of an embodiment, according to the present invention. 
         FIG. 2  is a perspective diagram showing first and second housings of a Faraday cage of the embodiment, according to the present invention, the first and second housings being separated from each other. 
         FIG. 3  is an exploded perspective diagram of the Faraday cage shown in  FIG. 1 . 
         FIG. 4  is a cross sectional view showing the Faraday cage of the embodiment, according to the present invention. 
         FIG. 5  is a cross sectional view taken along the line V-V in  FIG. 3 . 
         FIG. 6  is a perspective diagram of the second outer cover shown in  FIG. 2  and shows that a projection forming member is detached from the cover. 
         FIG. 7  includes (a) which is a perspective diagram of the filter cartridge viewed from the upstream relative to the suction direction, and (b) which is a perspective diagram of the filter cartridge viewed from the downstream relative to the suction direction. 
         FIG. 8  is a cross sectional view of the filter cartridge shown in  FIG. 3 . 
         FIG. 9  is a cross sectional view at the time of putting the filter cartridge together with the first and second housing. 
         FIG. 10  is a schematic cross sectional view showing the status of charge on the filter cartridge, at the time of measuring the electric charge of the fine particles sucked in by using the Faraday cage. 
         FIG. 11  is a schematic cross sectional view showing the status of charge on the filter cartridge, at the time of measuring the weight of the filter cartridge with a use of the electronic balance, after the electric charge of the fine particles sucked in is measured. 
         FIG. 12  shows a modification of the filter cartridge of the embodiment, according to the present invention, and is a schematic cross sectional view showing the status of charge on the filter cartridge, at the time of measuring the electric charge of the fine particles sucked in by using the Faraday cage. 
         FIG. 13  shows a modification of the filter cartridge of the embodiment, according to the present invention, and is a schematic cross sectional view showing the status of charge on the filter cartridge, at the time of measuring the weight of the filter cartridge with a use of the electronic balance, after the electric charge of the fine particles sucked in is measured. 
         FIG. 14  includes (a) which is a cross sectional view showing a main part of the modification of the leading end portion of the first inner cover, and (b) which is a cross sectional view showing a main part of the modification of a portion at which the first inner cover and the second inner cover contact each other. 
     
    
    
     BEST MODE FOR CARRYING OUT THE INVENTION 
     The following describes a preferable embodiment of the present invention, with reference to the attached documents. 
     As shown in  FIG. 1 , a device  100  includes: a Faraday cage  1  which is for measuring an electric charge of fine particles sucked in from the outside; a piping members  6  connected to the Faraday cage  1  through a connection member  70 ; a filter unit  5  provided between the piping members  6 ; a suction pump  12  connected to the Faraday cage  1  through the piping members  6 ; an electric potential meter  14  connected to the coaxial cable  13  which is connected to the Faraday cage  1 ; and a weight gauge  18 . Note that a known electric potential meter and a known weight gauge are adopted as the electric potential meter  14  and the weight gauge  18  (an electronic balance  18  in the present embodiment), respectively. The suction pump  12  and the electric potential meter  14  may be integrally structured. Further, as the piping members  6 , a flexible tube made of rubber or a synthetic resin is used. 
     As shown in  FIG. 2 , the Faraday cage  1  has a casing  1   a  having substantially a cylindrical outline, and a filter cartridge  4  accommodated in the casing  1   a . The casing  1   a  has a first housing  2  and a second housing  3  which are separable from each other. Note that the filter cartridge  4  is disposed between the both housings  2  and  3 . Since the casing  1   a  is separable into two pieces, the filter cartridge  4  is easily placed in or taken out from the casing  1   a . Further, the casing  1   a  is provided with the connection member  70  for connecting one of the piping members  6  to the casing  1   a . Thus, driving of the suction pump  12  enables the Faraday cage  1  to suck in fine particles along with the air from the outside in a direction parallel to the axis of the Faraday cage  1 , from the left to right of  FIG. 4  (hereinafter, suction direction A). This suction causes the air, which is to be output from the casing  1   a  (the air sucked into the casing  1   a  from the outside), to flow from a hole  70   a  of the connection member  70  towards the suction pump, through the piping members  6  and the filter unit  5 . That is, the connection member  70 , the piping members  6 , and the filter unit  5  provided between the piping members  6  structure an air outputting route. 
     As shown in  FIG. 3  and  FIG. 4 , the first housing  2  has: a cylindrical first outer cover  21  made of an aluminum alloy; a cylindrical first holder  22  made of polycarbonate resin; and a cylindrical first inner cover  23  made of stainless steel. The first inner cover  23  is disposed so as to sandwich the first holder  22  between the first inner cover  23  and the first outer cover  21 . The first outer cover  21  and the first inner cover  23  are electrically insulated from each other. Note that the first outer cover  21  may be structured by a conductive material such as aluminum, copper, and a magnesium alloy. Further, the first inner cover  23  may be structured by a conductive material other than stainless steel. The first holder  22  may be structured by an insulative material other than polycarbonate resin. 
     At the upstream end of the first outer cover  21  relative to the suction direction A is an annular flange  31 . The annular flange  31  has a hole  31   a  through which the first holder  22  and the first inner cover  23  are partially in communication. At the downstream end of the first outer cover  21  relative to the suction direction A is an annular projection  33  which is structured by forming a groove  32  extending in the circumferential direction nearby that downstream end. The annular projection  33  has two notches  34   a  and  34   b . These two notches  34   a  and  34   b  are point-symmetrical with respect to the center axis of the first outer cover  21 . That is, the notch  34   a  is formed in a position 180° displaced from the notch  34   b . Note that, as shown in  FIG. 4 , the annular projection  33  project from the outer circumferential surface which, when the first outer cover  21  and the second outer cover  51  of the second housing  3  are fit together, overlaps the second outer cover  51  of the first outer cover  21  relative to the fitting direction. 
     On the entire surface of the first outer cover  21  is an anode oxide layer, i.e., an insulative hard coating  10 , which is formed by anodizing and which is harder than the aluminum alloy used as the base material of the first outer cover  21 . In the present embodiment, anodizing is adopted as the surface treatment, and the thickness of the hard coating  10  is approximately 30 μm. The thickness of the hard coating  10  however may be any thickness within a range from 10 μm, inclusive, and 100 μm, inclusive. In other words, the entire surface of the first outer cover  21  is made harder than the base material with the hard coating  10  of 10 μm or more in thickness, and the hard coating  10  can be formed as long as the thickness thereof is 100 μm or less. 
     Further, a surface treatment other than anodizing is adoptable as long as a hard coating harder thane the base material of the first outer cover  21  is formed on the entire surface of the first outer cover  21 . Examples of adoptable surface treatment include various platings such as hard chromium plating, electroless nickel plating; a conversion treatment, an LD (antirust black conductive thin coating) treatment; or any combination of these treatments. Note that the hard coating, which is formed by any of the various plating treatments such as hard chromium plating, electroless nickel plating, or the like, a conversion treatment, LD treatment, or any combination of these surface treatments, is conductive. The other possible treatments may be application of a material that forms the hard coating onto the surface of the first outer cover  21 , or dipping the surface into such a material, and then subjecting the material to a curing treatment thereafter. Further, ion plating, a laser irradiation, or quenching are also adoptable. The coating film described in the present invention encompasses a film or layer formed over the base material surface or a film or a layer formed within the surface of the base material, which has a property (curing property or the like) that is different from the base material. 
     As shown in  FIG. 5 , there are two contact areas  11  where no hard coating  10  is formed, on a surface  33   a  of the annular projection  33  of the first outer cover  21  facing the groove  32 . These two contact areas  11  are point-symmetrical to each other about the center axis of the first outer cover  21 . Note that each of the two contact areas  11  is in a position 90° displaced from the notches  34   a  and  34   b . Further, the two contact areas  11  are formed in an fitting area (areas where two surfaces overlap each other in the fitting direction) where the first outer cover  21  and the later described second outer cover  51  are fit together, and are positioned such that the two contact areas  11  contact later-described projection forming members  66  and  67  respectively, when the both housings  2  and  3  are coupled with each other. 
     The contact areas  11  of the present embodiment are formed by forming the hard coating  10  on the entire surface of the first outer cover  21  except for the portions corresponding to the contact areas  11 . Specifically, anodizing is conducted while masking the portions to become the contact areas  11  and then the masking is removed. That is, the contact areas  11  are the surface of the base material of the first outer cover  21 , which is not anodized. Note that the contact areas  11  may be formed by: forming the hard coating  10  on the entire surface of the first outer cover  21 , and then machining the hard coating  10  to expose, in the areas corresponding to the contact areas  11 , the surface of the base material of the first outer cover  21  which is yet to be anodized. The present embodiment deals with a case where the hard coating  10  is formed on the entire surface of the first outer cover  21 , except for the contact areas  11 ; however, the hard coating  10  may be formed only in the fitting areas except for the contact areas  11 , or formed only on the groove  32  except for the contact areas  11 . Further, the hard coating  10  may be formed only on the surface  33   a  except for the contact areas  11 . 
     As shown in  FIG. 3  and  FIG. 4 , the first holder  22  has: a cylindrical leading end portion  22   a  projecting outwardly from the hole  31   a ; a cylindrical main body part  22   b  which is mostly covered by the first outer cover  21 ; and an annular flange  22   c  connecting the leading end portion  22   a  and the main body part  22   b . As shown in  FIG. 4 , the first holder  22  is structured by closely attaching and fixing the annular flange  22   c  to the annular flange  31  of the first outer cover  21  with screws from the outside the first outer cover  21 . 
     At the downstream end of the main body part  22   b  relative to the suction direction A is formed an annular projection  36  which projects in radial directions of the main body part  22   b . The annular projection  36  has an outer diameter which is substantially the same as the outer diameter of the annular projection  33 . Further, on the annular projection  36  are formed two notches  37   a  and  37   b . These notches  37   a  and  37   b  are disposed so that, when the first holder  22  is fixed to the first outer cover  21 , the notch  37   a  faces the notch  34   a , and the notch  37   b  faces the notch  34   b . The notch  37   a  and the notch  34   a  form a single large notch  38   a , and the notch  37   b  and the notch  34   b  form a single large notch  38   b.    
     Further, annular projection  36  has two press-fit plungers  39   a  and  39   b  each of which has a resin ball outwardly biased in a radial direction of the main body part  22   b . These two press-fit plungers  39   a  and  39   b  are point-symmetrical to each other about the center axis of the main body part  22   b . That is, the press-fit plunger  39   a  is in a position 180° displaced from the press-fit plunger  39   b . Note that the press-fit plungers  39   a  and  39   b  are in positions 90° displaced from the positions of the notches  37   a  and  37   b.    
     As shown in  FIG. 3  and  FIG. 4 , the first inner cover  23  has a lengthy part  23   a  extending along the suction direction A; an increased-diameter part  23   b  having a larger diameter than the inner diameter of the lengthy part  23   a  at the downstream end; and a cylindrical collar  45  disposed within the increased-diameter part  23   b . The lengthy part  23   a  has a leading end portion  41   a  which projects from the first holder  22  when the first inner cover  23  is attached to the first holder  22 ; and a jointing portion  41   b  whose diameter is increased in steps along the suction direction A, which joints the leading end portion  41   a  and the increased-diameter part  23   b . Note that when the first inner cover  23  is put through the first holder  22 , the first inner cover  23  is prevented from detaching from the first holder  22  with a use of a C-shaped retaining ring  42 . Note further that a short flexible tube  99  made of an insulative material is fit into the leading end portion  41   a ; however, this short flexible tube  99  is not particularly necessary. 
     The diameter of the increased-diameter part  23   b  is increased in steps along the suction direction A. On the inner circumferential surface of the collar  45  is formed a taper surface  45   a  which is tilted from the suction direction A. The collar  45  is fixed to the increased-diameter part  23   b  with a screw, while being closely attached to the annular flange  43  formed at the upstream end of the increased-diameter part  23   b  relative to the suction direction A. Note that the collar  45  is made of brass which is conductive; however, the collar  45  may be made of any other conductive materials. 
     Inside the jointing portion  41   b  is a biasing member  47 . This biasing member  47  is disposed between a step portion  48  of the jointing portion  41   b  and the collar  45 , and biases the filter cartridge  4  inserted into the first inner cover  23  in the suction direction A. That is, the biasing member  47  biases the filter cartridge  4  in a direction away from the first inner cover  23 . The biasing member  47  is structured by a coil spring  47   a  and a pedestal  47   b  disposed between the coil spring  47   a  and the collar  45 . However, for example, the biasing member  47  may be structured by an elastic member such as rubber, instead of the coil spring  47   a . Further, the pedestal  47   b  may be omitted. 
     Inside the main body part  22   b  of the first holder  22  is disposed a biasing member  49 . The biasing member  49  is disposed between the annular flange  22   c  and the increased-diameter part  23   b , and biases the first inner cover  23  inserted into the first holder  22  in the suction direction A. Note that the first inner cover  23  is provided with a C-shaped retaining ring  42 , and is supported by the first holder  22  in such a manner that the first inner cover  23  is able to slide in the suction direction A. The biasing member  49  is structured by a coil spring; however, may be structured by an elastic member such as rubber. 
     As shown in  FIG. 3  and  FIG. 4 , the second housing  3  has: a cylindrical second outer cover  51  made of an aluminum alloy; a second holder  52  made of polycarbonate resin; a cylindrical second inner cover  53  made of stainless steel, which is disposed so as to sandwich the second holder  52  between the second inner cover  53  and the second outer cover  51 ; and a joint holder  54  made of polycarbonate resin, which is disposed so as to sandwich the second outer cover  51  between the joint holder  54  and the second holder  52 . The second outer cover  51  and the second inner cover  53  are electrically insulated from each other. Note that the second outer cover  51  may be made of a conductive material such as aluminum, copper, and a magnesium alloy. Further, the second inner cover  53  may be made of a conductive material other than stainless steel. The second holder  52  and the joint holder  54  may be made of an insulative material other than polycarbonate resin. 
     At the downstream end of the second outer cover  51  relative to the suction direction A is an annular flange  61 . The annular flange  61  has a hole  61   a  into which a part of the second holder  52  is inserted. The annular flange  61  has a hole  61   b  into which the coaxial cable  13  connected to the electric potential meter  14  is inserted. The outer shield line of the coaxial cable  13  is connected to the second outer cover  51  and the core line is connected to the second inner cover  53 . 
     As shown in  FIG. 4 , the second outer cover  51  has, at its upstream end relative to the suction direction A, two projections  51   a  and  51   b  which project from the inner circumferential surface. These projections  51   a  and  51   b  are structured by the two projection forming members  66  and  67  fixed to the base material of the second outer cover  51 . The projection forming members  66  and  67  are made of a conductive material such as stainless steel. Specifically as shown in  FIG. 6 , two notches  68  and  69  are formed at the upstream end of the second outer cover  51 . These notches  68  and  69  are point-symmetrical to each other about the center axis of the second outer cover  51 . As shown in  FIG. 4 , the projection forming members  66  and  67  are fit into the notches  68  and  69  and are fixed to the second outer cover  51  by screws, in such a manner that the leading end portions (projections  51   a  and  51   b ) project from the inner circumferential surface of the second outer cover  51 . The respective shapes of the portions of the projection forming members  66  and  67  projecting from the inner circumferential surface of the second outer cover  51  are shapes that correspond to those of the notches  38   a  and  38   b , respectively. Thus, after the first and second outer covers  21  and  51  are fit together, the first and second outer covers  21  and  51  are coupled with each other by rotating the first housing  2  by 90° in the circumferential direction so that the projections  51   a  and  51   b  are engaged with the annular projection  33  in the fitting direction parallel to the suction direction A. As should be understood, the projections  51   a  and  51   b  and the annular projection  33  constitute a lock mechanism for locking the both housings  2  and  3 . 
     Further, the second outer cover  51  also has on its entire surface, an insulative hard coating  15  formed by anodizing. As is the case with the hard coating  10 , the hard coating  15  is also approximately 30 μm in thickness; however the thickness of the hard coating  15  may be any thickness within a range from 10 μm, inclusive, to 100 μm, inclusive. Note that the hard coating  15  may be also formed by various surface treatments or other treatments, as is the case with the hard coating  10 . 
     As is hatched in  FIG. 6 , there are two contact areas  16  having no hard coating  15 , which are formed on the surfaces  68   a  and  69   a  of the notches  68  and  69 , respectively. These surfaces  68   a  and  69   a  contact the projection forming members  66  and  67 , when the projection forming members  66  and  67  are fixed on the notches  68  and  69 . Thus, the projection forming members  66  and  67  and the base material of the second outer cover  51  are electrically connected. The projection forming members  66  and  67  of the present embodiment are fixed to the base material of the second outer cover  51  on which the hard coating  15  is formed by anodizing, with the contact areas  16  being masked. Therefore, no hard coating  15  is formed on the projection forming members  66  and  67 . When the both housings  2  and  3  are coupled with each other, the contact area  11  of the first outer cover  21  and the projection forming members  66  and  67  contact each other and are electrically connected. Note that the hard coating  15  is not formed on the entire surfaces of the projection forming members  66  and  67 , and the surfaces themselves serve as the contact areas. These surfaces are made of stainless steel and are harder than the base material of the second outer cover  51 , as such. Thus, when the covers  21  and  51  are coupled with each other, it is possible to restrain chipping off, galling, or the like, which is attributed to friction of the covers at the fitting area. Further, the contact areas  16  may be formed by conducting a machining process after anodizing, as in the case described above. 
     In the present embodiment, the projection forming members  66  and  67  and the second outer cover  51  may be formed in one piece. In that case however, the contact areas without the hard coating is formed in the areas within the fitting area in which the first and second outer covers  21  and  51  are fit together, when the both housings  2  and  3  are coupled with each other, so that the contact areas face the contact areas  11 . In other words, the hard coating  15  may be formed in the entire fitting area, except for the contact areas. Further, the hard coating may be formed only on the leading end surface of the projections  51   a  and  51   b  which face the bottom surface of the groove  32 . As long as the hard coating is formed, it is possible to restrain the chipping off, galling, or the like, which is attributed to the friction of the covers in the fitting area at the time of coupling the both covers. 
     As shown in  FIG. 3 , the second holder  52  has substantially a disc-like shape, and the surface thereof facing the second inner cover  53  has a recess  63  in which the downstream end of the second inner cover  53  relative to the suction direction A is fit. At the center of the bottom surface of the recess  63  is a hole  64  which constitutes the air outputting route for the air output from the filter cartridge  4 . As shown in  FIG. 4 , the surface of the second holder  52  opposite to the surface on which the recess  63  is formed has an annular projection  52   a  having the hole  64 , which is inserted into the hole  61   a . Further, the second holder  52  has a hole  52   b  having substantially the same diameter as that of the hole  61   b . The hole  52   b  faces the hole  61   b  when the second holder  52  is fixed to the second outer cover  51 . Into this hole  52   b , too, is inserted the core line of the coaxial cable  13  connected to the second inner cover  53  and the insulative member covering the core line. 
     As shown in  FIG. 4 , the second inner cover  53  has an inner circumferential surface having: a taper surface  53   a  which is tilted from the suction direction A; a straight surface  53   b  which extends in the suction direction A; and a curved surface  53   c  connecting the taper surface  53   a  and the straight surface  53   b . Further, at the end portion on the outer circumference of the second inner cover  53  opposite to the second holder  52 , an annular projection  65  projecting in the radial directions is formed. The outer diameter of the annular projection  65  is the same as the largest outer diameter of the increased-diameter part  23   b  of the first inner cover  23 . The second inner cover  53  is fixed onto the second holder  52  with a screw from the outside the second holder  52 , while the downstream end of the second inner cover  53  relative to the suction direction A is fit in the recess  63 . 
     As shown in  FIG. 4 , the joint holder  54  has an inner tube  71  fit between the hole  61   a  of the annular flange  61  and the annular projection  52   a  of the second holder  52 ; an outer tube  72  disposed outside the inner tube  71 ; and an annular flange  73  connecting the inner and outer tubes  71  and  72 . On the inner circumferential surface of the inner tube  71  is a female thread which is formed from the vicinity of the middle portion to the downstream end of the inner tube  71  relative to the suction direction A, and a connection member  70  is screwed into this female thread. 
     Further, the annular projection  52   a  of the second holder  52  is fit into the upstream end portion of the inner tube  71 . On the outer circumferential surface of the annular projection  52   a  is formed an annular groove on which an O-ring is disposed. With this, the sealing property between the inner tube  71  and the second holder  52  is improved. Note that screwing the second holder  52  from outside the joint holder  54  fixes the second outer cover  51 , the second holder  52 , and the joint holder  54 , while sandwiching the annular flange  61  between the joint holder  54  and the second holder  52 . 
     The filter unit  5  is provided outside the casing  1   a  through the connection member  70  and the piping members  6 . The filter unit  5  has a cylindrical resin case  7 , a filter member (second filter)  8  accommodated in the resin case  7 . The resin case  7  has a transparent cylindrical main body part  7   a ; connecting portions  7   b  connecting the piping members  6  at both ends of the main body part  7   a . Each of the connecting portions  7   b  is structured so that the piping member  6  is attached or detached through a simple operation. Thus, the filter unit  5  is easily attached to or detached from the piping members  6 . 
     The filter member  8  is accommodated in the main body part  7   a . By accommodating the filter member  8  in the main body part  7   a  which is entirely the light-transmissive area, it is possible to know at one glance whether or not the filter unit  5  has collected the fine particles which are essentially supposed to be collected by the later-described filter  83 . This way, a damage to the filter  83  or an absence of the filter  83  is surely confirmed. Further, the filter member  8  is a hollow fiber membrane filter and the filtration accuracy thereof is higher than that of the filter  83  accommodated in the filter cartridge  4 . That is, for example, when the filter  83  is a filter paper of 1.0 μm or 0.7 μm in particle retention capacity, the filter member  8  is a hollow fiber membrane filter of 0.01 μm in filtration accuracy. 
     Next, the filter cartridge  4  is described below. As shown in  FIG. 7  and  FIG. 8 , the filter cartridge  4  includes two housings  81  and  82  which fit each other, and the filter (first filter)  83  accommodated in the both housings  81  and  82 . These two housings  81  and  82  are fit together to structure a single container. 
     The two housings  81  and  82  are made of polypropylene resin which is an insulative material. However, the two housings  81  and  82  may be entirely conductive by forming them with polypropylene resin to which fine metal particles as a conductive material are added. The container of the filter cartridge  4  is made of a synthetic resin, and therefore the weight thereof is made approximately 2 to 3 g. That is, the weight of the filter cartridge  4  is made closer to that of the fine particles to be sucked in. Supposing that a filter is provided in a metal container, the weight is heavy and is approximately 100 g. If the measurement range of the electronic balance  18  is set to 0.01 mg, to accurately measure the total weight of the fine particles collected by the metal container and the metal container itself, the total weight is too heavy to measure. Setting the measurement range to a greater value for measuring the total weight of the metal container and the fine particles, the measurement accuracy will drop. However, in the present invention, the weight of the filter cartridge  4  is made light and is approximately 2 to 3 g, the total weight of the filter cartridge  4  and the fine particles is measurable even if the measurement range is set to 0.01. 
     In the present embodiment, polypropylene resin to which a metallic fine powder is added is used as the material for the housings  81  and  82  instead of the insulative material. In this case, the material is not particularly limited provided that the conductivity is achieved. For example, a conductive material other than the fine metal particles such as metal fiber or carbon black may be used. Further, the resin may be a synthetic resin other than the polypropylene resin such as polyethylene resin or any styrene based resin. 
     The surface conductivity of the synthetic resin in general is 10 −14  Scm 2 . This surface conductivity is preferably made 10 −11  Scm 2  or more in the present embodiment, by adding a conductive material in the synthetic resin. It is further preferable to achieve the surface conductivity of 10 −9  Scm 2  more. 
     Further, the synthetic resin forming the housings  81  and  82  of the present embodiment may be any synthetic resin, as long as the synthetic resin is a material having transparency that the inside status can be seen from the outside. For example, when a milk white resin is used, making the thickness of the filter cartridge  4  relatively thin makes the filter cartridge  4  semi-transparent which enables confirmation of whether the filter cartridge  4  is a used filter cartridge  4  having sucked in the fine particles. In this case, substantially the entire filter cartridge  4  is the light-transmissive area. This enables confirmation of the status of the filter  83  accommodated in the filter cartridge  4 . 
     Note that the amount of conductive material added to the synthetic resin is an amount that achieves a suitable surface conductivity, and achieve a suitable transparency that enables confirmation of the inside status of the filter  83  from the outside. The above mentioned effect can be achieved also by forming at least one of the housings  81  and  82  by using a transparent synthetic resin (e.g. polycarbonate resin). It is also possible to make at least a part of the housings  81  and  82  transparent. This also achieves the above mentioned effect. 
     The filter cartridge  4  has a cylindrical lengthy part (first cylindrical part)  4   a  extending along the suction direction A; a cylindrical increased-diameter part  4   b  whose diameter is expanded larger than the inner diameter of the downstream end of the lengthy part  4   a ; and a cylindrical shorter part (second cylindrical part)  4   c  which is shorter than the lengthy part  4   a . The length of the lengthy part  4   a  is longer than the outer diameter of the increased-diameter part  4   b . More specifically, it is preferable that the length of the lengthy part  4   a  be approximately the same as the outer diameter of the increased-diameter part  4   b  or a double of the outer diameter. More preferably, the length of the lengthy part  4   b  is approximately 1.2 to 1.8 times, and even more preferably 1.8 times, the outer diameter of the increased-diameter part  4   b . This way, the leading end portion of the lengthy part  4   a  projects from the first housing  2  of the Faraday cage  1 , when the filter cartridge  4  is inserted into the Faraday cage  1 . This makes it easier to perform an operation of sucking in the charged fine particles from outside. 
     The filter  83  is a filter paper having a disc-like shape, which is selected according to the particle diameter of the fine particles to be measured. For example, a filter paper having a particle retention capacity of 1.0 μm is adopted for measuring the electric charge of the toner used in electrophotographic technology, and a filter paper having a particle retention capacity of 0.7 μm is adopted for measuring the electric charge of even finer particles. 
     As shown in  FIG. 7 , the outer circumferential side surface which is farthest apart from the center of the increased-diameter part  4   b  is chamfered to form a polygonal shape (e.g. hexadecagon). This way, the filter cartridge  4  removed from the casing  1   a  is restrained from rolling, when placed on a plane in such a manner that the outer circumferential side surface contacts the plane. Therefore, the fine particles are less likely spilled from the filter cartridge  4 , and the weight of the filter cartridge  4  having collected the fine particles is stably measured. 
     The housing  81  has a lengthy part  4   a  and a upper half portion  85  constituting a half of the increased-diameter part  4   b  on the upstream side. The lengthy part  4   a  has: a leading end portion  86  whose outer diameter is slightly smaller than the inner diameter of the leading end portion  41   a ; and a jointing portion  87  for jointing the leading end portion  86  and the upper half portion  85 , which has an outer diameter slightly smaller than the smallest inner diameter of the jointing portion  41   b.    
     As shown in  FIG. 8 , the leading end portion  86  has a degression area  84   a  whose inner diameter is gradually decreased in the suction direction A. That is, the inner diameter d 1  of the suction port  86   a  formed at the upstream end of the leading end portion  86  relative to the suction direction A is slightly larger than the inner diameter d 2  at the downstream end. Further, the length of the leading end portion  86  in the suction direction A is substantially the same as that of the leading end portion  41   a . When the filter cartridge  4  is inserted into the first inner cover  23 , the upstream end of the leading end portion  86  and the upstream end of the leading end portion  41   a  are substantially in the same position. This prevents adhesion of the fine particles to the inner circumferential surface of the leading end portion  41   a , when being sucked into the Faraday cage  1 , and most of the fine particles are taken into the filter cartridge  4 . Therefore, the total weight of all the fine particles whose respective electric charges have been measured is measurable by measuring the weight of the filter cartridge  4 . 
     The jointing portion  87  has a progressive area  84   b  whose inner diameter is gradually increased in the suction direction A. That is, the inner diameter d 3  at the upstream end of the jointing portion  87  relative to the suction direction A is smaller than the inner diameter d 4  at the downstream end. Thus, a die for forming this lengthy part  4   a  including a leading end portion  86  having the degression area  84   a  and the jointing portion  87  having the progressive area  84   b  can be separated at the portion corresponding to the boundary between the leading end portion  86  and the jointing portion  87 . This contributes to reduction of the manufacturing cost of the filter cartridge  4 . Further, the jointing portion  87  has substantially the same length as the jointing portion  41   b , relative to the suction direction A. On the outer circumferential surface at the downstream end of the jointing portion  87  relative to the suction direction A, there are three abutting portions  88  and three projections  89 , each of which projects from the outer circumferential surface and extends in the suction direction A. These abutting portions  88  and the projections  89  are alternately disposed apart from one another at equal intervals, along the circumferential direction. 
     Each of the three abutting portions  88  has a shape of a rectangular column extending in the suction direction A, and the height thereof from the outer circumferential surface of the jointing portion  87  is lower than those of the projections  89 . Specifically, when the filter cartridge  4  is inserted into the first inner cover  23 , each of the abutting portion  88  passes the collar  45  without contacting the inner circumferential surface of the collar  45 , and abuts the pedestal  47   b . This way, the filter cartridge  4  is biased by the biasing member  47  in the suction direction A. Since the filter cartridge  4  is biased in the suction direction A, the downstream end of the filter cartridge  4  is pressed against one side of the second holder  52 . This improves the sealing property of the connecting portion between the outlet port  98  of the filter cartridge  4  and the hole  64 , and the suction force at the suction port  86   a  of the filter cartridge  4  is ensured. As the result, the fine particles are reliably sucked and collected in the filter cartridge  4 , and the fine particles are kept from being sucked into a gap between the first inner cover  23  and the filter cartridge  4  from the leading end portions  41   a  and  86 . To further improve the sealing property between the filter cartridge  4  and the second inner cover  53 , a 2 mm thick packing made of silicon resin may be disposed on the surface of the second inner cover  53  to contact the downstream end of the filter cartridge  4 . 
     Each of the three projections  89  has a shape of a triangular column extending in the suction direction A, and the sharp leading end of the projection  89  is positioned farthest from the outer circumferential surface of the jointing portion  87 . Further, each of the projections  89  has a height such that, when the filter cartridge  4  is inserted into the first inner cover  23 , the leading end of the projection  89  contacts and is crushed by the inner circumferential surface of the collar  45 . Since the projections  89  and the collar  45  are engaged with each other, the filter cartridge  4  hardly falls from the first housing  2 , when taking out the filter cartridge  4  from the casing  1   a . Therefore, the fine particles are less likely spilled from the filter cartridge  4 . Further, the upstream end surface of each projection  89  relative to the suction direction is slanted. This facilitates crushing of the leading end of the projection  89 , when the filter cartridge  4  is inserted into the first inner cover  23 . Note that, since the projections  89  and the collar  45  contact each other when the filter cartridge  4  is inserted into the first inner cover  23 , the filter cartridge  4  and the first inner cover  23  are electrically connectable if the filter cartridge  4  is conductive. 
     As shown in  FIG. 7(   a ) and the  FIG. 8 , there are four ribs  91  in the upper half portion  85 . Each of these ribs  91  has substantially a triangular shape which extends from the vicinity of the entrance of the upper half portion  85  to the vicinity of the outer circumference end of the upper half portion  85 , along a slanted surface  85   a . These ribs  91  are disposed about the center axis of the upper half portion  85 , at intervals of 90°. The downstream end surface of each rib  91  is positioned to be able to contact the upstream side surface of the filter  83  accommodated in the filter cartridge  4 , so as to regulate the range of the movement of the filter  83 , while the fine particles are being sucked in. This prevents the filter  83  from being largely deformed and damaged, and at the same time enables the filter  83  to reliably collect the fine particles. 
     Further, at the vicinity of the outer circumference end of the upper half portion  85 , there is an annular weld portion  85   b  for combining the housings  81  and  82  by welding, which extends in the circumferential direction. Thus, after the housings  81  and  82  are engaged with each other with the filter  83  being sandwiched therebetween, the housings  81  and  82  can be welded by heating the weld portion  85   b  from the outside the filter cartridge  4 . Note that the housings  81  and  82  may be fixed to each other by using an adhesive agent. In that case, there is no need for the weld portion  85   b.    
     The housing  82  has a shorter part  4   c  and a down half portion  92  which constitute a half of the increased-diameter part  4   b  on the downstream side. The shorter part  4   c  has an outer diameter which is slightly smaller than the smallest inner diameter of the second inner cover  53 . Further, at the downstream end of the shorter part  4   c  is formed an annular projection  95 . The annular projection  95  has a slanted surface  96  formed on the upstream side relative to the suction direction A, and a slanted surface  97  on the downstream side. The outer diameter of the annular projection  95  is substantially the same as the smallest inner diameter of the second inner cover  53 . The tilt angle of the slanted surface  97  with respect to the outer circumferential surface of the shorter part  4   c  is smaller than that of the slanted surface  96 , and forms a relatively gradual slope. With this annular projection  95  on the shorter part  4   c , the annular projection  95  is suitably caught by user&#39;s fingers, when a user holds the shorter part  4   c  by his/her fingers. This makes it easier to hold the filter cartridge  4  when the filter cartridge  4  is placed in or taken out from the casing  1   a . Further, since the slanted surface  97  forms a gradual slope, the outer peripheral leading end of the annular projection  95  suitably sink into the fingers. As the result, the filter cartridge  4  is easy to hold. 
     The shorter part  4   c  has a progressive area  84   c  whose inner diameter is increased in the suction direction A. That is, the inner diameter d 5  at the upstream end of the shorter part  4   c  relative to the suction direction A is slightly smaller than the inner diameter d 6  at the outlet port  98  which is formed at the downstream end. The outlet port  98  has a larger diameter than the suction port  86   a . This strengthens the suction force from the suction port  86   a.    
     As shown in  FIG. 7(   b ) and  FIG. 8 , the down half portion  92  has two substantially trapezoidal ribs  93  which perpendicularly cross each other. Each of the rib  93  extends from the boundary portion between the shorter part  4   c  and the down half portion  92  to the vicinity of the outer circumference end of the down half portion  92 , along the slanted surface  92   a . The upstream end surface of each rib  93  is in a position to be able to contact the downstream side surface of the filter  83  accommodated in the filter cartridge  4 , and regulates the movement of the filter  83 , while the fine particles are being sucked in. This prevents the filter  83  from being largely deformed and damaged, and at the same time enables the filter  83  to reliably collect the fine particles. Further, at the downstream end of each rib  93  is formed a notch  93   a.    
     Next, the following describes with reference to  FIG. 1  and  FIG. 9  to  FIG. 11 , an operation performed in the device  100 , for deriving the per-unit weight electric charge of the fine particles having been sucked into the Faraday cage  1 . 
     First, the user confirms the inside status of the filter cartridge  4  from the outside to check whether the filter cartridge is an already-used cartridge. Then, the weight of a non-used filter cartridge  4  alone is measured by the electronic balance  18 . The filter cartridge  4  is then placed by the user between the first and second housings  2  and  3  which are separated from each other, as shown in  FIG. 9 , and the lengthy part  4   a  of the filter cartridge  4  is inserted into the lengthy part  23   a  of the first inner cover  23 . At this point, the abutting portions  88  abut the pedestal  47   b , and the respective leading ends of the projections  89  contact and are crushed by the inner circumferential surface of the collar  45 . This way, the projections  89  are engaged with the collar  45 . By having the three projections  89  engage with the collar  45 , the center axis of the filter cartridge  4  parallel to the suction direction A substantially coincides with the center axis of the casing  1   a . At this time, the filter cartridge  4  and the first inner cover  23  are electrically connected through the collar  45 , and the filter cartridge  4  and the second inner cover  53  are electrically connected through direct contact to each other. 
     Next, when the both covers  21  and  51  are fit together, the both covers  21  and  51  are positioned so that the projections  51   a  and  51   b  are able to pass the notches  38   a  and  38   b . The first and the second housings  2  and  3  are moved towards each other to fit the covers  21  and  51  together. As shown in  FIG. 9 , before the covers  21  and  51  are fit together, the downstream end of the first inner cover  23  relative to the suction direction A is projected from the downstream end of the first holder  22  relative to the suction direction A, due to the biasing force applied by the biasing member  49 . However, as shown in  FIG. 4 , by fitting the covers  21  and  51  together, the downstream end surface of the first inner cover  23  contacts the upstream end surface of the annular projection  65  of the second inner cover  53 , and the first inner cover  23  is pushed into the first holder  22 . At this time, the biasing force from the biasing member  49  increases the pressure for pressing the downstream end surface of the first inner cover  23  against the upstream end surface of the annular projection  65 . Therefore, the electric contact between the first inner cover  23  and the second inner cover  53  is made further reliable. When the first and second outer covers  21  and  51  are fit together, and the projections  51   a  and  51   b  are engaged with the annular projection  33 , there could be a problem such as rattling of the covers  21  and  51 , which would lead to decrease of the pressure for pressing the second inner cover  53  against the downstream end surface of the first inner cover  23 . With the biasing member  49  however, the rattling of the both covers  21  and  51  is absorbed by the biasing member  49 , and such a decrease in the pressure for pressing the second inner cover  53  against the first inner cover  23  is restrained. 
     When the both covers  21  and  51  are fit together, the biasing force from the biasing member  47  acts on the filter cartridge  4 . This increases the pressure for pressing the downstream end surface of the filter cartridge  4  against the side surface of the second holder  52 . As a result, the fine particles are reliably sucked and collected in the filter cartridge  4 , as is hereinabove mentioned, and the fine particles are no longer inadvertently sucked into a gap between the first inner cover  23  and the filter cartridge  4 . In addition, the biasing member  47  also absorbs a certain level of variation in the size of the filter cartridge  4  relative to the fitting direction. 
     After the first and second outer covers  21  and  51  are fit together by the user, the first housing  2  is rotated by 90°. This causes the respective resin balls of the press-fit plungers  39   a  and  39   b  to go into two curved grooves. The curved grooves are formed on the inner circumferential surface of the second outer cover  51  and are in the respective positions so as to face the annular projection  33  and overlap the projection forming members  66  and  67  in the suction direction A, respectively. Note that there are four curved grooves on the second outer cover  51 , which are disposed about the center axis of the second outer cover  51 , at intervals of 90°. This inhibits rotation of the first housing  2  with respect to the second housing  3 . In other words, the first and the second housings  2  and  3  do not rotate with respect to each other, unless rotational forces of a certain level are applied thereto, respectively. 
     Further, the projections  51   a  and  51   b  are rotate by 90° from the respective positions after passing the notches  38   a  and  38   b . This engages the projections  51   a  and  51   b  with the annular projection  33 , relative to the fitting direction, and the both housings  2  and  3  are coupled with each other, thus forming the casing  1   a . As should be understood, when the first and second outer covers  21  and  51  are fit together and rotated by 90°, the projections  51   a  and  51   b  and the annular projection  33  contact each other and the annular projection  33  and the inner circumferential surface of the second outer cover  51  contact each other. However, with the hard coating  10  or  15 , occurrence of chipping off, galling, or the like is restrained. Further, the hard coating  10  or  15  is also formed on the inner circumferential surfaces of the both covers  21  and  51 . This prevents continuity between the first outer cover  21  and the first inner cover  23 , and between the second outer cover  51  and the second inner cover  53 , even a conductive foreign matter or a water droplet enters between the first outer cover  21  and the first inner cover  23 , or between the second outer cover  51  and the second inner cover  53 . Therefore, the electric charge in the first and second inner covers  23  and  53  is accurately measured. Further, the respective contact areas of the both covers  21  and  51  contact each other and the covers  21  and  51  are electrically connected to each other. As is understood, the both covers  21  and  51 , when coupled with each other, are electrically connected to each other, although the covers  21  and  51  have the hard coatings  10  and  15 , respectively. Therefore, an influence from the external electric field is effectively eliminated, in the measurement of the electric charge inside the first and second inner covers. Note that, when the hard coating is conductive, the hard coating may be formed on the entire surfaces of the covers  21  and  51 . That is, the contact areas  11  and  16  may not be formed on the covers  21  and  51 . This structure also electrically connects the covers  21  and  51  when the covers  21  and  51  are coupled with each other, and brings about the above mentioned effects. 
     When the filter cartridge  4  is inserted, the interior surfaces of the both covers  21  and  51  having the hard coatings  10  and  15  may be charged by contacting the filter cartridge  4  or by a friction between the filter cartridge  4  and the interior surfaces of the covers  21  and  51 . However, this is not a concern as long as that electric charge is taken into account, in the measurement of the electric charge. Specifically, the zero point of the electric potential meter may be adjusted. Further, the biasing force from the biasing member  49  acts in directions (parallel to the fitting direction) in which the covers  51  and  21  separate from each other. Thus, the biasing force increases the force for engaging the projections  51   a  and  51   b  with the annular projection  33  and achieves more reliable electric contact. From this standpoint, the biasing member  49  constitutes a part of the lock mechanism. 
     Next, the user drives the suction pump  12  to generate a suction force in the suction port  86   a  of the filter cartridge  4  coupled with the Faraday cage  1 , and sucks the fine particles along with the air from the outside into the filter cartridge  4 . The fine particles having been sucked in at this time are collected by the filter  83 , and the air is output from the outlet port  98  towards the suction pump end through the air outputting route (the holes  64  and  70   a , the piping members  6 , and the filter unit  5 ). If the filter  83  of the filter cartridge  4  is not duly attached, or if the filter  83  is damaged during the operation of the suction pump  12 , the fine particles having been sucked in pass the filter cartridge  4  and are output to the suction pump end. However, with the filter unit  5  on the air outputting route, the fine particles are collected by the filter member  8  of the filter unit  5  and are kept from being output to the suction pump end. Therefore, it is possible to reliably eliminate, for example, the risk of scattering, from the suction pump  12  to the outside, the fine particles having been sucked in. Further, since the main body part  7   a  of the filter unit  5  is transparent, whether or not the fine particles have been collected by the filter unit  5  is confirmed at one glance. Note that, when the fine particles are collected by the filter unit  5 , the filter cartridge  4  is exchanged with the one without any defect, and the fine particles are sucked in again. 
     Next, the user stops driving the suction pump  12 , and if the filter unit  5  collects no fine particles, the total electric charge of the fine particles inside the filter cartridge  4  is measured. As shown in  FIG. 10 , for an electric charge E 1  (e.g. negative charge) of the fine particles collected in the filter cartridge  4 , an equal quantity of opposite charge E 2  (positive charge) occurs in the filter cartridge  4  due to electrostatic induction. When the fine particles are sucked in, triboelectric charging occurs to the fine particles on the interior wall of the filter cartridge  4 . As a result, charges E 3   a  and E 3   b  (e.g. negative charges) occur on the interior surface of the filter cartridge  4  and charges E 4   a  and E 4   b  (positive charges) occur on the fine particles. As shown in  FIG. 10 , the charges E 3   a  and E 4   a  are paired with a closed electric line of force (arrowed with a broken line in  FIG. 10 ). The charges E 3   b  and E 4   b  on the other hand do not practically pair with each other and do not close an electric line of force. The latter case is believed to occur because a fine particle having the charge E 4   b  separates, due to the movement of that particle inside the cartridge, from the part of the interior wall of the filter cartridge  4  where the charge E 3   b  occurred. For the two charges E 3   b  and E 4   b , equal quantities of opposite charges E 5   b  and E 6   b  occur in the filter cartridge  4  due to electrostatic induction. For each of the charges E 2 , E 5   b , and E 6   b  occurred in the filter cartridge  4 , an equal quantity of charge occurs at the capacitor C formed between the first and second inner covers  23  and  53  of the Faraday cage  1  and the first and second outer covers  21  and  51  (earth in the figure). Meanwhile, since the respective quantities of two charges E 5   b  and E 6   b  are equal to each other and the respective polarities thereof are opposite to each other, the charges E 5   b  and E 6   b  are canceled, and an equal quantity of charge to the charge E 2  is accumulated at the capacitor C at the end. The charge E 1  of the fine particles collected in the filter cartridge  4  is measured by using the electric potential meter  14  to measure the electric charge occurred at the capacitor C, whose quantity is equal to the charge E 2 . With the above measurement, the original electric charge E 1  of the fine particles is measured without an influence from the charges E 3   a , E 3   b , E 4   a , and E 4   b  which are caused by triboelectric charging. 
     Next, the total weight of the filter cartridge  4  and the fine particles collected is measured. In this case, the filter cartridge  4  with the fine particles being collected therein is taken out from the casing  1   a  by reversing the above described procedure. That is, the first housing  2  is rotated by 90°, and the both housings  2  and  3  are moved in directions to separate from each other. Then, the filter cartridge  4 , which is supported by the first inner cover  23  by having the projections  89  engaged with the collar  45 , is taken out from the first inner cover  23 . 
     Next, as shown in  FIG. 11 , the user places the filter cartridge  4  on a measuring plate  18   a  of the electronic balance  18 . At this time, the filter cartridge  4  is placed on the measuring plate  18   a  so that the lengthy part  4   a  of the filter cartridge  4  is at the higher level than the shorter part  4   c , i.e., the downstream end of the shorter part  4   c  and the outer circumference end of the increased-diameter part  4   b  contact the measuring plate  18   a  of the electronic balance  18 . This prevents scattering of the fine particles collected by the filter  83 , from the suction port  86   a.    
     Then, the user measures the total weight of the filter cartridge  4  and the fine particles by the electronic balance  18 . At this time, the status of the charge in the filter cartridge  4  as shown in  FIG. 10  is maintained. That is, as shown in  FIG. 11 , equal quantities of opposite charges E 2 , E 5   b , and E 6   b  occur in the filter cartridge  4  due to electrostatic induction caused by the charge E 1  of the fine particles, the charge E 3   b  on the interior surface of the filter cartridge  4  and the charge E 4   b  occurred on the fine particles which are caused by triboelectric charging, respectively. Thus, the electric lines of force are closed between the charge E 1  and the charge E 2 , the charge E 3   b  and the charge E 5   b , and the charge E 4   b  and the charge E 6   b , respectively, and these charges E 1 , E 3   b , and E 4   b  do not influence the outside the filter cartridge  4 . Note that the electric line of force from the charge E 4   a  is closed by the charge E 3   a . Therefore, even when the filter cartridge  4  is placed on the measuring plate  18   a , a charge causing a coulomb attraction between the filter cartridge  4  and the measuring plate  18   a  does not occur on the measuring plate  18   a . The charge on the windshield member of the electronic balance, which is constituted by an insulative glass, also causes no coulomb attraction between windshield member and the filter cartridge  4 . Therefore, the filter cartridge  4  is highly accurately measured by the electronic balance  18 . Note that, because the container of the filter cartridge  4  is made of a conductive material, the charge E 3   b  and the charge E 5   b  may be discharged from the filter cartridge  4  through the measuring plate  18   a . Further, the charge E 3   b  and the charge E 5   b  may be discharged from the filter cartridge  4  to the user or the air, when the user holds the filter cartridge  4 . 
     Next, from the total weight of the filter cartridge  4  resulting from the above measurement, there is subtracted the weight of the filter cartridge  4  itself which is measured before the filter cartridge  4  is attached to the casing  1   a . This way a difference in the weight is calculated. Then, to calculate per-unit weight electric charge of the fine particles, the total electric charge of the fine particles is divided by the above calculated difference in the weight. 
     As described, with the present embodiment, simply by fitting the both covers  21  and  51  together and rotating the first housing  2  by 90° in a circumferential direction, the lock mechanism is able to engage the both housings  2  and  3  with the filter cartridge  4  being attached thereto. Since the structure of this lock mechanism for engaging the both housings  2  and  3  is simplified, the filter cartridge  4  is easily placed in or taken out from the casing  1   a . Further, preparation for a subsequent measurement simply requires replacement of the filter cartridge  4  with a new one. There is no longer a need for disassembling the suction nozzle or the like and clean the same. Therefore, the workability of the measurement is improved. 
     Further, when the both housings  2  and  3  are engaged with each other, the electric contact between the first and second outer covers  21  and  51  and the electric contact between the first and second inner covers  23  and  53  are firmly maintained. It is therefore possible to accurately measure the electric charge in the space closed by the first and second inner covers  23  and  53 . At the same time, with the electrically contacted first and second outer covers  21  and  51 , the influence of an external electric field is effectively eliminated, when measuring the electric charge in the space closed by the first and second inner covers  23  and  53 . 
     Further, since the hard coatings  10  and  15  are formed on the fitting areas of the covers  21  and  51 , respectively, it is possible to restrain the chipping off, galling, or the like, which is attributed to the friction of the covers  21  and  51  in the fitting area at the time of coupling the both covers  21  and  51 . The covers  21  and  51  can be repetitively coupled with or separated from each other. Further, since the covers  21  and  51  are made of an aluminum alloy and the hard coatings  10  and  15  are formed by anodizing, the respective weights of the both covers  21  and  51  are made relatively light, and therefore the entire weight of the Faraday cage is reduced. 
     Since the filter unit  5  is provided outside the casing  1   a , the casing  1   a  is downsized as compared with the case of providing the filter unit in the casing. Further, the provision of the filter unit  5  outside the casing  1   a  is confirmed at one glance, and exchanging of the filter unit  5  is made easier. The filtration accuracy of the filter member  8  of the filter unit  5  is higher than that of the filter  83 . Therefore, it is possible to reliably collect particles having passed the filter cartridge  4 , the fine particles having a particle diameter which is too small for the filtration accuracy of the filter  83 . 
     Further, since substantially the entire filter cartridge  4  is a light-transmissive area, it is possible to easily confirm whether the filter cartridge  4  is a new one or one which is already used and have collected the fine particles. This prevents inadvertent usage of an already-used filter cartridge. 
     In the present embodiment, substantially the entire filter cartridge  4  is the light-transmissive area. However, the light-transmissive area may be formed partially on any one of the lengthy part  4   a , the increased-diameter part  4   b , and the shorter part  4   c . Such a structure also enables confirmation of whether or not the filter cartridge is a used one. Further, the light-transmissive area may be formed at a portion of the increased-diameter part  4   b , upstream from the filter  83 . This structure enables confirmation of the fine particles collected by the filter  83  through the light-transmissive area, in addition to the above-described effect. Therefore, it is possible to reliably prevent a usage of an already-used filter cartridge. To partially provide the light-transmissive area on the filter cartridge, an opening is formed on a part where the light-transmissive area is to be provided, and a transparent film or resin plate for covering the opening is welded or adhered to the filter cartridge. 
     When measuring the weight of the filter cartridge  4  having sucked in the fine particles, the charges E 1 , E 3   b , and E 4   b  do not influence the outside the filter cartridge  4 . Further, the electric line of force from the charge E 4   a  is closed by the E 3   a . Therefore, the weight of the filter cartridge  4  is highly accurately measured. This enables accurate calculation of the per-unit electric charge of the fine particles. 
     The following discusses, as a comparative example, a case where no conductive material is added to the filter cartridge  4 ; i.e., where the housings  81  and  82  are made of only a synthetic resin. In this case, since no electrostatic induction takes place in an insulator, the charges E 2 , E 5   b , and E 6   b  shown in  FIG. 10  will not occur inside the filter cartridge  4  but in the first and second inner covers  23  and  53  of the Faraday cage  1 . For the charges E 2 , E 5   b , and E 6   b , equal quantity of charges occur in the first and second inner covers  23  and  53  of the Faraday cage  1 , and then at the capacitor C formed between the first and second inner covers  23  and  53  and the first and second outer covers  21  and  51 , as is the case of  FIG. 10 . At the end, an amount of charge that equals to the charge E 1  remains at the capacitor C, and hence measurement of the charge E 1  of the fine particles collected in the filter cartridge  4  is possible. 
     The difference will be seen from the status of  FIG. 11 , when the user places the filter cartridge  4  on the measuring plate  18   a  of the electronic balance  18 . Specifically, there will be no charges E 2 , E 5   b , and E 6   b  which are present in the filter cartridge  4  shown in  FIG. 11 , and the electric lines of force from the charges inside the filter cartridge are not closed within the filter cartridge. As a result, the charges E 1 , E 3   b , and E 4   b  in the filter cartridge  4  cause electrostatic induction to cause occurrence of opposite charges on the measuring plate  18   a . As a result, the coulomb attraction occurs between these charges. The coulomb attraction also takes place between the charges inside the filter cartridge  4  and the charges occurring on the windshield member of the electronic balance, which is made of an insulative glass. Due to these coulomb attractions, the measurement value is unstable. As is understood from the above, a filter cartridge made of an insulative material does not have the advantage of confining therein the electric line of force of the charges inside the filter cartridge; however, such a filter cartridge is advantageous in terms of cost, because there is no need for adding a conductive material to the synthetic resin, or no need of applying such a material on the exterior surface of the synthetic resin. The filter cartridge made of an insulative material is therefore suitable for occasions of measuring only the electric charge of all the fine particles sucked in, and not the per-unit weight electric charge of the fine particles. 
     The housings  81  and  82  of the filter cartridge  4  of the present embodiment is made of a synthetic resin to which a conductive material is added. However, the filter cartridge  204  may be such that the conductivity is realized on the entire exterior surface of a container constituted by the housings  81  and  82 . This modification is described below with reference to  FIG. 12  and  FIG. 13 . Note that the elements and parts that are identical to those of the above embodiment are given the same reference numerals, and no further explanation therefor is provided below. 
     As shown in  FIG. 12 , in the filter cartridge  204  of the present modification, two housings  281  and  282  constituting the container is made of a polypropylene resin to which no conductive material is added, and a conductive film  283  is formed on the entire exterior surface of these housings. The conductive film  283  may be, for example, a material made by mixing conductive fine particles such as fine metal particles in a binder material. However, the material of the conductive film  283  is not particularly limited as long as the material is conductive. The surface conductivity is preferably 10 −11  Scm 2  or higher, and more suitably 10 −9  Scm 2  or higher, as is mentioned hereinabove. 
     With the formation of the conductive film  283  on the entire exterior surface of the filter cartridge  204 , the surface of the conductive film  283  facing the housings  281  and  282  is in the same status as those of the housings  81  and  82  of  FIG. 10 , at the time of measuring the total electric charge of the fine particles. That is, as shown in  FIG. 12 , the electric charge E 1  of the fine particles collected in the filter cartridge  204  causes electrostatic induction to cause occurrence of an equal quantity of opposite charge E 2 . Further, of the charges E 3   a , E 3   b , E 4   a , and E 4   b  which are caused by triboelectric charging, the charges E 3   b  and E 4   b  cause electrostatic induction to cause occurrence of equal quantities of opposite charges E 5   b  and E 6   b , respectively. 
     When the total weight of the filter cartridge  204  and the fine particles collected therein is measured, the user places the filter cartridge  204  on the measuring plate  18   a  of the electronic balance  18 , as shown in  FIG. 13 . At this time, the charge in the filter cartridge  204  is maintained at the status as shown in  FIG. 12 . In this modification, the charge E 3   b  is in the housings  281  and  282  which are insulative, and is not able to move. Meanwhile, the charge E 5   b  is a charge occurring due to electrostatic induction caused by the charge E 3   b , and is not able to move. This status therefore is different from that of  FIG. 11  in that the charges E 3   b  and E 5   b  are not discharged from the filter cartridge  204  through the measuring plate  18   a , the user, and the air. However, the electric line of force from the charge E 3   b  is closed by the charge E 5   b . Therefore, it is possible to highly accurately measure the weight of the filter cartridge  204  by the electronic balance  18 , as in the case of the above embodiment. 
     In the above modification, the conductive film  283  is formed on the entire exterior surface of the housings  281  and  282 . However, for example, the conductive film may be formed only on the entire exterior surface of the part of the increased-diameter part  4   b  constituting the shorter part  4   c . That is, the conductive film needs to be formed only on a part that contacts or be in the vicinity of the measuring plate  18   a  or the insulative glass. In this case, as in the above modification, the charges occurring to the increased-diameter part  4   b  and on the interior surface of the shorter part  4   c  are canceled. Therefore, the similar effects are achieved. Note that the charge on the interior surface of the lengthy part  4   a , which is caused by triboelectric charging, is relatively apart from the measuring plate  18   a . Therefore, an influence of this charge to the measurement of the weight is very unlikely. 
     Further, in the above described embodiment and the modification, the electric charge of the fine particles sucked in the filter cartridge  4  or  204  is measured, while the filter cartridge  4  or  204  is electrically connected to the first and second inner covers  23  and  53  of the Faraday cage  1 . However, the electric charge of the fine particles may be measured after electrically insulating the filter cartridge  4  and  204  from the first and second inner covers  23  and  53 . In this case, charges occur on the exterior surface of the filter cartridge  4  or  204  and the interior surfaces of the first and second inner covers  23  and  53 . The charge occurred on the exterior surface of the filter cartridges  4  or  204  and the charge occurred on the interior surfaces of the first and second inner covers  23  and  53  are due to electrostatic induction caused by the charges E 2 , E 5   b , and E 6   b . The charge on the exterior surface of the filter cartridge  4  or  204  is discharged from the filter cartridge  4  or  204 , before the measurement of the weight, through the user and the air while the user holds the filter cartridge  4  or  204 . The charge is also discharged from the filter cartridge  4  or  204  through the measuring plate  18   a . Therefore, highly accurate measurement of the weight of the filter cartridge  4  or  204  by the electronic balance  9  is possible, as is the case described hereinabove. 
     Note that the filter cartridge is not limited to one made of a synthetic resin to which a conductivity is realized, and may be one made of an insulative material such as glass fiber, fabric, paper, or trees to which conductivity is realized. Further, the above embodiment provided an explanation on a filter cartridge which is attached to a Faraday cage for measuring the electric charge of fine particles sucked in from the outside, and which has a filter for collecting the fine particles sucked in. The filter cartridge may be, for example, a saclike meshed container made of synthetic resin, glass fiber, fabric, paper, or wood to which conductivity is realized by, for example, spraying a conductive material. 
     Thus, a preferable embodiment of the present invention is described hereinabove. It should be noted that the present invention is not limited to the above embodiment, and may be altered in various ways within the scope of claims. For example, the above embodiment and modification deal with a case where the container of the filter cartridge has the lengthy part, the increased-diameter part, and the shorter part. However, the filter cartridge may be a cylinder whose shape relative to the suction direction is the same, or rectangular or polyangular tube. Further, the outer shield line of the coaxial cable  13  may be connected to the first outer cover  21 , and the core line to the first inner cover  23 . It is possible to adopt a wiring member other than the coaxial cable  13 . 
     Further, the above described embodiment deals with a case where the lock mechanism is constituted by the projections  51   a  and  51   b  and the annular projection  33 . It is however possible to form a male thread on the outer circumferential surface of the cover  21  and a female thread part on the inner circumferential surface of the cover  51 . This way, the both housings are engaged with each other by rotating one of the housings in a circumferential direction so that the male thread is screwed into the female thread. Further, the projections  51   a  and  51   b  may be formed on the first housing  2 , and the annular projection  33  may be formed on the second housing  3 . In other words, the arrangement of the projections  51   a  and  51   b  and the arrangement of the annular projection  33 , on the housings  2  and  3  may be other way around. 
     Further, the lock mechanism is not limited to the one described above, as long as the first and second housings  2  and  3  are coupled with or separated from each other reliably and easily. For example, the lock mechanism may be a hook or a coupler which engages with the groove  32 , simply by fitting the first housing  2  to the second housing  3 , so that the both housings  2  and  3  are not separated. Further, the lock mechanism may be a plunger type lock mechanism such that the both housings  2  and  3  are locked and not separable, simply by the press-fit plungers  39   a  and  39   b.    
     Further, in the above embodiment, the downstream end of the filter cartridge  4  is pressed against a side surface of the second holder  52  to prevent the fine particles from being sucked in, from between the leading end portion  41   a  of the first inner cover  23  and the leading end portion  86  of the filter cartridge  4 , into a gap between the first inner cover  23  and the filter cartridge  4 . This prevention is made further reliable by forming the leading end portion of the first inner cover  23  in a shape as shown in  FIG. 14(   a ). 
     As shown in  FIG. 14(   a ), the inner diameter of the suction port  241   c  of the leading end portion  241   a  is gradually reduced from the upstream end towards the downstream end. The inner diameter at the downstream end is made smaller than that of the suction port  86   a  of the filter cartridge  4 . This structure more reliably prevents adhesion of the fine particles to the outer circumference of the suction port  86   a  of the filter cartridge  4 . 
     Further, in the above embodiment, the downstream end surface of the first inner cover  23  and the upstream end surface of the annular projection  65  of the second inner cover  53  are brought into contact with each other, to electrically contact the first inner cover  23  and the second inner cover  53 . This electric contact is made more reliable by having the downstream end of the first inner cover and the upstream end of the second inner cover contact each other as shown in  FIG. 14(   b ). 
     As shown in  FIG. 14(   b ), a projecting stair-like part  265   a  in the shape of ring is formed on the upstream end surface of the annular projection  265  of the second inner cover  253  which surface contacts the downstream end surface of the increased-diameter part  223   b  of the first inner cover. This projecting stair-like part  265   a  has an outer diameter that matches with the inner diameter of the increased-diameter part  223   b . In this case, when the first inner cover and the second inner cover are brought into contact with each other, the downstream end surface of the increased-diameter part  223   b  and the upstream end surface of the annular projection  265  contact each other, and the outer circumferential side surface of the projecting stair-like part  265   a  and the inner circumferential surface of the increased-diameter part  223   b  contact each other. Thus, even if contamination occurs between the downstream end surface of the increased-diameter part  223   b  and the upstream end surface of the annular projection  265 , the contact of the projecting stair-like part  265   a  to the inner circumferential surface of the increased-diameter part  223   b  is ensured. This way the reliability of the electric contact is further improved. 
     Further, the filter unit may be provided in the casing  1   a . In this case, the filter unit may be provided on the air outputting route which is downstream from the outlet port  98  of the filter cartridge  4 . Further, the filter unit  5  may have a partially transparent or non-transparent main body part, instead of the main body part  7   a . Further, the filter member  8  in the filter unit  5  is not particularly limited as long as the filtration accuracy thereof is at least equal to that of the filter  83 . 
     Further, the biasing members  47  and  49  may be omitted. Further, a plurality of projections  89  does not have to be formed on the outer circumferential surface of the filter cartridge  4 . Further, the upstream end of the first inner cover  23  and the upstream end of the filter cartridge  4  relative to the suction direction A do not necessarily have to be coincided with each other. Further, the outer circumferential side surface of the filter cartridge  4  does not have to be chamfered. Further, the ribs  93  do not have to be formed on the housing  82 . 
     Further, in the above embodiment, the lengthy part  4   a  has the degression area  84   a  and the progressive area  84   b . These two areas  84   a  and  84   b  however are not necessary. That is, the lengthy part  4   a  may have a single progressive area (degression area) in which the inner diameter gradually increases (decreases) in the suction direction A, or a straight area where the inner diameter is constant. Note that the shorter part  4   c  may also have a degression area in which the inner diameter gradually decreases in the suction direction A, or a straight area where the inner diameter is constant. Further, the diameter of the suction port  86   a  may be larger than or equal to that of the outlet port  98 . Further, the annular projection  95  may be formed on the downstream end of the shorter part  4   c.    
     REFERENCE NUMERALS 
     
         
         
           
               1  Faraday Cage 
               1   a  Casing 
               2  First Housing 
               3  Second Housing 
               4 ,  204  Filter Cartridge 
               4   a  Lengthy Part (First Cylindrical Part) 
               4   b  Increased-Diameter Part 
               4   c  Shorter Part (Second Cylindrical Part) 
               5  Filter Unit 
               7  Resin Case 
               8  Filter Member (Second Filter) 
               10 ,  15  Hard Coating 
               11 ,  16  Contact Area 
               14  Electric Potential Meter 
               18  Electronic Balance (Weight Gauge) 
               21  First Outer Cover 
               23  First Inner Cover 
               33  Annular Projection 
               34   a ,  34   b  Notch 
               47  Biasing Member (Second Biasing Member) 
               49  Biasing Member (First Biasing Member) 
               51  Second Outer Cover 
               51   a ,  51   b  Projection 
               53  Second Inner Cover 
               81 ,  82 ,  281 ,  282  Housing (Container) 
               83  Filter (First Filter) 
               84   a  Degression Area 
               84   b ,  84   c  Progressive Area 
               86   a  Suction Port 
               89  Projection 
               95  Annular Projection 
               98  Outlet Port 
               100  Device 
               283  Conductive Film