Abstract:
A powder inhaler for administering powder by inhalation, comprising a chamber ( 58 ) which includes a base member ( 12 ) and a peripheral wall member ( 48 ), wherein the chamber ( 58 ) includes a first inlet ( 36 ) through which a first stream of air containing powder is in use drawn on inhalation by a user and a plurality of second inlets ( 66, 67, 68, 69, 100 ) at the junction between the base member ( 12 ) and the peripheral wall member ( 48 ) through which a plurality of second streams of air of combined lesser flow than the first stream of air are in use drawn on inhalation by the user so as to minimize retention of powder at the junction between the base member ( 12 ) and the peripheral wall member ( 48 ).

Description:
BACKGROUND OF THE INVENTION 
     The present invention relates to a powder inhaler for administering powder by inhalation. 
     A number of powder inhalers are known which use different systems for introducing a dose of powder into an air stream. Typically, the powder is inhaled into the lungs of a patient in order to treat, for example, asthma. 
     EP-A-0237507 discloses one such powder inhaler. This inhaler comprises an inhalation channel and a mouthpiece which includes an air chamber and an outlet nozzle, which together define a flow path through which a stream of air is drawn during inhalation by a user. This inhaler further comprises a dosing mechanism for providing a dose of powder to the inhalation channel. During inhalation, air is first drawn into and through the inhalation channel so as to pick up powder. The stream of air containing powder is then drawn through the air chamber and out of the outlet nozzle of the mouthpiece. 
     FIG. 1 illustrates such a powder inhaler. The inhaler comprises an inhaler unit  1  and a cap  2  which is a screw fit to the same. The inhaler unit  1  comprises a mouthpiece  3  which includes an outlet nozzle  4 , an inhaler body  6  and a rotatable grip portion  8  for operating a dosing mechanism for providing doses of powder for inhalation. The inhaler body  6  is provided with an opening  10  which is filled with a window  11  through which an indicating wheel  42  is visible so as to provide an indication as to the usage of the inhaler. 
     FIG. 2 illustrates in exploded view component parts disposed within and to the inhaler body  6 . The inhaler body  6  is capped with a divider  12  which is fixed thereto. For aesthetic reasons the inhaler body  6  is an opaque moulding. The divider  12  is a transparent moulding which includes a peripheral groove  13  and an adjacent peripheral ridge  14  by which the mouthpiece  3  is attached thereto. The divider  12  further includes a depending tongue  15 , a part of which forms the window  11 . 
     Within the inhaler body  6  are housed the component parts of the dosing mechanism. These component parts include a dosing unit  16  which comprises a member  17  having a planar surface in which a plurality of dosing elements  18  are provided and a s haft  20  which extends axially from the centre of the member  17 , a distribution unit  22  which comprises an inhalation channel  24  and a storage unit  26  which comprises a storage chamber  28  for storing powder. The above-mentioned component parts of the dosing mechanism are assembled by passing the inhalation channel  24  through an opening  30  in the storage unit  26  and passing the shaft  20  through central openings  32 ,  34  in the distribution unit  22  and the storage unit  26  respectively. When so assembled, the upper ends of the inhalation channel  24  and the storage chamber  28  pass respectively through first and second openings  36 ,  38  in the divider  12 . In this way, the distribution unit  22  and the storage unit  26  are fixed in position in relation to one another and the dosing unit  16  can be rotated relative thereto. 
     In this inhaler the storage chamber  28  is open at the bottom such that in use powder is provided to the dosing unit  16  under the action of gravity and the distribution unit  22  further comprises scrapers  40  which are resiliently biased against the surface of the member  17  of the dosing unit  16  in which the dosing elements  18  are provided. In this way, as the dosing unit  16  is rotated, the dosing elements  18 , which in this inhaler comprise a plurality of through holes, are filled with powder by the scrapers  40 . Powder is prevented from passing through the dosing elements  18  by a plate (not illustrated) which is disposed beneath the dosing unit  16 . 
     The divider  12  further includes a supporting member  41  for rotatably supporting an indicating wheel  42 . The indicating wheel  42  includes a plurality of teeth  44  disposed around the periphery thereof which engage with a spiral groove or protrusion  46  on the end face of the shaft  20  of the dosing unit  16 . The supporting member  41  is configured to align the indicating wheel  42  such that a part of the periphery thereof is disposed adjacent the inner surface of the window  11 . In use, as the dosing unit  16  is rotated, the spiral groove or protrusion  46  engages with one or more of the teeth  44  on the indicating wheel  42  so as to rotate the same. In this way, by providing a coloured marking on the periphery of the indicating wheel  42 , it is possible to provide the user with a visible indication at the window  11  as to the usage of the inhaler. 
     As illustrated in FIG. 4, the mouthpiece  3  is fixed to the divider  12 . The mouthpiece  3  comprises first and second parts  48 ,  50 , the first part  48  being the part which is gripped in the lips of a user and the second part  50  being an insert fitted within the first part  48 . The second part  50  comprises a tubular section  52  which includes one or more spirally or helically shaped projections  54  that act to deflect the air drawn therethrough and thereby deagglomerate any larger particles of entrained powder and a substantially radially-directed flange  56  which provides a surface that together with upper surface of the divider  12  defines an air chamber  58  that is in fluid communication with the inhalation channel  24  through which air containing powder is drawn on inhalation by a user. The flange  56  includes a downwardly-depending annular skirt  59  which is a tight fit within the first part  48  and acts to fix the second part  50  to the first part  48 . 
     In use, as described hereinabove, powder is transferred from the storage chamber  28  to one of the dosing elements  18 , and, with rotation of the dosing unit  16 , the one dosing element  18  provides a dose of powder to the inhalation channel  24 . In this inhaler the dosing unit  16  is rotated by rotating the grip portion  8  in the clockwise sense when viewed from below between first and second angularly-spaced positions. For this purpose, the dosing unit  16  includes wedge-shaped elements  60  disposed around the periphery of the member  17  thereof and the grip portion  8  comprises a resilient member (not illustrated) which is configured to engage with an axially-directed surface  60   a  of a respective one of the wedge-shaped elements  60  so as to rotate the dosing unit  16  by pushing the respective wedge-shaped element  60 . On rotation of the grip portion  8  in the opposite, counter-clockwise sense between the second and the first angularly-spaced positions, the dosing unit  16  remains stationary and the resilient member is located behind the axially-directed surface  60   a  of the adjacent wedge-shaped element  60 ; the resilient member riding over an inclined surface  60   b  of the adjacent wedge-shaped element  60 . 
     SUMMARY OF THE INVENTION 
     Whilst this powder inhaler functions perfectly adequately, it is an aim of the present invention further to improve the transfer of powder through the air chamber. 
     Accordingly, the present invention provides a powder inhaler for administering powder by inhalation, comprising a chamber which includes a base member and a peripheral wall member, wherein the chamber includes a first inlet through which a first stream of air containing powder is in use drawn on inhalation by a user and a plurality of second inlets at the junction between the base member and the peripheral wall member through which a plurality of second streams of air of combined lesser flow than the first stream of air are in use drawn on inhalation by the user so as to minimise retention of powder at the junction between the base member and the peripheral wall member. 
     In this way, the supplementary streams of air drawn through the second inlets pass over an inner peripheral surface of the chamber, preferably along the peripheral wall member towards the outlet thereof. These supplementary streams of air ensure that there are no dead spots or stagnant regions in the chamber. As a result, the likelihood of any retention of powder in the chamber is greatly reduced. 
     Preferably, the chamber includes a third inlet through which a third stream of air is in use drawn into the chamber on inhalation by the user. 
     More preferably, the third inlet is in the base member of the chamber. 
     Preferably, the first and third inlets are disposed to one side of the chamber. 
     In this way, the streams of air drawn through the first and third inlets interact particularly effectively and good flow characteristics are achieved in the chamber. 
     Preferably, the base member and the peripheral wall member are separate components which are attached to one another and the second inlets comprise channels in one or both of the base member and the peripheral wall member. 
     Providing supplementary streams of air by means of such channels is particularly advantageous since it is only necessary to make minor alterations to the shape of the component parts. Further, additional dedicated openings are unnecessary. In addition, such channels necessarily provide streams of air at the junction between the base member and the peripheral wall member of the chamber where stagnant regions are most likely to occur. 
     More preferably, the channels comprise recesses in an outer peripheral edge of the base member. 
     Preferably, the plurality of second inlets are disposed in a peripheral region of the chamber remote from the first inlet. 
     In a preferred embodiment the first inlet is disposed to one side of the chamber and the plurality of second inlets are disposed to the generally opposite side of the chamber. 
     In this way, the regions of the chamber, in which the stream of air drawn through the first inlet is not likely to pass directly, are provided with streams of air drawn through the second inlets. 
     Preferably, the inhaler further comprises a powder dislodging member disposed within the chamber for dislodging powder from an inner surface thereof and wherein at least one of the plurality of second inlets is disposed adjacent the powder dislodging member so as to minimise retention of powder thereon. 
     As stagnant regions are likely to develop near the powder dislodging member, these stagnant regions can be prevented and retention reduced by providing at least one of the plurality of second inlets near the powder dislodging member. 
     More preferably, the inhaler further comprises a cover plate which covers the base member and includes at an outer peripheral edge thereof at least one channel which generally positionally corresponds to the plurality of second inlets. 
     In a preferred embodiment the cover plate includes the powder dislodging member. 
     As should be understood, the function of the second inlets is not for the mixing or deagglomeration of powder, but merely to prevent stagnant regions or dead spots developing in the chamber. As such, the second inlets need only allow a relatively small amount of air through into the chamber. 
     Preferably, the plurality of second inlets are configured such that from 10 to 40 percent of the air drawn into the chamber is drawn therethrough. 
     More preferably, the plurality of second inlets are configured such that from 20 to 35 percent of the air drawn into the chamber is drawn therethrough. 
     Still more preferably, the plurality of second inlets are configured such that from 25 to 30 percent of the air drawn into the chamber is drawn therethrough. 
     Preferably, the inhaler further comprises a housing, one end of which provides the base member, which includes at least one groove which is in fluid communication with the plurality of second inlets. 
     Preferably, the at least one groove extends circumferentially. 
     Preferably, the housing includes at least one flute which is in fluid communication with the at least one groove. 
     More preferably, the at least one flute is disposed in an outer surface of the housing. 
     In a preferred embodiment the housing includes a plurality of grooves and a plurality of flutes which are in fluid communication with respective ones of the plurality of second inlets. 
     With this configuration, air is easily and efficiently drawn from the atmosphere into the chamber. 
     Preferably, the peripheral wall member includes at least one inwardly-extending flange which is clipped over the peripheral ridge of the base member. 
     Preferably, the base member is circular, the peripheral wall member is rotatable around the base member and the peripheral wall member includes a plurality of inwardly-extending flanges which are spaced and dimensioned such that at least three-quarters of the cross-sectional area of the second inlets is open for any relative position of the base member and the peripheral wall member. 
     In this way, the peripheral wall member, which may form part of the mouthpiece, can be freely rotated without interfering with the function of the second inlets. At the same time, the flanges hold the peripheral wall member securely in place. 
     Medicaments suitable for administration by the powder inhaler of the present invention are any which may be delivered by inhalation and include, for example, β2-adrenoreceptor agonists, for example, salbutamol, terbutaline, rimiterol, fenoterol, reproterol, adrenaline, pirbuterol, isoprenaline, orciprenaline, bitolterol, salmeterol, formoterol, clenbuterol, procaterol, broxaterol, picumeterol, TA-2005, mabuterol and the like, and their pharmacologically acceptable esters and salts; anticholinergic bronchodilators, for example, ipratropium bromide and the like; glucocorticosteroids, for example, beclomethasone, fluticasone, budesonide, tipredane, dexamethasone, betamethasone, fluocinolone, triamcinolone acetonide, mometasone and the like, and their pharmacologically acceptable esters and salts; antiallergic medicaments, for example, sodium cromoglycate and nedocromil sodium; expectorants; mucolytics; antihistamines; cyclooxygenase inhibitors; leukotriene synthesis inhibitors; leukotriene antagonists; phospholipase-A2 (PLA2) inhibitors; platelet aggregating factor (PAF) antagonists and prophylactics of asthma; antiarrhythmic medicaments; tranquilisers; cardiac glycosides; hormones; antihypertensive medicaments; antidiabetic medicaments; antiparasitic medicaments; anticancer medicaments; sedatives; analgesic medicaments; antibiotics; antirheumatic medicaments; immunotherapies; antifungal medicaments; antihypotension medicaments; vaccines; antiviral medicaments; proteins; polypeptides and peptides, for example, peptide hormones and growth factors; polypeptide vaccines; enzymes; endorphines; lipoproteins and polypeptides involved in the blood coagulation cascade; vitamins; and others, for example, cell surface receptor blockers, antioxidants, free radical scavengers and organic salts of N,N′-diacetylcystine. 
     Preferred embodiments of the present invention will now be described hereinbelow by way of example only with reference to the accompanying drawings. 
    
    
     DESCRIPTION OF THE DRAWINGS 
     FIG. 1 illustrates a perspective view of a known powder inhaler, illustrated with the cap separated; 
     FIG. 2 illustrates an exploded perspective view of component parts of the inhaler of FIG. 1; 
     FIG. 3 illustrates the underside of the mouthpiece of the inhaler of FIG. 1; 
     FIG. 4 illustrates a part-sectional side view of the inhaler of FIG. 1; 
     FIG. 5 illustrates an exploded perspective view of a powder inhaler in accordance with a first embodiment of the present invention, illustrated with the cap separated; 
     FIG. 6 illustrates an exploded perspective view of the component parts of the mouthpiece of the inhaler of FIG. 5; 
     FIG. 7 illustrates a perspective view of the body part and the cover plate of the inhaler of FIG. 5; 
     FIG. 8 illustrates a fragmentary vertical sectional view (along section I—I in FIG. 7) of the inhaler of FIG. 5; 
     FIG. 9 illustrates a fragmentary vertical sectional view (along section I—I in FIG. 7) of the inhaler of FIG. 5 with a modified mouthpiece; 
     FIG. 10 illustrates an exploded perspective view of the body part and the cover plate of an inhaler in accordance with a second embodiment of the present invention; and 
     FIG. 11 illustrates a perspective view of the body part and the cover plate of the inhaler of FIG.  10 . 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     Structurally, the powder inhalers in accordance with the preferred embodiments of the present invention have many features in common with the known powder inhaler described hereinabove in relation to FIGS. 1 to  4 . For this reason, and in order to avoid unnecessary duplication of description, only the structural differences will be described in detail and reference is made to the preceding description of the known powder inhaler; like reference signs designating like parts. 
     FIGS. 5 to  7  illustrate a powder inhaler or components thereof in accordance with a first embodiment of the present invention. This embodiment is a modification of the above-described known powder inhaler and differs from the above-described known powder inhaler in the construction of the mouthpiece  3 , the inhaler body  6  and the divider  12  and in further comprising a cover plate  61  which is disposed over the divider  12 . 
     In this embodiment the inhaler unit  1  comprises a body part  62  which is a single component of an opaque material that comprises both the inhaler body  6  and the divider  12 . 
     As in the above-described known powder inhaler, the inhaler body  6  includes the opening through which the indicating wheel  42  is visible, with the storage unit  26  being modified in comprising a tongue and being formed of a transparent material, one part, in this embodiment the distal end, of which tongue is shaped and dimensioned such that when the storage unit  26  is fitted in the inhaler body  6  that part of the tongue fills the opening  10  so as to provide the window  11 . Further, as in the above-described known powder inhaler, the indicating wheel  42  is rotatably supported to the underside of the divider  12  such that at least a part of the periphery of the indicating wheel  42  is visible through the window  11 . 
     The inhaler body  6  of this embodiment differs from that of the above-described known powder inhaler in including first and second flutes  63 ,  64  at the upper edge and outer surface thereof which are normally in fluid communication with the atmosphere. 
     As in the above-described known powder inhaler, the divider  12  includes a peripheral ridge  14  by which the mouthpiece  3  is attached to the body part  62 . The divider  12  of this embodiment differs from that of the above-described known powder inhaler in including first and second peripherally-extending grooves  13   a ,  13   b  which together extend around a major part of the periphery of the divider  12  adjacent the peripheral ridge  14  and are in fluid communication with the first and second flutes  63 ,  64  in the inhaler body  6  respectively. The divider  12  of this embodiment further differs from that of the above-described known powder inhaler in that the peripheral ridge  14  includes first to fourth channels  66 ,  67 ,  68 ,  69 , in this embodiment in the form of concave recesses at the outer peripheral edge of the peripheral ridge  14 , with the first and second channels  66 ,  67  being in fluid communication with the first groove  13   a  and the third and fourth channels  68 ,  69  being in fluid communication with the second groove  13   b . The channels  66 ,  67 ,  68 ,  69  in the peripheral ridge  14  provide air flow paths through which controlled amounts of air are, on inhalation by the user, drawn from the atmosphere into the air chamber  58  at the periphery thereof. In this embodiment the channels  66 ,  67 ,  68 ,  69  in the peripheral ridge  14  are disposed generally diametrically opposite the outlet to the inhalation channel  24  which is that region of the air chamber  58  in which stagnant volumes are most likely to exist. The divider  12  of this embodiment further differs from that of the above-described known powder inhaler in including a supplementary air inlet  71  which is in fluid communication with the atmosphere, in a preferred embodiment through an opening in the peripheral wall of the inhaler body  6 . 
     The cover plate  61  comprises a substantially planar member  72  which covers the relatively complex structural features at the upper surface of the divider  12 , a radially outwardly-biased catch element  74  which is connected to the planar member  72  by a resilient arm  76  and radially movably disposed in a peripheral recess  77  and a powder dislodging member  78 , in this embodiment a scraper, which is configured to contact a part of the lower surface of the flange  56  which defines the upper surface of the air chamber  58 . In this embodiment the cover plate  61  is of a smaller radial dimension than the peripheral edge of the peripheral ridge  14  of the divider  12 . The cover plate  61  further includes first and second openings  80 ,  82  which correspond in position respectively to the outlet of the inhalation channel  24  and the supplementary air inlet  71  and first to fourth channels  84 ,  85 ,  86 ,  87  which are disposed at the outer peripheral edge thereof and correspond in position to the first to fourth channels  66 ,  67 ,  68 ,  69  in the peripheral ridge  14  of the divider  12 . By providing channels  84 ,  85 ,  86 ,  87  in the cover plate  61 , air flow paths are ensured between the channels  66 ,  67 ,  68 ,  69  in the peripheral ridge  14  of the divider  12  and the air chamber  58 , which flow paths could be blocked if, for example, the manufacturing tolerances of the mouthpiece  3  were such that a peripherally-extending part of the mouthpiece  3  were to abut the peripheral edge of the upper surface of the cover plate  61 . In this embodiment the first to fourth channels  84 ,  85 ,  86 ,  87  in the cover plate  61  are disposed upstream, in a rotational sense, of the powder dislodging member  78  such as to provide an air flow at a peripheral region of the air chamber  58  behind the powder dislodging member  78 , which peripheral region is that region where powder dislodged by the powder dislodging member  78  would preferentially accumulate. In this embodiment one, the third, channel  86  in the cover plate  61  is provided by the recess  77  in which the catch element  74  is movably disposed and the other, the first, second and fourth, channels  84 ,  86 ,  87  comprise concave recesses at the outer peripheral edge of the cover plate  61 . With this configuration, the channels  84 ,  85 ,  86 ,  87  in the cover plate  61  act to communicate air drawn through the channels  66 ,  67 ,  68 ,  69  in the peripheral ridge  14  of the divider  12  to the periphery of the air chamber  58 . In this embodiment the effective total length of the first to fourth channels  84 ,  85 ,  86 ,  87  in the cover plate  61  is about one-quarter of the length of the peripheral edge of the cover plate  61 . 
     In this embodiment, similarly to the above-described known powder inhaler, the mouthpiece  3  comprises first and second parts  48 ,  50 . The mouthpiece  3  of this embodiment differs from that of the above-described known powder inhaler in that the mouthpiece  3  is rotatable relative to the body part  62  and in that the first part  48  further includes a plurality of radially inwardly-directed projections  90  and a plurality of radially outwardly-directed projections  92 . Each of the inwardly-directed projections  90  comprises a first flank  90   a  which faces in one sense, in this embodiment in the counter-clockwise sense when viewed from above, and a second flank  90   b  which faces in the opposite, clockwise sense. The inwardly-directed projections  90  act by engagement with the catch element  74  on the cover plate  61  to prevent the mouthpiece  3  from being freely rotated relative to the inhaler body  6 , which free movement would be undesirable when a user was trying to grip the mouthpiece  3  in the lips. In order to rotate the mouthpiece  3  a sufficient rotational moment has to be applied thereto such as to cause the catch element  74  to ride over the inwardly-directed projections  90  on the mouthpiece  3 . In use, on rotating the mouthpiece  3  relative to the body part  62 , the lower surface of the flange  56  of the second part  50  of the mouthpiece  3  is rotated relative to the powder dislodging member  78 , thereby causing any powder which may have accumulated on that part of the lower surface of the flange  56  immediately upstream, in a rotational sense, of the powder dislodging member  78  to be removed. The mouthpiece  3  of this embodiment further differs from that of the above-described known powder inhaler in that the first part  48  thereof further comprises a plurality of radially inwardly-directed flanges  96  disposed about the lower edge thereof by which the mouthpiece  3  is clipped to the divider  12 ; the inwardly-directed flanges  96  being clipped beneath the lower surface of the peripheral ridge  14  of the divider  12 . In this embodiment, as illustrated in FIG. 8, where the flanges  96  have a radial width such as to extend to the inner radial edge of the channels  66 ,  67 ,  68 ,  69  in the peripheral ridge  14  of the divider  12 , the flanges  96  are peripherally spaced such that at least three of the channels  66 ,  67 ,  68 ,  69  in the peripheral ridge  14  of the divider  12  are in fluid communication with a respective one of the first and second grooves  13   a ,  13   b  for any relative position of the mouthpiece  3  and the body part  62 . In a modified mouthpiece  3 , as illustrated in FIG. 9, the flanges  96  can be configured so as to have a smaller radial width such as to extend only partially over the channels  66 ,  67 ,  68 ,  69  in the peripheral ridge  14  of the divider  12 . In this modified mouthpiece  3  the flanges  96  need not have any particular peripheral spacing and indeed the mouthpiece  3  could include a single, continuous flange. 
     The cap  2  of this embodiment differs from the cap  2  of the above-described known powder inhaler in being configured to rotate the mouthpiece  2  on removal. In this embodiment the cap  2  includes a plurality of resilient members  98  disposed about the inner periphery thereof which engage with the outwardly-directed projections  92  on the first part  48  of the mouthpiece  3 . The resilient members  98  extend axially and enclose an acute angle with the inner periphery of the cap  2 ; the distal ends of the resilient members  98  being directed in the counter-clockwise sense when viewed from above. By having an axial length the resilient members  98  engage the outwardly-directed projections  92  on the first part  48  of the mouthpiece  3  for the entire period that the cap  2  is being removed, in this embodiment by unscrewing in the counter-clockwise sense when viewed from above. In use, when the cap  2  is removed, one or more of the resilient members  98  engage respective outwardly-directed projections  92  on the first part  48  of the mouthpiece  3  and cause the mouthpiece  3  to be rotated relative to the body part  62 , thereby causing the powder dislodging member  78  to remove any powder which may have accumulated on that part of the lower surface of the flange  56  of the second part  50  of the mouthpiece  3  upstream, in a rotational sense, of the powder dislodging member  78 . In this way, whenever a user removes the cap  2  the mouthpiece  3  is automatically rotated. In fitting the cap  2 , in this embodiment by screwing in the opposite, clockwise sense, the resilient members  98  are deflected and ride over the respective outwardly-directed projections  92  on the first part  48  of the mouthpiece  3 . 
     FIGS. 9 and 10 illustrate component parts of a powder inhaler in accordance with a second embodiment of the present invention. This embodiment is a modification of the inhaler of the above-described first embodiment and differs from the first embodiment in the construction of the divider  12  and the cover plate  61 . The divider  12  of this embodiment differs from the divider  12  of the first embodiment in including a single peripheral groove  13  adjacent the peripheral ridge  14  thereof and in that the peripheral ridge  14  includes first to fifth channels  66 ,  67 ,  68 ,  69 ,  100 . As in the first embodiment, the channels  66 ,  67 ,  68 ,  69 ,  100  in the peripheral ridge  14  comprise concave recesses at the outer peripheral edge of the peripheral ridge  14 . The cover plate  61  of this embodiment differs from the cover plate  61  of the first embodiment in including first to third channels  84 ,  85 ,  86 . As in the first embodiment, in this embodiment one, the second, channel  85  in the cover plate  61  is provided by the recess  77  in which the catch element  74  is movably disposed. The cover plate  61  of this embodiment further differs from the cover plate  61  of the first embodiment in that the first and third channels  84 ,  86  comprise elongate recesses which extend along the peripheral edge of the cover plate  61 . In this embodiment the first, second and third channels  84 ,  85 ,  86  in the cover plate  61  are disposed adjacent one another and the effective total length of the first, second and third channels  84 ,  85 ,  86  is about one-third of the length of the peripheral edge cover plate  61 . 
     Finally, it will be understood that the present invention has been described in its preferred embodiments and can be modified in many different ways without departing from the scope of the appended claims.