in fig1 of the drawings is shown a base 1 upon which is seated a bellows 2 of the wedge - type . overlying the bellows , and mounted on the base at 3 & amp ; 4 for pivotal movement about axis 5 , which lies adjacent to the apex of the bellows , is a rectangular plate 6 formed with large rectangular aperture 7 . an arm 8 mounted upon plate 6 carries a stylus 9 for marking a trace upon a chart 10 carried by a chart - carrier 11 ( fig2 ). chart - carrier 11 is mounted for movement perpendicular to the plane of the paper as described in our u . s . pat . no . 3 , 363 , 260 . to the side 12 of the rectangular aperture , adjacent to arm 8 is hinged a rectangular platen 13 . upon platen 13 is mounted an arm 14 terminating in a roller 15 which engages a track 16 constituted by the ends of a set of elements 17 carried by a bracket 18 . as will be seen from fig2 the said ends of elements 17 are sloped at progressively changing angles . when the bellows is inflated via air inlet 19 , stylus 9 is moved for marking chart 10 , and roller 15 moves along track 16 . to adjust the apparatus as a final step in its manufacture , the bellows is inflated stepwise , by introducing a series of equal portions of air from a metering device such as a syringe . this causes the stylus to move clockwise , as seen in fig2 and also causes roller 15 to traverse the track 16 . the position of the stylus after the introduction of each portion of air is adjusted by moving axially that of the elements 17 which is in contact with the roller . in this way , the pressure of air within the bellows 2 is varied by varying the reaction between platen 13 and the bellows 2 . the adjustment of the position of the stylus is dependent only upon the adjustment of that of the elements 17 involved for the particular position of roller 15 . with previously known arrangements , an adjustment has tended to interfere with adjustments previously made for previous positions of the stylus and satisfactory overall adjustment has been a time - consuming operation involving acquired skill . the track assembly of fig3 and 4 is part of a spirometer which is otherwise similar to that shown in fig1 and 2 of our said prior specification . instead of having a track 16 constituted by the ends of elements 17 as described therein , the spirometer has a track for the roller 15 presented by the concave face of a strip 16 &# 39 ; of resilient synthetic resinous material curved as shown to approximate to the locus of roller 15 . strip 16 &# 39 ; is carried at its ends by wire hangers 20 which engage notches formed in the strip at 21 and have their ends 22 inturned and engaged within recesses 23 of curved mounting bar 24 . the length of the strip between the notches is greater than the distance between the ends 22 of the hangers by such an amount as to provide the curvature . bar 24 is formed at equidistant positions along its length with tapped holes within which are threadably engaged stops 25 . each stop has a knurled head 26 for adjustment purposes and is formed with a smoothly convex end 27 for bearing upon face 28 of the strip 16 &# 39 ;. to adjust the spirometer as a final stop in its manufacture , the bellows is inflated stepwise as previously described and the stops 25 are adjusted in their turn . with the spirometer in use , the strip 16 &# 39 ; is pressed into contact with the ends 27 of stops . it is not necessary that all of the ends 27 should engage the strip simultaneously . the roller 15 may , when between a pair of stops , press the strip into contact only with that pair . in such a case the remainder of the strip , ie . the part or parts beyond the pair of stops may be spaced away from the remaining stops . alternatively stated , the length of the strip between the notches need not be identical with the length of the track traversed by the roller . as the roller 15 passes along the track it bears against the strip 16 &# 39 ; which effectively bridges the spaces between the stops 25 . the essential discontinuity of a finite number of the stops is substantially concealed by the bridging action of the strip 16 &# 39 ;. it will be noted that whereas the embodiment of fig1 and 2 has eight of the elements 17 ( as shown in fig2 ) an improved result is obtained with only six stop 25 . in the example shown in fig3 and 4 , the strip 16 &# 39 ; is formed by cutting from a sheet of low density polyethylene . it has a thickness of 0 . 7 mm , a width of 7 mm and a length of 100 mm . the part of the third embodiment shown in fig5 differs from the part of the first embodiment shown in fig1 in that instead of the apertured rectangular plate 6 and the platen 13 there are provided a frame 6a pivotally mounted at 3 and 4 and an enlarged rectangular platen 13a which extends over the whole of the adjacent face of the bellows 2 . brackets 29 and 30 secured to platen 13a are connected to frame 6a for pivotal movement about an axis 12 &# 39 ;. arm 8 is formed as part of frame 6a . in fig5 platen 13a is shown in its extreme position in contact with frame members 31 and 32 . resistance to the movement of roller 15 produced by the track , causes the platen to pivot out of contact with members 31 and 32 . roller 15 is arranged to run in engagement with a track in the manner shown in fig2 and 3 . a track constructed as described with reference to fig3 and 4 is preferred . it is found that , compared with the arrangement of fig1 the arrangement of fig5 provides a more stable assembly . additionally , the dynamic characteristics of the apparatus are improved by the reduction of oscillatory movement of the stylus 9 . it is found that , compared with the arrangement of fig1 the arrangement of fig5 provides a more stable assembly . additionally , the dynamic characteristics of the apparatus are improved by the reduction of oscillatory movement of the stylus 9 . it will be understood that the form of construction described by reference to the drawing is given by way of example only and that various departures may be made therefrom within the scope of the invention claimed .
a spirometer having an inflatable bellows 2 , is provided with a movable member 15 which runs along a track 16 . by adjusting parts 17 , the configuration of the track is set locally to give a variable resistance to movement of member 15 and , consequently , inflation of the bellows 2 . calibration is achieved in a simple manner by adjusting parts 17 as the bellows is inflated progressively with measured amounts of air . adjustments made correctly are not upset by adjustments made later in the calibration operation .