US2622177A

US2622177A - Angle-swept potentiometer

Info

Publication number: US2622177A
Application number: US226890A
Authority: US
Inventors: Alfred J Klose
Original assignee: G M Giannini & Co Inc
Current assignee: G M Giannini & Co Inc
Priority date: 1951-05-17
Filing date: 1951-05-17
Publication date: 1952-12-16
Anticipated expiration: 1969-12-16

Description

DCC. 16, A- J KLOSE ANGLE-SWEPT POTENTIOMETER 2 SHEETS-SHEET 1 Filed May 17, 1951 LFQD M0553 INVENToR.

TTOQ/veys.

A. 1.l KLosE ANGLE-SWEPT POTENTIOMETER Dec. 1 6, 1952 2 SHEETSSHEET 2 Filed May 17, 1951 IN VEN TOR.

Trae/VEYS.

Patented Dec. 16, 1952 AGLE-SWEPT POTENTIOMETER Alfred J. Klose, Rolling Hills, Calif., assignor to G. M. Giannini & Co. Inc., Pasadena, Calif., a

corporation of New York Application May 17, 1951, Serial No. 226,890

3 claims. 1

This invention has to do with means for 1 developing an electrical voltage or current that corresponds in a definite manner to the mechanical movementl of an element, such, for example, as the sensitive element of a measuring instrument.v Such an electrical representation of a mechanical movement may be utilized typically for controlling a circuit in accordance with the movement, and for indicating at a distancaas by anielectrical instrument, the instant position of the movable element.

Previously available devices for translating a mechanical movement into a corresponding elecf trical variable commonly utilize a resistance coil and abrush that is movable along the coil in response to the mechanical movement to be represented. Such devices are of limited usefulness, particularly in connection with .instruments in which the sensitive element has relatively short travel.` Not only is it diiiicult in many instances to obtain a sufficient range of effective electrical resistance with the brush movement available, but it is often impossible to provide enough coil turns in the resistance coil to give the desired smoothness of response. Mechanical amplification of the available travel, as by a linkage, has the serious disadvantage of correspondingly amplifying the load imposed upon the movable element.

An important object of the present invention is to overcome such diiculties, and .thereby to provide means for developing a voltage or current that varies substantially smoothly in definite correspondence with linear movementof an element, even though the travel of that 'element be very small.' `Moreover, the invention accomplishes that result without imposing upon the moving element 'any appreciable additional load.

."Ihat Yis accomplished typically in accordance withthe invention by mounting a resistance coil and its cooperating brush in such a way that .the relativemovement of coil and brush, in response tornovement of the movable element to be represented, is not parallel to the axis of the coil, but is oblique thereto. That is made possible by providing a novel type of brush, which has an elongated contact surface.

The invention further provides, in its preferred form, a coil structure that includes a longitudinal contact edge that is relatively sharp in transverse section, the wire of each coil turn making a relatively sharp bend as it passes the contact edge of the coil.

` A further object of the inventionv is the pro-j vision of a special mountingfor` the coil 'and brush which maintains the necessary spatial relationship between them while permitting relative adjustments of special types.

A full understanding of the invention and of its further objects and advantages will be had from the following description of certain embodiments of it in instruments of illustrative types. That description and the accompanying drawings, which form a part of it, are intended only as illustrations, and not as limitations upon the scope of the invention.

In the drawings:

Fig. 1 is a section of an illustrative embodiment of the invention, taken on line I-I of Fig. 3;

Fig. 2 is a section on line 2 2 of Fig. 1;

Fig. 3 is a section on line 3-3 of Fig. 1;

Fig. 4 is a section on line 4-4 of Fig. 1;

Fig. 5 is a fragmentary section on line 5-5 of Fig. 1, shown at enlarged scale;

Fig. 6 is a longitudinal section of another illustrative embodiment of the invention, taken on line 6-6 of Fig. 8;

Fig. 7 is a vtransverse section on line 'l-l of Fig. 6;

Fig. 8 is a transverse section on line 8--8 of Fig. 6;

Fig. 9 is a side elevation of another illustrative embodiment of the invention; and

Fig. 10 is a plan corresponding to Fig. 9.

Figs. 1-5 represent an illustrative embodiment of the invention in an instrument for developing a voltage corresponding to the difference between two uid pressures. The instrument comprises a pressure sensitive element, shown as a metal bellows 20, the movement of which in response to changes of fluid pressure differential is caused to develop a corresponding voltage by movement of the brush 22 over potentiometer coil 50. The

instrument is housed in a case which comprises typically three principal sections. The generally cylindrical case body 30 is closed at one end by the flexible diaphragm 32 and cover plate`34 and at the other end by the bellows housing 36. All moving parts of the instrument are supported as a movement assembly 40 on the circular mounting plate 42, apertured at 44, which is rigidly clamped, as by the screws'4l, between case body 30 and bellows housing 36. The junctions between those two case members and between each of them and mounting plate 42A are sealed by a single 0ring 38, while diaphragm' 32 effectivelyseals cover plate 34 to the case bodyi K Upon removal of screws 4| and bellows housing 36, the entire movement assembly 40 is immediately bodily removable from the case,

curvature.

3 rendering the parts to be described readily accessible, as for adjustment.

The inner end of bellows 28 is open, and is edge mounted upon the outer face of mounting plate 42, to which it is soldered or otherwise secured in fluid-tight fashion, surrounding plate aperture 44. The outer end of the bellows, which is free, is closed in the usual manner by a cap 46 which carries on its inner face a rigid post 48 extending internally along the axis 2| of the bellows to a point inward of plate 42. The inner end of post 48 carries brush 22, which thus moves, relatively to plate 42, along the axisv2| of bellowsv 2n as the latter expands or contracts.

The contact edge 24 of brush 22 comprises an elongated cylindrical surface of relatively sharp That surface is tangent to a plane 23 that is parallel to the brush movement, that is, parallel to bellows axis 2|. Plane 23, which will be referred to as the contact plane, preferably passes approximately through axis 2|, and

may be visualized as being normal to the planesy of Figs. 2, 4 and 5 at the lines 23. The line in which the relatively sharp contact edge of the brush is tangent to contact plane 23 is preferably with respect to movement parallel to contact plane 23, but to be capable of yielding movement normal to that plane. A preferred manner of construction by which that may be attained is shown Illustratively in the figures and comprises a mounting bar 25, rigidly mounted transversely of post 48 and insulated therefrom as by a sheet 28 of dielectric material, and a leaf spring 26, having one edge rigidly mounted longitudinally of bar 25 and extending therefrom in a flat curve generally parallel to contact plane 23. The free edge of spring 26 is stiffened as by a flange 21, which may be, and preferably is, integral with the spring. The brush contact surface 24 is parallel to flange 21, and may comprise a fine wire 24a of a suitable noble metal secured rigidly to the flange at its junction with the spring body. A wire somewhat longer than the contactsurface to be formed is preferably stretched taut between suitable jaws and brought into parallel relation adjacent the elbow of the spring flange, and solder is then flowed between the wire and the spring and allowed to harden as at 29, flxedly mounting the wire as a strictly rectilinear contact edge of smoothly and uniformly rounded section. The radius of curvature of that contact surface is readily controllable by selection of the wire diameter, which may typically be five to twelve thousandths of an inch. The brush may be electrically connected, as by a light and flexible wire lead 64 to a brush terminal 65 which extends through and is insulated from the wall of case body 3U.

The pressure to be represented is admitted to the chamber 31, surrounding bellows 20 and between mounting plate 42 and bellows housing 36, through a suitable port in that housing, shown typically at 31a. with threads for connection of a pressure line. A reference pressure may be similarly admitted through port 3 la to the chamber 3| between cover plate 34 and flexible diaphragm 32. The entire space 33 between diaphragm 32 and mounting plate 42, including also the interior of bellows 20 is preferably filled with ai chemically inert, substantially incompressible fluid,kv as through a port provided forthat 4pur- 4 pose in the wall of case body 38. Such a port is shown in Fig. l sealed by the screw plug 35. The reference pressure in chamber 3| is then transmitted to the interior of bellows 20, and the position of brush 22 longitudinally of bellows axis as is determined by the differential between the pressure in chamber 31 and that reference.

Potentiometer coil 58. as shown, is of flat section and is' longitudinally tapered, as shown best in Fig. 4. The coil has a longitudinal contact edge 52 which is preferably accurately straight and sharp in a sense to be further explained. The coil is mounted on plate 42, on the opposite face of the plate from bellows 20. The illustrativeV mounting means shown includes a fixed bracket 54, which may be rigidly mounted on plate 42 as by soldering. Bracket 54 includes a plate-like body portion 55 which lies in a plane normal to mounting plate 42 and parallel to the working edge of brush 22, and which is provided with a track parallel to the movement axis-2| of bellows 20. That track is represented in the present embodiment by the slot 56 in the plate-like body of the bracket. A flat slide member 60 is supported on, and in parallel relation to, bracket 54 for adjustable movement in translation in its own plane longitudinally of slide` 56. As illustrated, two threaded studs 6| are fixed in slide member 60 and are' slidingly received by slot 56, the nuts G2 being turned down on the.

studs to clamp bracket 54 in adjusted position between one face of l slide 60 and a clamping washer 63.

A swinging bracket 10 has a fiat base portion;

1| and a plate-like armportion 12 `forming la right angle with the base. Bracket base 1| lies flatly against the opposite face of slide member 60, and is adjustably swingable through an angle of about 30 in the plane of that face about an axis defined by pivot stud 13, fixed in bracket base 1| normal to movement axis 2|. A parallel lock ing stud 14 is received by a curved slot 61 in slide member 66. permitting convenient locking of the swinging movement of bracket 18 by tightening of the nuts 15 and 16 on the respective studsv 13 and 14. Arm portion 12 ofbracket 10 carries a track, shown as two parallel slots 18,

parallel to the pivot axis about which the bracket is adjustable.

A mounting block is mounted on bracket 10 in a'manner to be adjustable in translation along track 18 and also in rotation throughr a small angle about an axis normal to the track. Such dual adjustability of block 80 may be guided and clamped, for example, by the two screwsV 82,

threaded into a flat face of the block and received.

with adequate clearance in the respective slots 18. To facilitate relative adjustment of the block and bracket when screws 82 are loose, adjusting screws 84 may be provided, screw-threaded into ears 11 of bracket 18 parallel to track 18 and freely received in bores in block 88. Springs 85 hold the block against the heads of Vscrews 84.

Resistance coil 50 is fixedly-mounted, as by thel screws 81, against the face of block 88 op? aesaair'zv 23, opposing the described contact edge 24 of brush 22, with which it .makes electrical contact. The exact relative positions of the coil and brush are so adjusted by the screws 84, already described, that contact of the surfaces slightly deforms leaf spring 26, producing a light yielding contact that is substantially uniform regardless of the longitudinal position on the coil of the point of contact. The opposite ends of resistance coil 50 are electrically connected, as by flexible leads 66 and 61, to the two coil terminals 68 and 69, set in case body 30 and extending through its wall in insulated relation.

The rotational position of coil 50 and bracket about pivot stud 13 is adjusted to give the desired angle b between the length of coil 50 and brush 22, as seen, for example, in Fig. l. The approximate value of that angle, for the present illustrative embodiment in which brush 22 is approximately normal to the direction of brush movement, is such that the sine of angle b equals the ratio of the maximum travel o-f brush 33 to the usable length of coil 50. In accordance with the invention, 'that ratio is appreciably less than unity, and may be as small as 0.2 or less. In other words, although the total travel of the movable element (here represented by the free end of bellows may be relatively small, the usable length of resistance coil 50 over which brush contact edge 24 moves may be relatively great. That is in marked contrast to previous devices, in which the range of movement of the potentio-meter brush along the coil is no greater than the available travel of the movable element to be portrayed For example, if the total brush travel between the highest and the lowest anticipated pressures is 0.02", that distance is also the maximum usable length of the resistance coil in previously available instruments of the type here described. If the resistance coil is wound with wire of 0.001 diameter, no more than 20 turns can be provided within the usable length of such a coil, since the `average lead of the coil .must be at least equal to the wire diameter. That means that only 20 steps are available by which to represent the smoothly varying pressure, leading necessarily to a relatively crude approximation. Tha-t cannot well be remedied merely by magnifying the mechanical movement by a linkage, since the force required of the instrument to move the brush over the coil is thereby magnied at least in the same ratio, leading in practice to uneven and unreliable response.

The diiculty is solved in the present embodiment of the invention by setting the resistance coil at a relatively small angle b with the elongated brush. If that angle is 11.5, for example, so that sin b is about 0.2, then the usable length of the coil is five times greater than the linear travel of the brush. Thus, with a brush travel of 0.02", the point of contact with the coil moves over a range of 0.10 on `the coil. That coil length may include typically about 100 turns, providing satisfactorily smooth electrical representation of the brush movement. Moreover, the force required of the bellows to move the brush is not increased by such oblique position of the coil. Y

The exact degree of effective amplification of the brush movement is very conveniently adjustable by Variation of angle b, which is accomplished by release of nuts 15, 16 and rotation of bracket 10 about the axis of stud 13. The coil may be adjusted bodily longitudinally of bellows 6. axis 2l by releasing nuts 62 and moving slide member 60 along track 56 in fixed bracket 54. By that adjustment the brush contact point may be brought to any desired zero position on the coil contact surface for a definite selected pressure differential. That rero point is preferably taken at the intersection of the axis of pivot stud 13 with the coil contact surface. The coil can then be rotated about that axis to adjust angle b without disturbing the zero adjustment.

A preferred manner of forming the contact surface of a resistance coil in accordance with the invention is shown best in Fig. 5. The coil form 5I is preferably of at section, as indica-ted, and made of insulative material. One longitudinal edge of the coil form is tapered as indicated at 53. That taper may be carried to a relatively sharp edge, and the wire of the coil may be wound directly over that edge. However, in the preferred embodiment illustrated, the taper leaves a thin flat edge, which is slotted to receive a relatively thin insert 51. The exposed edge of insert y51 is smoothly rounded in section, and is accurately straight in a direction longitudinal of the coil. Such an insert is preferably of a relatively hard dielectric material that can be accurately worked. Glass has been found to be a suitable material. The wire with which the coil is wound passes over insert 51. The thickness of the insert and the shape of its exposed longitudinal edge and also the tapered form 53 of the coil form proper are so chosen that the coil wire bends sharply as it passes over the edge of the insert. That bend is quite sharp, even in proportion to the relatively small diameter of the wire. For example, the radius of curvature of the wire at the point of maximum curvature is preferably only 2 to '7 times the diameter of the wire itself, which may typically be about one mil. That relatively sharp curvature of each wire turn at the contact surface of the coil, combined with the relatively sharp contact edge 24 of the brush, permits the angle b between coil and brush to be made quite small without causing such irregularities of contact as simultaneous contact of the brush with more than two turns, or with non-adjacent turns, of the coil.

Figs. 6-8 show another illustrative embodiment of the invention, comprising a feeler gage adapted for translating small movements of the feeler head into corresponding electrical Voltages. As illustrated, the instrument is mounted in a cage having a tubular body portion and forward and rear caps 9| and 92 closing the respective ends of the tube. A lever 04 is mounted longitudinally within the case body for pivotal movement aboutI a pivot axis 05 transverse of the case near `its forward end. In the preferred embodiment shown, that mounting comprises two pairs of crossed leaf springs 9S opposite ends of which are secured to block 91 and to cross arm 98, extending transversely from lever 94 parallel to pivot axis S5. That axis is a line that passes through all four springs. The swinging movement of lever 9d about axis S5 may be limited by abutment stops adjustably mounted with respect to case body 9S. Such abutments are indicated typically as screws 05 radially threaded in fixed brackets |01 and accessible for adjustment through access apertures 109 in the case body. Springs 90 establish an equilibrium, or zero, position of lever 94, which may be substantially midway between abutment stops 05, or may be relatively close to one of the stops. The lever is resiliently swingable from that equilibrium position in response to a relatively slight torque.

Lever 94 projects forwardly from pivot axis 95 through a clearance aperture 93 in forward cap 9|, and carries at its forward end a spherical feeler head 90. The lever extends rearwardly from pivot axis 95 along the cylindrical axis of the case and carries at its rearward end a brush |00. That brush may be constructed in a manner similar to that already described for brush 22 of the embodiment of Figs. l-5, but as shown is of a somewhat different type. A leaf spring |02 of Y form is mounted on lever 94 with its arms |03 extending generally parallel to each other on opposite sides of the longitudinal axis of lever 94 and in a plane approximately normal to that axis. A wire |04 extends between the ends of the two springarms, and is stiff enough in relation to the flexibility of the spring arms that it may be considered for all practical purposes to be a straight rigid rod. As illustrated, that rod, which forms the contact edge of brush |00, is parallel to pivot axis 95. Brush is preferably electrically insulated from lever 94 on which it is mounted, and is connected, as by a relatively fine and flexible insulated wire |06 to 'a pin of an electrical connector |00, which is mounted in the case wall, for example in the wall of rear cap 92.

A resistance coil ||0 is supported as by a bracket on the inner face of rear cap 92. The coil axis is normal to the longitudinal axis of the case and oblique with respect to pivot axis S and to brush |00, Coil l0, as shown, is of irregular section with a relatively sharp contact edge ||2 directed toward brush |00. The plane tangent to the brush and coil at their point of contact is normal to the longitudinal axis of case body 90 and is parallel to pivot axis 95. That contact plane is indicated by the line ||5 in Fig. 6. The illustrated brush structure is typical of a great variety of detailed structures, in that it produces relatively great flexibility of the brush in a direction normal to contact plane H5, and relatively great stiffness parallel to that plane. That results in a uniformly yielding contact with the coil, and yet maintains in full effect a definite and positive position of the brush relative to lever 04 in the direction of movement of that lever. Opposite ends of coil ||0 are electrically connected to pins of connector |08 as by wires, one of which is indicated at The angle between contact edge 2 oi' the coil and brush |00 may be definitely xed during assembly `of the device, or may be adjustable, for

example by rotation of cap 02 on case body 90, the adjusted position then preferably being locked by any suitable means such as a locking screwl I4 which is slidably received in a circumferentially slotted hole in the cap flange. Other adjustments of the type described in connection with Figs. l-5 may be provided, although for many purposes it is sudicient to rely upon accurate manufacture and assembly of the parts to give correct cooperation of brushy and coil. That is particularly true if spring arms |03 are relatively fiexible in a direction normal to contact plane I5. Such exibility of the brush structure` compensates, for example, for the fact that the movement of brush |00 is not strictly limited to translation, but includes a small component of rotation about pivot axis 05.

As illustrated, the length of theV forward arm of lever 94, from pivot axis 05 to the center of head 99 is only about one quarter that of the 8. other arm, measured from axis to contact plane H5. The ratio of those lengths may typically be 1:4. That has the effect of amplifying the resultant brush movement, responsive to a given displacement of head 09, by a factor of four. As already indicated, another effect of such leverage is to amplify the force required at head 09 to move brush |00 over coil ||0 in opposition to the surface friction. For some purposes, of which the present instrument is illustrative. the load on the sensitive element need not be held to an absolute minimum, and a corresponding increase of load due to friction can be tolerated. However, even under such conditions, that increase is usually limited, and the amplification factor that is tolerable for the friction may still be insuiiicient toV give the required range of brushmovement. Moreover, the degree of amplication that can be obtained by means of a lever system is limited in practice by backlash and friction at the pivot when a conventional pivot bearing is used. In the spring type of pivot of the present embodiment, friction may be virtually eliminated. However, unless the angle f of deflection is held within a limitv of about 10,

an appreciable force is required to overcome the spring stiffness, and difficulties may be encountered from hysteresis effects. `All of those difli-y culties, as well as the space requirements of a lever system, are most serious at relatively high lever ratios. At relatively low lever ratios the cited disadvantages may be limited to acceptable proportions.

In accordance with the present invention it is possible to obtain a relatively large effective amplification of the potentiometer brush movement without incurring the serious disadvantages of a lever system of corresponding ratio. That is done by combining a lever system of moderate ratio with suitable relative orientation of the brush and coil. By utilizing the amplifying effect of a small oblique angle between brush and coil, in combination with a lever system of moderate ratio, it becomes for vthe first time possible to obtain relatively great values of the overall amplification, while retaining high accuracy and sensitivity and relatively low loading of the sensitive element; It is found desirable to limit the leverage amplification to a factor between about two and about'six, while it is practicable to make the overall amplifica-tion of the combination from about two to about six times that of the lever system alone. As an illustration, anglefb as shown in Fig. 8, is approximately 19.5, giving an effective 3-fold amplification of the brush movement. The overall amplification, with the combination of leverage system and coil angle shown, is therefore` twelve. That is, movement of feeler lhead 9-9 through a distance of 0.001 normal to the longitudinal axis of the instrument and to pivot axis 95 displaces the electrical contact point of brush |00 along coil ||0 bya distance of 0.012. That coil. length may readily accommodate about 12 `coil turns, so that the instrument can be counted on to respond to a movement of the sensitive element through only 0.0001".

Another illustrative embodiment of the invention is shown in Figs. 9 and l0. A curved Bour-- don tube is represented generally by the numeral l30. The'tube is mounted midway of its length vupon a threaded fitting |3| bymeans of a short side tube |32 which provides a fluid pressure conmecticn between fitting` llll and `the interior of the Bourdon tube. AAn electrical resistance coil |40 is mounted bodily upon one of the free ends |34 of the Bourdon tube, and a cooperating brush |50 upon the other free end |35 of the tube. As the curvature of the Bourdon tube changes in response to changes of internal Iiuid pressure, altering the distance between its free ends, the brush is moved longitudinally of the coil.

An advantage of the type of instrument shown ,is in its inherent freedom from disturbance by vibration or by other types of acceleration to which it may be exposed. The two arms of the Bourdon tube are preferably at least roughly balanced dynamically. The counterweight |37 rigidly mounted on the brush end |35 of the tube represents illustrative means of substantially balancing the resonant periods of the two arms of the tube.

The particular mounting means illustrated in Figs. 9 and 10, `although extremely simple, provides three distinct types of adjustment for the relative positions of coil and brush. Coil Mt, as shown, is wound on a form |4| of insulative material. That form is of flat section and lies in a substantially vertical plane as seen in Fig. 9, that position of the instrument being purely illustrative. The ccil is rigidly connected to a light frame |42, which is centrally mounted upon bracket |44, rigidly connected to end |35 of tube |3. rhe connection between frame M2 and bracket HM permits rotational adjustment of the bracket with respect to the frame about an axis normal to the length of coil |43. As shown, that connection is the single screw Het, threaded vertically into bracket IM.

Brush |50, as shown, comprises a wire |52 stretched between the two arms |53 and |54 of a U-shaped mount of nat spring material |55. The body ci mount |55 is slotted at |56 parallel to the axis of the U. The single screwY |51', freely received in slot E56 and threaded vertice-ily into the xed bracket 58, secures mount |55 to brachet |58 in a manner permitting rotational adjustment of the brush about the vertical axis of screws il and also translational adjustment along the length of slot |58. The spring arms |53 and 54 are relatively flexible in 'a direction normal to their plane, providing yielding contact between brush and the upper edge of coil |463. At the same time, the flexibility of the brush mounting is relatively slight in the plane of mount |55, so that the position oi the brush is effectively positively donned in that plane with respect to end |35 of the Bourdon tube upon which it is mounted. Electrical connection may be made to the brush through dtting 53|, and to the coil through insulated leads, not shown.

The angle b between coil and brush is shown illustratively in Y' g. lo as approximateiy 14.5, giving substanta four-fold amplification of the relative movement of the two ends o' the Bourdon tube. lt will be seen that such amplincation is attained by extremely simple and economical means, and without destroying the advantages inherent in the illustrated type or instrument.

lt will be understood that many alterations may be made in the specic embodiments dee scribed without departing from the spirit of the invention, and that those embodiments are not by any means intended to represent the' only types of instrument in which the invention may have utility.

I claim:

l. In an instrument for comparing two iiuid e pressures, the combination of twohousingmembers essentially of cup shape, an apertured mounting plate, assembly means releasably securing the housing members in opposed relation on opposite sides of the mounting plate and in hermetically sealed relation therewith to form a generally cylindrical housing that is partially divided transversely into two chambers by the mounting plate, a generally cylindrical pressure responsive bellows closed at one end and open at the other end, the open end of the bellows being rigidly secured in hermeticallysealed relation to the mounting plate, with the length of the bellows extending transversely thereof in one of the chambers and with the aperture in the mounting plate forming a passage between the other chamber and the interior of the bellows, structure forming passages for admitting iluid pressures to be compared to the said one chamber externally of the bellows and to the other chamber, respectively, an elongated brush support having one end rigidly mounted on the closed end of the bellows and extending longitudinally and internally thereof with its free end projecting in spaced relation through the aperture in the mounting plate into the other chamber, a potentiometer brush mounted on the free end of the brush support for movement therewith in a direction axially of the bellows in response to differences between the pressures to be compared, a potentiometer coil, bracket structure supported solely on the mounting plate and mounting the potentiometer coil in the other chamber in position to be engaged by the potentiometer brush, the length of the coil forming an acute angle with the said direction of movement oi the brush, and the said bracket structure including means for adjustably varying that angle, release of the said assembly means providing full access to the bellows and to the said bracket structure while maintaining normal relationship of the potentiometer brush and coil.

2. In a potentiometer, in combination, a potentiometer coil comprising a coil form of relatively soft dielectric material having a rlat transverse section and having a longitudinal edge that is approximately straight, a longitudinal slot in the form edge substantially midway between the flat form faces, an insert of relatively hard dielectric material mounted in the slot and having a longitudinal edge that extends beyond the form edge and that is accurately straight and of uniform predetermined transverse section, and a coil winding wound on the form, each turn of the winding passing over the exposed edge of the insert and taking a definite curvature corresponding to the transverse section thereof; a potentiometer brush having an elongated Contact edge, and means supporting the brush for relative sliding movement with respect to the coil, the contact edge of the brush forming an acute angle with the length of the coil and engaging the winding where it passes over the insert.

3. A feeler gage comprising an elongated generally cylindrical housing having an aperture at one end and having the other end closed by a cap that is adjustably rotatable about the longitudinal axis of the housing, an elongated lever mounted intermediate its length within the housing for swinging movement about a pivot axis normal to the housing axis and adjacent the apertured end of the housing, one end of the lever extending through the said aperture and carrying an abutment at a relatively small radius from the pivot axis, two mutually engageable potenti- I1 ometer elements comprising respectively a potentiometer coil and brush, each of said elements having an elongated contact edge, means mounting one potentiometer element on the inner face of the housing cap with its contact edge in a plane normal to the housing axis, means mounting the other potentiometer element on the other end of the lever with its contact edge extending longitudinally of the pivot axis and at a relatively large radius therefrom and in sliding `engagement with the contact edge of the said one potentiometer element, and electrical connections to the respective potentiometer elements, the angle between the contact edges of the said ele- 12 ments being adjustable by rotation or the cap relative to the housing.

ALFRED J. KLOSE.

REFERENCES `CITED The following references are of record in the file of this patent:

UNITED STATES PATENTS