US3497858A

US3497858A - Potentiometer

Info

Publication number: US3497858A
Application number: US703023A
Authority: US
Inventors: Clare A Norris
Original assignee: Welwyn Electric Ltd
Current assignee: Welwyn Electric Ltd
Priority date: 1968-02-05
Filing date: 1968-02-05
Publication date: 1970-02-24
Anticipated expiration: 1987-02-24
Also published as: FR2001316A1; NL6901839A; GB1216634A; DE1905260A1

Description

Feb; 24, 1970 l c, A, NORRIS 3,497,858

FIG. 2

/N VEA/70,@ Clare H. War/is Feb. 24, 1970 I C. A. NORRIS 3,497,858

POTENMOMETER Filed Feb. 5, 1968 i l 2 sheets-Sheet a g; *T2642 20 w) e? f2 msm ll l

United States Patent O 3,497,858 POTENTIOMETER Clare A. Norris, London, Ontario, Canada, assigner of seven-twelfths interest to Welwyn Electric Limited, Bedlington, Northumberland, England, and ive-twelfths interest to Societe Francaise de lElectro-Resistance, Nice, France Filed Feb. 5, 1968, Ser. No. 703,023 Int. Cl. H01c 5/02 U.S. Cl. 3318-180 15 Claims ABSTRACT OF THE DISCLOSURE A potentiometer construction of the type having a resistance element, a collector strip, a movable contact for engaging the element and the strip, and contact driving means. The improvements include a supporting structure for the contact comprising a carrier block and a carrier block spring. The contact is attached to the carrier block, and the carrier block spring is directly connected to the driving means. The carrier block is maintained out of contact with the driving means.

The carrier block and carrier block spring ride directly on the surface of the housing for the structure and means are provided for precisely spacing the resistance element relative to the carrier block. A high degree of uniformity can then be maintained.

This invention relates to a potentiometer construction, and the invention will be specifically described with respect to devices of the so-called trimmer potentiometer type. The invention is specifically concerned with the interior structure of the potentiometer and the invention is particularly directed to supporting means for the movable contact and resistance element employed in the construction whereby a device characterized by reliable operation, accuracy and durability can be eiiciently and economically produced.

The internal comp-onents of potentiometers comprise a resistance element and a collector strip with terminals connected to both the element and the strip. A movable contact is provided for engaging both the element and the strip whereby a circuit will be completed between terminals connected to the element and strip. As the position of the contact changes, the amount of resistance included within a particular circuit will increase or decrease.

The mechanical design of potentiometers can greatly aliect the electrical characteristics, particularly in instruments which require a high degree of accuracy. The problems are magnified where miniaturized instruments are involved. Thus, even slight tendencies toward vibration of parts, irregularities, loose fitting parts, or other mechanical deficiencies can result in significant changes in resistance due to the fact that only very small distances need be traversed by a movable contact in order to effect such changes. In addition to significant changes in resistance which occur as a result of mechanical deficiencies rather than as a result of intended movement of contacts, the mechanical design of potentiometers can eiect various other performance characteristics. Specifically, the resolution at low resistance values and such other characteristics as the temperature coeiiicient of resistance and noise generation can be adversely aliected through failure to achieve uniform and consistent contact pressure during operation.

The prior art provides for the production of potentiometers of desired characteristics, however, the cost of producing these is extremely high. Where production costs are kept low, inferior quality results.

ice

It is a general object of this invention to provide a trimmer potentiometer design which will provide a high degree of reliability in performance, even in miniature form, while having only a relatively few parts of a relatively uncomplicated design whereby the potentiometer can be produced without undue expense.

It is a more specific object of this invention to provide a design for a trimmer potentiometer, which can be eiiiciently produced while still being characterized by substantially infinite resolution even at low resistance values While at the same time providing a low temperature coefiicient of resistance and negligible generation of noise during the normal operation of the unit.

It is a further object of this invention to provide a potentiometer of the type described which is characterized by a mechanical design which permits uniform and consistent contact pressure during operation to thereby provide accuracy and reliability, and which can also be assembled by means of relatively simple techniques.

These and other objects of this invention will appear hereinafter, and for purposes of illustration, but not of limitation, specific embodiments of the invention are shown in the accompanying drawings in which:

FIGURE 1 is a vertical, sectional view of a potentiometer design characterized by the features of this invention;

FIGURE 2 is a bottom plan View of the potentiometer with the base portion removed;

FIGURE 3 is a plan View of the potentiometer with the main housing portion removed;

FIGURE 4 is a bottom plan view taken about the line 4 4 of FIGURE 1;

FIGURE 5 is an end view taken about the line 5 5 of FIGURE l;

FIGURE 6 is an end view taken about line 6 6 of FIGURE l;

FIGURES 7 through l0 each comprise vertical, sectional views taken about the line 7 7, 8 8, 9 9, and 10 10, respectively of FIGURE l;

FIGURE ll is a fragmentary vertical, sectional view taken about the line 11 11 of FIGURE l0; and,

FIGURE l2 is a horizontal sectional view taken about the line 12 12 of FIGURE 11.

The potentiometer design of this invention includes a housing having the usual elements located therein. Thus, a resistance element and a collector strip are mounted within the housing with a movable contact being provided for engagement therewith. Terminals are connected to the element and the strip, and a driving means is provided for adjusting the position of the contact to thereby vary the resistance included within the circuits between the terminals.

The specific improvements of the invention generally relate to the means for supporting the various elements within the construction. One particularly important aspect of the invention involves the supporting structure for the movable contact, said structure including a carrier block and a carrier block spring. The carrier block spring is in direct driving engagement with the driving means, and this spring in turn drives the carrier block. The carrier block does not have physical contact with the driving means. However, the contact which is attached to the carrier block is moved by the driving means since the carrier block spring will transmit the desired driving action.

The carrier block spring includes a portion which engages the housing. This engagement serves to resiliently hold the carrier block spring in driving engagement with the driving means. For example, Where the driving means comprises a screw, a detent is formed on the carrier block spring for interlitting relationship with the threads of this screw. The carrier block spring is pressed between the housing and the screw to provide positive driving engagement.

The carrier block itself is in engagement with the housing, and this block is, therefore, precisely positioned with respect to the housing. The resistance element is supported by pillars formed in the housing and the pillar position and carrier block size can be very accurately controlled so that a precise spaced relationship is provided between the surface engaged by the carrier block and the surface of the resistance element. The contact pressure on the resistance element is then precisely controlled.

The design is also of importance from the standpoint of ease of assembly. The carrier block and contact spring are easily put into position as a unit just prior to the placement of the resistance element on the pillars. The use of the pillars to locate the resistance element eliminates any critical operation in this regard.

The accompanying drawings illustrate the above noted features as well as other features of the invention. In describing the illustrated structure, and in the appended claims, reference is made to upper and lower surface, and to downwardly and upwardly extending portions, etc. It will be understood that this terminology is used solely for convenience in describing the relative location of the parts, and that the orientation of the potentiometer is not to be considered to be limited in any respect.

The potentiometer construction comprises a main housing portion and a base portion 12. The main housing includes a top wall 14, downwardly extending side walls 16 and end walls 18. The base portion is substantially rectangular, and includes various cut-outs and protruding areas having functions which will be later described.

T-he base portion supports a resistance element 20 which may be of any conventional coniiguration. A conductive strip 24 is secured at one end of the resistance element, and a conductive tab 26 which includes conductive extension 28 is secured at the other end of the resistance element. Lead

wires

30 and 32 are connected to the end portion 29 of the extension 28 and the tab 24, respectively.

A collector strip 34 is mounted on one side wall 16. The collector strip includes an extension 36 which is connected to lead wire 38. The contact bridges between the collector strip and the resistance element. In accordance with conventional potentiometer operation, a circuit is thus provided between the lead wire 38 and each of the

lead wires

30 and 32. As the contact is moved, the resistance in one of these circuits will be increased while the resistance in the other circuit will be decreased.

Instead of lead wires, printed

circuit board terminals

30', 32 and 38 can be secured to the base. This represents an important advantage of the design since the different connecting means can be provided without any material change in the other components of the potentiometer.

The contact 40 is moved back and forth by means of lead screw 42. One end 44 of the lead screw is journalled in the bore 46 deiined by one end wall 18 of the housing. The opposite end of the lead screw extends through the bore 48 in the opposite end wall of the housing. The operating head I50 of the lead screw includes a slot 52 whereby rotation of the lead screw, and consequently adjustment of the potentiometer, can be undertaken by means of a screwdriver or other mechanical means. Obviously, direct manual adjustment is also possible.

The lead screw is maintained in position by means of retainer spring 54 which engages the shoulder 56 defined by the lead screw. The head 50 defines a shoulder 58 and a compressible O-ring 60 is located for engagement with this shoulder.

The contact 40 lconsists of a formed spring which 4 includes contact arm 62 for engagement with the collector strip 34. A cylindrical section 64 is formed at the end of the contact arm to provide the de'sired contact.

The second -contact portion of the contact 40 comprises three arms 66 which engage the resistance element 20. The contact design provides better resolution, complete assurance of contact, and maximum resiliency while minimizing longitudinal `deflection during operation. With respect to the latter point, the individual arms 66 are only formed in the contact for approximately half the flexural length of the contact. Therefore, the area at the bend Where flexing takes place is relatively strong Ito minimize longitudinal deflection while each of the individual fingers will have a relatively high degree of resilience.

The arm 62 in contact with the strip 34 is, as a practical matter, separated from the arms 66, and, therefore, irregularities in the movement of the arm 62 will not influence the arms 66. It will also be noted that the upper arm portion 67 serves as a means for properly aligning the Contact 40 with respect to carrier block 70. Thus, this upper arrn portion engages a protruding portion 71 of the carrier block lto provide for accurate alignment.

The intermediate portion 68 of the contact 40 interts with carrier block 70. As best shown in FIGURE 10, the carrier block portions 71 define a dovetail groove 72 for receiving the contact portion 68. An adhesive may be employed for preventing longitudinal shifting of the contact relative to the block. The contact is, however, cornpletely conned against lateral movement by the arrange ment shown.

The carrier block 70 comprises upstanding sections 74 defining a well therebetween. It will be noted that the opening between the arms 74 is greater than the diameter of the lead screw 42 so that lthere is no physical contact between the carrier block and lthe lead screw. The upper surface 76 of the carrier block sections 74 engage the inner surface 78 of the top wall 14 of the housing.

As best shown in FIGURE 12, the top surfaces 76 define ared edges `80 which extend outwardly from the mid-section of the carrier block. It has been found that this arrangement provides an ideal cleaning action during operation of the potentiometer. Thus, the flared edges 80 tend to drive foreign material which might collect on the surface 78 toward the center of this surface. Accordingly, the foreign material is moved to locations which are not traversed by the carrier block. This, therefore, insures a clean path for the carrier block thereby providing smooth operation. This vdesign for the surface 76 also provides minimum contact between this surface 78 to thereby reduce friction. It will be noted that vthe surface 76 is in intimate contact with the surface 78 while the side surfaces 77 of the block are confined by -the side walls of the housing to provide substantially complete stability for the carrier block.

A carrier block spring 82 is associated with the carrier block primarily within |the well defined by the block. The carrier block spring includes an upper portion 84 which, as best shown in FIGURE 12, iits within the well without contact with the edges 80. The opposed edges defined by the mid-section of the portion 76 of Ithe block restrict lateral movement of the spring so as to completely avoid contact of the spring with portions of the enclosure.

The portion v84 also defines a detent 86 for engagement with the threads 88 of the lead screw 42. The detent 86 is angularly disposed to correspond with the pitch of the threads on the lead screw.

Beyond lthe portion defining -the detent 86, the spring extends upwardly to points 90 which contact the upper surface 78 of the top wall 14. The spring 82 is initially assembled so that the detent is pressed into engagement with the threads when the housing is assembled with the lead screw to thereby provide positive driving engagement lbetween the carrier block spring and the lead screw.

The spring 82 includes four downwardly depending arms 92 which define bends 94 having only a small area of contact with the carrier block to provide the necessary driving force while avoiding build-up of significant friction between the contacting surfaces. It will be apparent tha-t as the lead screw turns, movements imparted to the spring 82 will be directly transmitted to the carrier block to thereby achieve movement of the contact 40 relative to the strip 34 and resistance element 20.

The housing denes four pillars 96 which are provided for purposes of supporting the resistance element. As best shown in FIGURE 8, the upper surface 98 of the resistance element is in direct contact with the lower face of the pillars l96. Since the housing 10 can be manufactured by precision molding techniques, it will be apparent that the extent of the pillars 96 can be precisely controlled. Therefore, the distance between the surface 78 of the top wall 14 and the surface 98 can be precisely controlled.

The carrier block 70 is also a molded part and can be precisely dimensioned. Therefore, the distance .between the bottom surface of the carrier block and the surface 98 can be precisely controlled since the carrier block is in direct contact with the surface 78. The thickness of the resistance element, which is more diilicult to control, is not a factor in determining the spacing since only the upper surface of the resistance element is a factor, and the position of this surface is determined solely by the pillars 96. Since the critical spacing is precisely controlled, the contact ngers will have a positive relationship to the surface 98 of the resistance element. The deflection applied to the contact fingers can be controlled to a very close tolerance resulting in uniform contact pressure which contributes to reliable performance and low noise generation.

Isolation provided to the contact 40 and the` carrier block 70 by the carrier block spring 82 also contributes to the ideal performance of the potentiometer. Thus, the carrier block is -completely out of contact with the lead screw, and, therefore, irregular motions of the lead screw due to clearance tolerances at its ends or due to distortions throughout its length are not transferred to the contact assembly. The carrier block spring may be affected by such motions, particularly of the lateral variety, however, this may only cause shifting of the points 94 of the spring on the surface of the `carrier block without imparting any actual movement thereto. In this connection, the four'contact points of the carrier block spring with the carrier block are approximately an equal distance between the enclosure surface 78, and the ltop surface of the element 98, such that pressure applied to move the carrier block and contact assembly by the carrier block spring, is applied -to the median point between the ltwo surfaces.

The potentiometer design is characterized by many advantages from the standpoint of assembly steps. During assembly, the carrier block spring 82 and the lead screw 42 are associated with the housing 10 before the carrier block 70 and contact 40 are put into place. Accordingly, the most diicult assembly operations are accomplished while the carrier block and contact 40 are out of the way so that there is no danger of distortion of the contact or pick up of dirt by the contact during these assembly operations.

The contact 40 is secured to the carrier block outside of the housing, and this assembly can then be snapped into place as a unit. The collector strip, resistance element, and base portion 12 can be quickly yand easily located so that there is a minimum possibility of any damage to any of the critical elements occurring. It will also be apparent that the critical spaced-relationship between the carrier block 70 and the resistance element 20 is very easily accomplished so that a high performance construction is provided even though assembly costs are quite low.

The base portion 12 can be secured to the main housing portion 10 by the use of adhesives between mating surfaces thereof, preferably in areas of the base which are beyond the ends of the element 20. The application of the adhesives is not in any way critical since the critical spacing referred to above will not be in any Way affected by the thickness of the adhesive coating applied.

The contact 40 is designed so that maximum resilience will be provided in the three contact arms. The resilience is achieved by providing a relatively large radius at the point where the arms are connected to the bridging portion of the contact. The effective length of the arms is also relatively long thereby adding to the resilience which can be achieved.

The provision of the collector strip 34 on a side wall 16 of the enclosure, and the use of a thin material for this strip, represents a further desirable feature of the invention since a large area is available which can be utilized by the total resistance element surface itselfand as a result provide a maximum of area on the element surface for keeping the temperature of this surface as low as possible. The collector strip is preferably formed of a combination comprising a gold inlay over a brass base to provide an extremely smooth surface, low contact resistance, and a surface which is compatible with precious metals which are preferably used in the contact 40.

In considering the carrier block spring 82, is will be noted that the extremities of the arms 92 extend outwardly toward the outward ends of the housing. In practice, a potentiometer is often adjusted to the extent that the contact is driven to an extreme position at either end of the housing. In the case of the design illustrated, the extremities 100 will engage the pillars 96 when the assembly is moved to an extreme position. It will be appreciated that continued rotation of the lead screw will cause the extremities 100 to press against the pillars. As a result of this contact, there is a tendency for the arms to rotate about a line passing through the points of contact of the carrier block spring to the carrier block. This will cause movement to occur in the carrier block spring with the result that the detent 86 will tend to be lifted away from the threads 88. This relieves pressure on the lead screw and assures that a more reliable overriding action will be provided.

This design feature serves an additional function in that as soon as the lead screw is rotated in the opposite direction to move the contact away from an extreme position, the extremities of the arms in contact with the pillars will immediately tend to force the contact assembly in the opposite direction. This tendency provides almost immediate engagement of the lead Screw with the detent 86 to virtually eliminate lost motion during turning of the lead screw.

The design of this invention virtually eliminates backlash between the screw movement and contact movement. The contact 40 exerts pressure on the carrier block and holds the carrier block against the surface 78 of the top wall 14. The arms 92 and the sloping surface of the carrier block adds somewhat to this pressure. As the carrier block rides across this surface, it will do so in a smooth manner with no tendency to rock or rotate about an axis at right angles to the direction of motion.

The manner in which the lead screw is mounted in the construction is also important insofar as the invention is concerned. 'Ihe O-ring 60 is preferably of the conventional type so that the pressure exerted on the O-ring by` the lead screw can be varied by varying the thickness of the retainer clip 54. This provides a convenient means for adjusting the torque which must be applied to the lead screw to achieve rotation.

Trimmer potentiometers are often used in high quality military electronic equipment where excessive vibration is an important consideration. The design described provides a minimum of mass in the contact assembly, particularly because the carrier block spring is supported by the lead screw and does not contribute to the mass of the contact assembly which is therefore only the carrier block and the weight of the contact itself. This is highly important from a vibration standpoint since under certain types of abnormal motion, for example, as experienced in shock or vibration testing, movement of the contact may be avoided. To provide minimum weight, all unnecessary material is eliminated, and material with a relatively low specific gravity is used where possible, as in the design of the carrier block.

It should also be noted that the O-ring seal at the end of the screw is under reasonable compression under normal circumstances, and it is not inconceivable that very small movements of the lead screw can occur upon application of longitudinal shock. However, these small movements may not cause movement of the carrier block even under severe vibration, since these movements of the lead screw would be of a very short pulse duration. Since the carrier block spring does have resilience, a time delay is involved before any movement of the mass of the contact and carrier block assembly can occur. Very instantaneous pulses applied through the carrier block spring will dissipate quickly and will not, in effect, cause movement of the carrier block and contact. This is, of course, due to the fact that the carrier block will only be moved when a sustained longitudinal force is applied by the spring.

The potentiometer of this invention comprises a unit which provides precision mechanical design while overcoming deficiencies in existing designs. The design also provides a simple method of production from the standpoint of consistent assembly practices which involve a minimum of skilled labor and expensive tools and gauges. Furthermore, from unit to unit close tolerance is provided in the critical areas of the trimmer potentiometer from a performance standpoint.

It will be understood that various changes and modifications `may be made in the construction described which provide the characteristics of this invention without departing from the spirit thereofv particularly as defined in the following claims.

That which is claimed is:

1. InI a potentiometer construction including a housing and means located within the housing comprising a resistance element, a collector strip, terminals electrically connected to said element and said strip respectively, a movable contact having contact arms individually engaging said element and said strip, said contact also having an intermediate portion bridging said arms whereby a circuit is provided between terminals connected to the element and strip respectively, and contact driving means supported in said housing for varying the position of said contact relative to said element and strip, the improvement comprising a supporting structure for said contact including a carrier block, said carrier block defining an opening dimensioned whereby said contact driving means can be received in said opening without physical contact with the carrier block, and a carrier block spring having a first portion in driving engagement with said driving means, a second portion engaging the carrier block for transmitting drive thereto, and a third portion engaging said housing, the engagement of said third portion with said housing serving to resiliently hold the carrier block spring in driving engagement with said driving means.

2. A construction in accordance with claim 1 wherein said driving means comprises a screw, and wherein said first portion of said carrier block spring comprises a detent for engagement with the threads of said screw.

3. A construction in accordance with claim 1 wherein said second portion of said carrier block spring comprises downwardly extending arms straddling said carrier block, said arms defining bent-in portions contacting said carrier block.

4. A construction in accordance with claim 1 wherein said carrier block is dimensioned to engage said housing, and wherein said contact is attached to said carrier block on the side opposite the side of the block engaging said housing, said -contact comprising a spring member operating to force the block against the housing.

5. A construction in accordance with claim 1 including pillars located within said housing for supporting said resistance element, said pillars extending between the surface of the housing engaged by the carrier block and that surface of the resistance element which is engaged by said contact.

6. A construction in accordance with claim 5 wherein said pillars are engaged by said carrier block spring when the spring is moved to the extreme end of the housing by said drive means, and wherein said spring is caused to rotate by a minor amount upon engagement with said pillars to thereby assist in releasing the first portion of the spring from driving engagement with said driving means to thereby avoid damage as the driving means continues to rotate after the carrier block has stopped in said extreme position.

7. A construction in accordance with claim l1 wherein said element is mounted on one wall of said housing with said collector strip being mounted on a wall which is perpendicular to said one wall, said contact including one portion extending downwardly and defining a first contact surface for engagement with said element, and a separate portion defining a second contact surface located at right angles to said first surface, said second surface engaging said collector strip.

8. A construction in accordance with claim 7 wherein said one contact portion includes three fingers, each of said fingers individually engaging said resistance element.

9. A construction in accordance with claim 1 wherein said driving means comprises a screw having an operating head at one end for rotation of said screw, and including an O-ring formed `of compressible material located in engagement with a shoulder defined by said head, and a snap ring located forwardly of said O-ring for securing said screw in place, the torque required for turning of said screw being dependent upon Ithe thickness of said snap ring.

10. A construction in accordance with claim 4 wherein said carrier .block defines a pair of upstanding sections located on either side of said driving means, the upper surfaces of said sections engaging said housing, said surfaces defining interior edges which flare outwardly from the mid section of the block and wherein said sections exert a cleaning action with the flared edges forcing foreign material from the housing surface to interior locations on the surface which are outside the area traversed by the surfaces of the carrier block.

11. A construction in accordance with claim 4 wherein said carrier block defines a groove extending longitudinally of the housing and having a dovetail configuration, and wherein said bridging portion of said contact is positioned in said groove.

12. In a potentiometer construction including a housing and means located within the housing comprising a resistance element, a collector strip, terminals electrically connected to said element and said strip respectively, a movable contact having contact arms individually engaging said element and said strip, said contact also having an intermediate portion bridging said arms whereby a circuit is provided between terminals connected to the element and strip respectively, and contact driving means supported in said housing for varying the position of said contact relative to said element and strip, the improvement comprising a supporting structure for said contact including a carrier block, said carrier block `defining an opening dimensioned whereby said contact driving means can be received in said opening without physical contact with the carrier block, and a carrier block spring having a first portion in driving engagement with said driving means, and a second portion engaging the carrier block for transmitting drive thereto, the opening in said carrier block extending completely through one wall of the block whereby the block comprises a closed base and two side Walls, said side walls straddling said driving means after assembly whereby the carrier block and contact can 'be put into place after said drive means and carrier block spring are assembled, and wherein said second portion of said car rier block spring comprises opposed pairs of downwardly extending, spaced apart spring arms, the lengthwise spacing between the opposed arms being normally less than the length of said carrier block whereby the opposed arms resiliently engage the outside end surfaces of said carrier block, the arms in each pair being laterally spaced apart by an amount sufficient to permit receipt of said driving means in the opening defined between the ends of the arms without engagement of the arms with said driving means.

13. A construction in accordance with claim 12 including resistance element locating means formed within said housing in spaced apart relationship with respect to the surface of the housing engaged 'by the carrier block, and wherein said resistance element is located in said housing by positioning of the element in contact with said locating means, and wherein said housing and carrier block comprise precision molded parts whereby the position of the resistance element locating means and the size of the carrier block are precisely maintained.

14. In a potentiometer construction including a housing and means located within the housing comprising a resistance element, a collector strip, terminals electrically connected to said element and said strip respectively, a movable contact having contact arms individually engaging said element and said strip, said contact also having an intermediate portion bridging said arms whereby a circuit is provided between terminals connected to the element and strip respectively, and contact driving means supported in said housing for varying the position of said contact relative to said element and strip, the improvement comprising a supporting structure for said contact including a carrier block, said carrier block defining an opening dimensioned whereby said contact driving means can be received in said opening without physical contact with the carrier block, and a carrier block spring having a rst portion in driving engagement with said driving means, and a second portion engaging the carrier block for transmitting drive thereto, the opening in said carrier block extending completely through one wall of the block whereby the block comprises a closed base and two side walls, said side walls straddling said driving means after assembly whereby the carrier block and contact can be put into place after said drive means and carrier lblock spring are assembled, and wherein said second portion of said carrier block spring comprises opposed pairs of `downwardly extending, spaced apart spring arms, the 1engthwise spacing between the opopsed arms being normally less than the length of said carrier block whereby the opposed arms resiliently engage the outside end surfaces of said carrier block, the arms/in each pair being laterally spaced apart by an amount suficient to permit receipt of said driving means in the opening defined between the ends of the arms without engagement of the arms with said driving means.

15. A construction in accordance with claim 14 wherein said carrier block is maintained in engagement with a wall of said housing directly opposite said surface of the resistance element, and wherein said carrier block and said resistance element locating means comprise precision molded parts whereby the spacing between said surface of the resistance element and the carrier block can be precisely maintained.

UNITED STATES PATENTS References Cited 2,926,324 2/1960 Barden et al. 338-183 XR '2,946,975 7/1960 Caddock 338-183 2,958,062 10/1960 Barden et al. 338-180 3,107,336 10/1963 Caddock et al. 338-180 -3,l24,780 3/1964 Hulbert 338--183 XR 3,237,141 2/1966 Bourns 33-8-180 3,414,862 12/1968 Ginsberg 338-180 THOMAS l. KOZMA, PrimaryExaminer U.S. Cl. X.R. 338-183