US2881295A

US2881295A - Potentiometer

Info

Publication number: US2881295A
Application number: US723819A
Authority: US
Inventors: Franklin M Brown
Original assignee: Individual
Current assignee: Individual
Priority date: 1958-03-25
Filing date: 1958-03-25
Publication date: 1959-04-07
Anticipated expiration: 1976-04-07

Description

F. M. BROWN POTENTIOMETER April 7, 1959 4 Sheets-Shree? 1 Filed March 25,v 1958 INVENTOR.

Franklin M.BI'oWn BY @M/Q @m A gen April 7, 1959 F. M. BROWN POTENTIOMETER Filed March 25, 1958 4 Sheets-snaail 2 INVENTOR.

Franklin Mrown BY @6N F. M. BROWN POTENTIOMETER April 7, 1959 4 Sheets-Sheei 3 Filed 'March 25, 1958 m T m .Q Lm. .FN w m 9 f: o@ N\ s, @n um YQ v MQ ,mn W N s QQ Q Nb OS 58S wm QQ @Q n fr w a Ag M m@ l. ,k m@ nw KQ @Q F. M. BROWN POTENTIOMETER April 7, 1959 4 sheets-smeet 4 Filed March 25, 1958 INVENToR. F rc1 nkll'n M.BI'oWrl BY @LQ @w United States. Patent-TO POTENTIOMETER Franklin M. Brown, Portland, Oreg. Application March 25, 1958, Serial No. 723,819 18 Claims. (Cl. 201-48) This invention pertains to potentiometers and relates particularly to a potentiometer of improved construction and performance characteristics.

It is a principal object of the present invention to provide a potentiometer which is characterized by having extremely high resolutio-n.

Another important object of this invention is the provision of a potentiometer which is capable of operation over a substantial range with a minimum of total relative movement between parts, whereby to minimize the timel involved in setting tordesired values within the range. i

Still another important object of this invention is the provision of a potentiometer construction which accommodates adjustment of the degree of resolution over a substantial range.

A further important object of the present invention is to provide a potentiometer which is characterized by having a wide frequency range.

' Another important object of this invention is the provision of a potentiometer construction which accommodates direct read out over the range of operation.

A `still further important object of this invention is the provision of a potentiometer which is capable of being mounted with speed and facility and which is of minimum size for convenient incorporation into electronic and other instruments.

, The foregoing and other objects and advantages of this invention will appear from the following detailed description, taken in connection with the accompanying drawings, in which:

Fig. 1 is a view in longitudinal section, showing details of construction of a potentiometer embodying the features of the present invention;

Fig. 2 is a rear end view of the potentiometer shown in Fig. l, viewed from the bottom of Fig. 1 with the end plate removed;

Fig. 3 is a fragmentary sectional view taken along the line 3 3 in Fig. 2;

Fig. 4 is a fragmentary plan view of the dial assembly shown in Fig. l;

Fig. 5 is a schematic diagram of the electric circuit incorporated in the potentiometer construction shown in Fig. 1;

Fig. 6 is a side elevation, partly in section, of a modified potentiometer construction embodying the features of the present invention;

Fig. 7 is a rear end view of the potentiometer shown in Fig. 6;

Fig. 8 is a schematic diagram of the electric circuit incorporated in the potentiometer construction shown in Fig. 6;

Fig. 9 is a view in side elevation, partly in section, showing a further modification of a potentiometer construction embodying the features of this invention;

Fig. 10 is a fragmentary rear end view of the potentiometer construction shown in Fig. 9;

; Fig. 1l is aschematic ydiagram Aofthe electric'circuit incorporated in the for support upon an instrument incorporated in the potentiometer construction shown in Fig. 9;

Fig. 12 is a rear end view, similar to Fig. 10, of a still further modification of the potentiometer construction embodying the features of this invention; and

Fig. 13 is a schematic diagram of the electric circuit potentiometer construction shown in Fig. l2.

Referring first to the embodiment shown in Figs. l to 5 inclusive, the potentiometer is adapted to be mounted panel through a single opening in the latter. To this end there is provided an electrically nonconducting potentiometer casing having a cylindrical wall 21 and a forward end closure 22. Av central hollow support bushing 23 projects forward from this end closure for reception in the opening in the instrument panel, and is externally threaded for reception of the locking nuts 24, by which the casing is secured to the panel.

Mounted against the inner surface of the cylindrical wall 21 is a continuous resistance wire 25 wound helically upon an electrically non-conducting support 26 formed in the shape of a split cylinder. This bilar construction effectively counteracts induced fields and thus enhances the frequency range. The

opposite ends

27 and 28 of the helix are connected by wires 27' and 28' to the

terminal posts

29, 30 mounted in the wall 21.

An intermediate electrically nonconducting casing having a cylindrical wall 31 and a centrally apertured forward end closure 32, is mounted upon the peripheral ange 33 of an elongated sleeve 34, as by means of the screws illustrated. The sleeve projects rearward through the central aperture in the end closure 30 and forward through the hollow bushing 23. A helically Wound resistance wire 35 is mounted on an electrically nonconducting support 36 formed in the shape of a split cylinder, and this assembly is mounted against the inner surface of the wall 31.

The terminal ends 37 and 38 of the resistance wire 35 are connected by wires 37', 3S which extend through openings in the wall 31 and then cross each other for connection to the

spaced contacts

39 and 40, respectively.

These contacts are mounted on plate 41 secured to the' wall 31 for rotation with the intermediate casing, and they project radially outward for engagement with thel outer helical resistance winding 25. It is to be noted that by virtue of this crossed arrangement the trailing end 37 of the intermediate winding 35, during counterclockwise rotation o-f the latter, is connected to the leading contact 39 which is engageable at its limit of counterclockwise rotation with the corresponding end 27 of the outer winding 25. During clockwise rotation of the winding 35 the trailing end 38 is connected to the leading contact 40 which is engageable at its limit of clockwise rotation with the corresponding end 28 of the winding 25. These limits of rotation of the intermediate casing are fixed by means of the spaced

upstanding pins

42 and 43, respectively, supported by the end closure 22 of the outer casing and arranged for abutment by the radially extending pin 44 projecting from the peripheral flange 33.

An inner electrically nonconducting casing having a cylindrical wall 45 and a centrally apertured forward end closure 46 is positioned concentrically within the intermediate casing and is connected for rotation with an inner sleeve 47 which extends forwardly through the hollow sleeve 34. An electrically nonconducting support 48 in the form of a split cylinder, supports a helically Wound resistance wire 49, and this assembly is mountedy 50', 51 which pass rearwardlyvthrough openings in the end closure 46 and then are. crossed for connection to the circumferentially spaced

contacts

52 and 53, respectively.

, 'As best shown in Fig. 3, each' of these contacts is formed from a single strip of electrically conducting material, such as copper alloy, the strip being bent intermediate its ends to form a substantially U-shaped section. The U-shaped section is inserted freely through an opening 54 in the inner casing, with the closed end of the section projecting into a recess 55 in the forward face of the end closure 46. The wires 50', 51 connecting the terminal ends of the inner resistance winding 49 cross within this recess, the wires being covered with insulation in this area, and the terminal ends of these wires are threaded through the closed end of the U-shaped section for electrical connection therewith. In this manner each wire effectively anchors the contact within the opening 54. The bent ends of the contact rest within a radial notch 56 formed in the wall 45, to prevent lateral movement of said ends. The outwardly projecting end of each contact resiliently engages the intermediate resistance winding 35.

By virtue of the crossed relationship of the terminal connections of the inner resistance winding 49 with the

spaced contacts

52, 53, the ends of the intermediate winding arey connected to the corresponding ends of the inner winding at the limits of rotation of the inner casing. These limits of rotation are established 'by means of spaced pins 57 and 58 which are mounted in the end closure 46 of the inner casing and project from the opposite surfaces thereof, for abutment of their forward projecting ends with a pin 59 projecting rearwardly from the end closure 32 of the intermediate casing.

A shaft 60 extends rotatably through the inner sleeve 47 and carries at its rearward end an electrically nonconducting plate 61 which supports an electrically conducting contact 62. One end 63 of this contact projects radially outward for sliding engagement with the inner resistanceA winding 49, while the opposite end thereof is bent in a loop 64k to intercept the longitudinal axis of the shaft 6G, for resilient engagement with a terminal post 65 which is mounted at the center of the rear end closure 66 of the outer casing. This rear end closure rests within an annular groove in the cylindrical wall, and is secured thereto by such means as the screws illustrated. The contact end 63 is rotatable -between the ends of the resistance winding 49 these limits being established by a radial pin 67 extending from the shaft 60 for abutment against the rearward projecting ends of pins 57 and 58.

A dial assembly preferably isy associated with the concentric sleeves and shaft to provide direct read out of the various settings of the contacts on the associated resistance windings. To this end there is provided a plurality of dials each associated with one of the sets of contacts. Thus, a circular dial member 68` is provided with an apertured hub 69 for receiving the forward end of the sleeve 34 which is associated with: the intermediate casing. A set screw 70 is threaded into the hub` for releasably securing the latter to theV sleeve. A split keeper ring 71 is mounted in an annular groove in the sleeve 34 adjacent the forward end of the bushing 23 for preventing longitudinal displacement of the sleeve relative to the bushing.

A second circular dial 72 of lesser diameter than the 'rst mentioned dial 68, overlies the latter and is provided with an apertured hub 73 adapted to receive the forward end of the inner sleeve 47, for releasable attachment thereto by means of the set screw 74. A split keeper ring 75 is mounted in an annular groove in the inner sleeve 47 adjacent the outer end` of the outer sleeve 34, for preventing longitudinal displacement of the inner sleeve relative to the outer sleeve.

A third circular dial 76, of lesser diameter than the second dial, is provided with a hub conveniently in the form of a control knob 77, as illustrated` The control knob is provided with a central opening adapted to receive therein the forwardly projecting end of the shaft 60, and

the knob carries a set screw 78 for releasably interconnecting the knob and shaft. Y

The exposed surface of the rst dial 68 is provided with a circumferentially graduated scale 79 which is referred to an indexed marker 80 secured to the instrument panel adjacent the outer periphery of the dial. A second Icircumferentially graduated scale 81 is provided on the rst dial 68 concentrically within the outer scale, in an area covered by the exposedportion of the second dial 72. The exposed portion of this second dial preferably is opaque over its entire area with the exception of a small window section 82 through which the second scale may be viewed. This window section may be provided with an index marker, such as the pointer 83 illustrated.

The second dial 72 also is provided with a circumferentially graduated scale 84 in an area which is covered by the third dial 76, the latter preferably being opaque over its entire area with the exception of a small window 8S through which to view the scale. An index pointer 86 also is provided on this window.

From the foregoing it will be apparent that operation of the potentiometer is effected by manipulation of a single control shaft, either manually by control knob 77 or by motor driven connection to shaft 60. Thus, as the control shaft is rotated, the contact 63 slides over the surface of the inner resistance winding 49 until it reaches one of the terminal ends of the winding. At this point the radially extending pin 67 carried by the shaft abuts against one of the rearwardly projecting ends of the pins 57 or'58. Upon further rotation of the control shaft in the same direction, the entire inner casing is caused to rotate, whereby simultaneously to move the pair of spaced

contacts

52, 53 slidably over the intermediate resistance winding 35. As one or the other of these spaced contacts reaches a terminal end of the intermediate winding, the forwardly projecting end of one of the pins 57 or 53 engages the rearwardly projecting pin 59 carried by the intermediate casing. Upon continued rotation of the control shaft in the same direction, the intermediate casing is caused to rotate, thereby simultaneously moving the outer pair of

contacts

39, 40 along the outer resistance winding 25 until a terminal end of the latter is reached and the radially extending pin 44 on the peripheral flange abuts against one of the

rearwardly projecting pins

42, 43 carried by the end closure of the outer casing. At this point of adjustment the potentiometer is at its minimum or maximum setting, which will be indicated on the dial assembly.

Operation of the control shaft in the reverse direction effects reverse movement of the several contacts in the same sequence as described above, namely movement rst of the contact 63,. then

contacts

52, 53 and

nally contacts

39, 40. This sequence in both directions of rotation is achieved by virtue of the progressively increasing radii of rotation of the'sets of contacts.

It will be apparent that the three-stage assembly illustrated. in Figs. l to 5- may be modified to include additional stages, or may be modified by reducing the stages to two. Alternative constructions also may be employed. One such modification is illustrated in Figs. 6, 7 and 8. As in the modification earlier described, this modification also includes an outer casing having a cylindrical wall 90, a forward end closure 91 and a forwardly projectingv bushing 92 for mounting the casing on an instrument panel. An. electrically nonconducting support 93 in the form of a split cylinder supports a helical resistance wind ing 94 and the assembly is mounted against the inner surface of theV cylindrical wall. The terminal ends 95, 96 of the winding are 'connected to

terminal posts

97, 98 extending through the wail.

An inner casing of electrically'nonconducting material and including. a cylindrical wall 100 and a centrally perforated forward end closure 101 is mounted concentrically within the outer casing. A sleeve 102 is secured tol the` end: closure 101; andextends freely through the bushing for connection at its forward end to acircular dial 103. For this purpose the dial is provided with a central hub 104 carrying a threaded set screw 105 for releasable engagement with the sleeve.

An electrically nonconducting support 106 in the form of a split cylinder supports a helically wound resistance wire 107 and this assembly is mounted against the inner surface of the cylindrical wall of the inner case. The terminal ends 108 and 109 of the resistance winding are connected to

wires

108 and 109 which extend through spaced openings in the wall 100, and then are crossed and extended radially outward, terminating in a. pair of circumferentially spaced resilient contacts 110 and 111 which slidably engage outer resistance winding 9 4. As in the modication previously described, it is by virtue 0f this crossed arrangement that the corresponding ends of the inner and outer windings are connected together. In manner similar to the embodiment described hereinbefore, the limits of rotation of the inner casing are established by a pair of circumferentially spaced abutment screw heads 112 mounted in the end closure 91 of the outer casing, for abutment by a projecting screw head 113 mounted in the end closure 1,01l of theinner casing. y, A shaft 114 extends through the hollow sleeve 102 and casing end closure 101, and releasably supports at its rearward end an electrically nonconducting head 115, which is secured to the shaft by such means as the set screw 116.l Mounted upon the head is a contact member 117, one end 118 of which projects radially outward for sliding engagement with the inner resistance winding 107. The opposite end 119 of the contact member is bent forwardly for resilient sliding contact with an electricallyl conducting ring 120 secured to the end closure 101. This ring is connected electrically to an electrically conducting annular disc 121 which is supported on the forward surface of the casing end closure 101, -as by means of the screws 122, at least one of which is connected electrically to the ring as by means of the conductor 123. Resilient contacts 124 are mounted on the end closure 91 of the outer casing, as by means of the screw 125, and .resiliently engage the annular disc 121. These contacts are connected electrically through a conductor 126 to a terminal post 127 mounted on the wall of the outer casing.

The limits of rotation of the shaft 114 and hence the contact 118 engaging the inner resistance winding, are established by means of circumferentially spaced pins 128 projecting rearwardly from the end closure 101 of the inner casing for abutment by a pin 129 projecting for- Wardly from the head 115. j The forward end of the shaft 114 projects from the hub of the dial 103 and is adapted to be received within a acentral opening in a control knob 130 and secured to the latter by means of the set screw 131. The control knob carries a circular dial 132 which preferably is 0f smaller diameter than the dial 103 secured to the sleeve 102. vThese dials may carry suitable scales, as will be understood.

In both of the modifications previously described it is to b e noted that the pairs of circumferentially spaced contacts are mounted in substantially fixed relationship. It may be desired in some instances to be able to vary either the spacing between the contacts of a pair and thus varythe amount of resistance being shunted, or to vary the shunting resistance, in either case for the purpose of varying the ratio between the shunting and shunted resistance, thereby varying the degree of resolution of the potentiometer. The modfiication illustrated in Figs.l 910 and 1l functions to vary the shunting resistance, and this is accomplished by modifying the structure shown in'Figs. 6 and 7 as follows: The shaft 114 and control knob 130 are provided with a longitudinal bore through which to receive an electrically nonconducting shaft,140. An auxiliary control knob 141 is secured to the forward end of this shaft, as byV means of the set screw 142. The rearward end of the shaft extends freely through the apertured forward end closure of an auxiliary casing 143 which is mounted in fixed position upon the inner casing as by means of the bracket 144. This bracket is of electrically conducting material and is secured to the inner casing in electrical con tact with one end 108 of the inner resistance winding 107. The opposite end of the bracket is perforated to receive the nonconducting shaft freely therethrough, and this end of the bracket is held firmly against the casing by means of the self-locking washer 145 which is pressed onto the shaft. This washer is provided with a resilient extension 146 which forms a contact adapted to slidably engage a resistance winding 147 mounted on an electrically nonconducting split cylinder support 148 and secured in the casing. One end 149 of this winding is connected directly to the corresponding end 109 of the inner winding 107 by means of the electrical conductor 150, which also functions as a supporting bracket for the casing 143, and the opposite end 151 of the winding is connected directly to the opposite end 108 of the inner winding by means of a conductor 152 engaging the electrically conducting bracket 144.

The4 limits of rotation of the contact 146 driven by the electrically nonconducting shaft 140, are established by means of a collar 153 secured to said shaft forwardly of the casing 143, said collar supporting a radially extending pin 154 which is adapted to abut against the opposite ends of an arcuate projection 155 extending forwardly from the casing 143. Thus, by rotation of the auxiliary control knob 141, the contact 146 is moved oyer the auxiliary resistance winding 147 within the limits defined by the projection and pin. Further rotation of the auxiliary control knob causes the auxiliary casing 143 to rotate, and since the latter is mounted in fixed position on the inner casing 100, the latter thereupon is caused to rotate. However, it is preferred that rotation of the inner casing be effected by direct manipulation of the main control knob 130. .t

The modification shown in Fig. l2 functions -to vary the spacing between the circumferentially spaced contacts which slidably engage the outer resistance winding. This arrangement is provided by modifying the structure shown in Fig. 9, as follows: The shaft 140 and auxiliary control knob 141 is provided as indicated in Fig. 9, but the auxiliary casing 143 and its associated assembly is replaced by a cam member secured to the rearward end of the shaft. A cam follower 161 is mounted pivotally intermediate its ends on an electrically conducting pin 162 which is secured to the end 108 of the inner resistance winding 107. A loop spring 163 interconnects the pin and cam follower for normally urging the latter into -engagement with the cam. The contact 110 is supported by the cam follower, and since the latter is constructed of electrically conductive material, said contact is connected electrically to the end 108 of the inner resistance winding 107. It will be understood that either the shaft or the cam, or both, may be constructed of electrically nonconducting material in order to insulate the cam follower from the remaining elements of the structure.

In the operation of the modification shown in Fig. l2, rotation of the auxiliary control knob 141 causes simultaneous rotation of the cam 160 and consequent movement of the cam follower 161 and the attached contact 110, whereby to vary the spacing between said contact and the associated contact 111 of the pair.

, From the foregoing description it will be apparent that the present invention provides a precision potentiometer having extremely high resolution which may be varied within substantial limits, and which is of simplified con.- struction for economical manufacture. Mounting of thel pairs of contacts directly on the resistance windings or on theirpcasings eliminates the complexity of assembly heretofore encountered in the use of slip rings and wafer switch components. Maximum compactness of assembly is achieved by virtue of the concentric arrangement of the casings, and a predetermined sequence of operation is assured by the stepwise variations of radii of rotation of the contact units.

It will be apparent to those skilled in the art that various changes may be made in the details of construction described hereinbefore. For example, the wire wound resistances may be replaced by those of the carbon type. Although it is preferred that the resistances be mounted in the concentric cylindrical manner illustrated in order to achieve maximum compactness, other circular arrangements may be employed. For example, carbon type resistances may be mounted on the end closures of the casings, in the form of annular rings. Other modifications may be made as desired, without departing from the spirit of this invention and the scope of the appended claims.

Having now described my invention and the manner in which the same may be used, what I claim as new and desire to secure by Letters Patent is:

l. A potentiometer comprising a first base member mounted for rotation, irst circular resistance means mounted on the iir'st base member and having spaced terminal ends, contact means mounted coaxially with respect to the iirst resistance means for rotation relative to the latter and arranged for sliding engagement therewith over a predetermined range, control means connected to the contact means for rotating the latter, means interengaging the contact means and first base member at the ends of said range for rotating the contact means and first resistance means simultaneously with continued rotation of the control means, a second base member, second circular continuous resistance means mounted on the second base member and having spaced terminal ends and arranged coaxially with respect to the iirst resistance means, a pair of spaced contact means mounted on the rst base member for rotation therewith and connected to the terminal ends of the rst resistance means and arranged for sliding engagement with the second resistance means, the spaced contact means being so arranged that the contact means leading in the direction of rotation of the iirst resistance means is connected to the trailing end of the first resistance means and the contact means trailing in said direction is connected to the leading end of the lirst resistance means, and support means mounting the second base member independently of the relative rotation of the iirst resistance means and associated contact means.

2. The potentiometer of claim 1 wherein each resistance means is mounted in a cylindrical casing and the casing for the second resistance means is arranged con` centrically about the casing for the first resistance means.

3. The potentiometer of claim 1 wherein each resistance means comprises a continuous resistance wire arranged in the form of a split cylinder and the second resistance means is arranged concentrically about the first resistance means.

4. The potentiometer of claim l including means for varying the ratio between the shunting resistance and the associated shunted resistance.

5. The potentiometer of claim 4 wherein the varying means comprises means associated with at least one of the contact means of said pair for varying the spacing therebetween.

6. The potentiometer of claim 4 wherein the varying means comprises cam operated means associated with at least one of the contact means of said 'pair for varying the spacing therebetween.

7. The potentiometer of claim 4 wherein the varying means comprises auxiliary resistance means arranged in shunt with the first resistance means.

8. A potentiometer comprising a rst base member mounted: for rotation, rst circular-continuous resistance means having spaced terminal ends, contact means mounted coaxially with respect to the iirst resistance means for rotation relative to the later and arranged for sliding engagement therewith over a predetermined range, control means connected to the contact means for rotating the latter, means interengaging the contact means and first base member at the ends of said range for rotating the contact means and first resistance means simultaneously with continued rotation of the control means, a second base member mounted for rotation independ` ently of the relative rotation of the first resistance means and associated contact means, second circular continuous resistance means mounted on the second base member and having spaced terminal ends and arranged coaxially with respect to the first resistance means, a first pair of spaced contact means mounted on the first base member for rotation therewith and connected to the terminal ends of the lirst resistance means and arranged for sliding engagement with the second resistance means over a predetermined range, the spaced contact means being so arranged that the contact means leading in the direction of rotation of the first resistance means is connected to the trailing end of the first resistance means and the Contact means trailing in said direction of rotation is connected to the leading end of the first resistance means, means interengaging the spaced contact means and second base member at the ends of said range for rotating the spaced contact means and second resistance means simultaneously with continued rotation of the control means, a third base member, third circular continuous resistance means mounted on the third base member and having spaced ends and arranged coaxially with respect to the second resistance means, a second pair ofspaced contact means mounted on the second base member for rotation therewith and connected to the terminal ends' of the second resistance means and arranged for slid-Y ing engagement with the third resistance means, the second pair of spaced contact means being so arranged that the contact means leading in the direction of rotation of the second resistance means is connected to the trailing end of the second resistance means and the contact means trailing in said direction of rotation is connected to the leading end of the second resistance means, and support means mounting the third base member independently of the relative rotation of the second resist-` ance means and associated contact means.

9. The potentiometer of claim 8 wherein each resistance means is mounted in a cylindrical casing and the casing for the second resistance means is arranged concentrically about the casing for the iirst resistance means and the casing for the third resistance means is arranged concentrically about the casing for the second resistance means.

10. The potentiometer of claim 8 wherein each resistance means comprises a continuous resistance wire arranged in the form of a split cylinder and the second resistance means is arranged concentrically about the rst resistance means and the third resistance means is arranged concentrically about the second resistance means.

ll. The potentiometer of claim 8 including means for varying the ratio between at least one of the shunting resistances and the associated shunted resistance.

12. The potentiometer of claim ll wherein the varying means comprises means associated with at least one of the contact means of at least one of said pairs for varying the spacing therebetween.

13. The potentiometer of claim ll wherein the varying means comprises cam operated means associated with at least one of the contact means of at least one of said pairs for Varying the spacing therebetween. i

14. The potentiometer of claim 1l wherein the varying means comprises auxiliary resistance means arranged in shunt with the iirst resistance means.

l5. A potentiometer comprising first continuous reA sistance means having terminal ends, contact means arranged for sliding engagement with the first resistance means, second continuous resistance means having spaced terminal ends, a pair of spaced contact means connected to the terminal ends of the rst resistance means and arranged for sliding engagement with the second resistance means, and adjustable means for varying the ratio between the shunting resistance and the associated shunted resistance.

16. The potentiometer of claim 15 wherein the varying means comprises adjustable support means for at least one of the spaced contact means for varying the spacing between said spaced contact means.

References Cited in the le of this patent UNITED STATES PATENTS 2,729,727 Malloy et al. Ian. 3, 1956