US2773966A

US2773966A - Variable electrical resistance structure

Info

Publication number: US2773966A
Application number: US373331A
Authority: US
Inventors: Alfred J Mastropole
Original assignee: G M Giannini & Co Inc
Current assignee: G M Giannini & Co Inc
Priority date: 1953-08-10
Filing date: 1953-08-10
Publication date: 1956-12-11
Anticipated expiration: 1973-12-11

Description

Dec, 11, 1956 A. J. MAsTRoPoLE VARIABLE ELECTRICAL RESISTANCE STRUCTURE 2 Sheets-Sheet Filed Aug. lO, 1953 4L/#QED el MAS-reclame,

IN1/wrox rro/QA/E Dec. 11, 1956 A. .1. MASTROPOLE VARIABLE ELECTRICAL RESISTANCE STRUCTURE 2 Sheets-Sheet 2 Filed Aug. l0, 1953 pm. e

E. L L @ff Am @o United States Patent() VARIABLE ELECTRICAL RESISTANCE STRUCTURE Alfred J. Mastropole, Allendale, N. J., assigner to G. M. Giannini & Co. Inc., Pasadena, Calif., a corporation of New York Application August 10, 1953, Serial No. 373,331

19 Claims. (Cl. 201-59) This invention has to do with variable electrical resistance and potentiometer structures of the general type in which a selected portion of the resistance winding is shiftable out of alignment with the body of the wind ing, and the brush electrically engages the winding only at the deflected portion.

Potentiometers of that general type are described and claimed in the copending patent application Serial No. 373,411, led on August 10, 1953, by Alfred J. Klose and entitled Potentiometer. The present application describes and claims certain features and improvements in such variable electrical resistance structures.

More particularly, the features and improvements with which the present application is concerned include means for preventing accidental damage to the resistance winding by movement of the brush beyond its intended range of operation, means for insuring the proper condition of tension and position of the winding, and particularly convenient types of brush structure.

A full understanding of the invention, and of its further objects and advantages will be had from the following detailed description of certain illustrative embodiments, of which description the accompanying drawings form a part. That description is intended only as illustration, and it will be understood that many changes may be made in the particulars of the described embodiments without departing from the scope of the invention, which is defined in the appended claims.

In the drawings:

Fig. l is a rear elevation of an illustrative potentiometer in accordance with the invention, with cover removed;

Fig. 2 is a section on line 2 2 of Fig. l; K

Fig. 3 is a fragmentary section on line 3 3 of Fig. l at enlarged scale and showing the brush rotated into the plane of the section;

Fig. 4 is a fragmentary section on line 4 4 of Fig. 3;

Fig. 5 is a fragmentary section on line 5 5 of Fig. 2;

Fig. 6 is a section corresponding to Fig. 2 and showing an illustrative modification;

Fig. 7 is a fragmentary portion of Fig. 6 at enlarged scale;

Fig. 8 is a fragmentary section, corresponding to a portion of Fig. 3 and showing an illustrative modification; and

Fig. 9 is a fragmentary section on line 9 9 of Fig. 8.

Figs. l-5 represent an illustrative embodiment of the invention. A supporting frame member, indicated generally at 20, includes a cylindrical sleeve portion 22 with an external tlange 24 at its forward end. An axial through bore 26 in frame member 2l) provides convenient mounting for the ball bearings 28 and 29 by which the control shaft 30 is journaled. Forward bearing 28 is preferably protected, as by the shield 27, which is retained by the plate 31 and dust seal 32. Threaded mounting holes 33 may be provided in the forward face of frame flange 24, that ange portion of the frame then constituting a mounting plate.

A resistance winding is carried on a drum 40 of di- 2,773,966 Patented Dec. 11, 1956 ICC electric material, which comprises a cylindrical body section 42 and a drum flange 44. An axial bore in the drum receives sleeve portion 22 of the frame member, and the frame and drum are secured rigidly `together by the nuts 45 which turn on threaded rods 46. The latter are threaded into the rear face of frame flange 24 and are received by holes in the drum ange. A protective cover for the drum and winding is shown as a light sleeve 50, the forward end of which fits over the periphery of drum ange 44 and abuts frame flange 24, and a back cover plate 52, which closes the rear end of sleeve 50 and is secured by the screws 53. As shown, those screws are internally threaded and grip the ends of rods 46.

The resistance element, indicated at 60, may be of any type that is somewhat flexible, such, for example, as a single resistance wire or an elongated and relatively finely wound coil of such wire. Resistance element 60 is helically wound on the outer cylindrical face of drum body 42. That face is preferably provided with a helical channel, which may comprise the threads 62, to receive and locate the several turns of the winding in axially spaced relation, so that electrical contact between adjacent turns is positively prevented without requiring more than a relatively slight physical spacing between them. As a typical example, which is not intended in any sense to limit the scope of the invention, the resistance wire may be only a few thousandths of an inch in diameter, and the threads 62 in which it is received may, for example, have a pitch corresponding to as many as 72 threads per inch. A great variety of thread forms may be used successfully, a thread having an included angle of about 60 to 90 being preferred. The threads preferably cover a portion of the drum surface of greater axial extent than the winding itself. Since it is sufficient that the winding be supported in a surface that approximates a figure of revolution about the drum axis, drum body 42 need not necessarily have a continuous outer face. An illustrative type of brush assembly in accordance with the invention is indicated at 80. A brush-supporting rotor plate 82 of insulating material is lixedly mounted on the rearward end of control shaft 30 in any convenient manner, as by the

mounting nuts

83 and 84, which are threaded directly on the shaft and clamp the rotor plate between them. The brush proper is shown at 85, and a pick-up contact shoe at 86, fabricated typically from a single piece 87 of resilient sheet material such as a resilient bronze alloy. The central portion of contact piece 87 is tixedly mounted on rotor plate 82, as by the screws 83. Pick-up shoe 86 extends from that point of connection radially inward across the cylindrical axis of the drum, and yieldingly engages the inner end of a contact pin 90, which is supported on the axis of the instrument as by the grommet 91 in insulated relation to cover plate 52. The outer end of pin 90 constitutes an external terminal post for making electrical connection to the potentiometer brush.

The brush proper extends radially outward from the point of connection at 88, and is bent forwardly to form a substantially straight contact arm extending approximately axially in spaced relatie-n to the outer cylindrical surface of drum body 42. The arm is so formed that when under no constraint it extends obliquely outward, as indicated in phantom lines at 85a. One turn of winding 60 extends tangentially outward from the drum threads 62, as indicated at 61, and slidingly engages the radially outer face of brush 85, as indicated at 63. The wire is held yieldingly taut by the outward spring of the brush, and in turn deects the brush radially inward from its natural position 85a to such a position as that shown in solid lines at 85 in the drawings. The axial position at which the lifted turn 61 contacts the brush is not defined byy the structure of the brush itself, which in the present embodiment presents a smoothly polished outer surface to the wire, but is determined rather by the action of threads 62 of the drum in guiding the wire at each side of the brush.

The shape of the brush proper has been found to be of basic importance to insure long life and reliable action, particularly when the resistance wire is of small diameter, for example of the order of one mil. As shown inFig. 3, the cross section of the brush arm is preferably bowed outwardly with a radius approximately equal to 0.4 times the radius of the resistance winding and about twice the width of the arm. The corners of the brush, `over which the winding slides, are slightly broken, and theentire Working surface of the brush is carefully polished.l

totation of brush S5 with respect to the winding support 42 is positively limited to a predeterminedrange of movement by a positive stop mechanism shown illustratively at 100. The rotation of shaft 30 with .respect to frame member 20 is utilized-to produce axial movement of two nuts, which arein threaded-relation'with one of those members and in splined relation withfthe'other. As illustratively shown, the

nuts

102 and 103 engage the threads .101, provided on shaft'lti'between the bearings 28 and 29. The pitch of threads 101' in the present ernbodiment need not be the same as that of winding channel 62. Rotational movement of the nuts withrespect to the frame member is prevented in any suitable manner, as by the radial pins 104 and 105, fixed in the periphery of the respective nuts and slidingly received in the axial slot 106 in the wall of bore 26. Accordingly, as the shaft is turned, the nuts move together axially along the shaft without rotation relative to the splined member, here taken as the frame.

Stop formations 110 and M1 are mounted in the nonadjacent axial faces of the

respective nuts

102 and 103 at definite rotational positions with respect to spline formations 104and 105; and cooperating stop pins 108 and 109 are set in the member with respect to which the nuts 'are threaded, here shown as shaft 30, in position to engage the respective stop formations in response to shaft rotation. The parts are so proportioned that as the shaft rotates in one direction the stop pin toward which the nuts are thereby moved clears the stop formation on the nearer of the two nuts by a comfortable margin on one rotation and positively engages that formation on the next following rotation. By suitable angular and axial spacing of the pins, any desired integral or fractional number of rotations of the shaft may be permitted between the points at which the shaftis positively stopped, that number ordinarily corresponding to the number of available turns of the resistive winding over which the brush can effectively travel.

A particular advantage of the type of stop mechanism illustrated is the convenience and economy with which it can be adapted for resistance windings of different lengths. Such adaptation of the stop mechanism can be made duringassembly, and requires no modification,ofstructural forrn of the individual parts. For example, by assembling the

nuts

102 and 103 on shaft 30 farther apart by one thread than the positions shown in Fig. 2, the range of rotational movement to which brush 85 is confined is reduced by one full revolution. Similarly, that range may be increased or decreased by any integral number of revolutions within the capacity of the structure by suitable relative axial spacing ofthe two nuts on their threads. Further modification of the range of movementmay be provided by varying the relative rotational positions of the several cooperating elements of the mechanism. For example, a plurality of holes 107, shown in Fig. 5 as six, may be provided in the upper face of the nut 103 in different rotational relations to spline pin 10S; and during assembly a stud may be mounted in a selected one of those holes to produce .the stop formation 111. trative manner, the` range of movement of .brush-85 may ln that illusl be varied conveniently from one instrument to the next by a fractional number of revolutions.

The ends of winding 60 are anchored with respect to the supporting drum 42 at points spaced beyond the respective ends of the range of movement to which brush 85 is limited by stop mechanism 100. lust enough wire is allowed between the points of extreme travel of the brush and the effective points of anchorage of the wire to the drum to permit normal and uniform interaction of the brush and wire over the Whole range of brush travel. Preferred anchoring means, as illustratively shown, coniprise a slotted stud 65, set firmly but rotatably in drum 42. The head of stud 65 is slotted and is preferably positioned ywithin a recess 66 and below the level of threads 62. The winding end is secured to stud 65 in any suitable manner, for example by solder, as indicated at 67. After the winding has been thus anchored, its length can be altered conveniently by turning stud 65 in its radial bore, as by a screwdriver applied to its slotted head. The tension of the winding is thereby adjustable much in the manner of a violin string.

Channels may be formed through the body of the drum, as indicated at 68 and 69, through which connection may be made between the ends of the winding and respectivel binding posts set in the rear face of the dielectric drum flange at 77 and 7S. In Figs. 3 and 4 axial channel 69 is shown with a connecting radial bore 73 which opens into the recess 66 at the forward end of the winding, and gives access for connection to stud 65 to which that winding end is anchored at 67'. A wire'of relatively low resistance is indicated at 74 leading from that stud through channels 73 and 69 and radially outward through a second connecting radial bore 75 closely adjacent the rearward wall of drum fiange 44. A similar connection 76 from the rearward end of winding 60 passes through channel 68 via suitably placed radial bores.

Wires

74 and 76 are connected to the respective

terminal posts

77 and 78, which are threaded into the drum flange and are guided in insulation relation through cover plate 52 as by the rubber grommets 79, and which provide external terminal posts for making electrical connection to the resistance element of the potentiometer.

In assembling the winding, it is found convenient first to connect the rearward end of the winding to its stud 65 and then to start Winding from the rear forward, the brush. being already assembled but free of the winding.. The drum is slowly rotated, with the brush held stationary, so that the brush in effect follows the wire as it falls-.into place in the threads of the drum. The last Aturn of the Winding is passed over the brush and then connected as at 67 to its stud 65, as by solder. Before that connection, the tension of the winding is adjustedroughly to draw the brush nearly into' parallel relation with .the vouter cylindrical face of the drum. Final adjustment of the' tension of the winding, preferablyV bringing the brush accurately into axial alignment with the drum surface, may then be made by turning the studsS, in the manner already described.

MeansV are preferably provided for convenient and accurate adjustment of the angular position of the brush on control shaft 30. 4A `particular' advanatage of such adjustment is yto facilitate productionof the desired relation between the'endcpointsof the brush-movement, as :limited `byv stop mechanism 100,` andA the anchored ends 'ofwinding 60. 'The windingends are preferably anchored 4at points spaced angularly (as measuredalong the Windinglby more than the angle through which the brush is allowed to turn by stop'mechanism 100, the excess correspondingsubstantially to the angular length of the portion61 of the winding that is supported by thebrushin spaced relation to mounting drum 42. The brush is then preferably-.so adjusted on its shaft that' its averages angular range of movement is centered within the free portion of winding 60 between its anchored ends.

In the present embodiment, such adjustment is provided by mounting brush support 82 on shaft 30 by frictional means that lock the rotational position of the brush on the shaft effectively positively during all normal operation of the potentiometer, and yet permit their manual relative yrotation when adjustment is required, as during assembly. For example, the shaft may be rotated to one end of its range of movement, coming to a definite position determined by stop mechanism 14MB, and brush holder 30 may then be turned manually against its frictional mounting on the shaft to bring the brush to the desired distance from the winding anchor point 67. As shown, such adjustment is made possible by interposing a spring washer 89 between brush support 82 and upper mounting nut 84. Before mounting the brush assembly, lower mounting washer 83 is adjusted on the shaft to retain bearing 29 in position to reduce shaft end play to a minimum. Nut 84 is turned down over brush support 82 to compress washer 89 appreciably, but not to its solid thickness, producing suiiicient friction to make brush holder 82 effectively rigid on the shaft during normal operation, but readily movable manually for adjustments of the type described.

In the embodiment of Figs. l-5, brush 85 moves in a circular path about the axis of the winding. Because of the helical form of the winding, the contact point at 63 between winding and brush progress longitudinally of the brush in an axial direction. Alternatively, the brush may be caused to move in a helical path, so that it progresses axially with a pitch equal to that of the winding. ln Figs. 6 and 7 is shown an illustrative embodiment providing that type of action, and in which axial movement of the brush is coordinated with the axial movement of the stop mechanism.

As illustratively shown in Fig. 6, Shaft 110 is journaled near its forward end in frame 100, and is threaded at 112 to receive the single stop nut 120 at the rear of the

journal bearings

116 and 117. Two stop nuts may be used if preferred, as already described. Nut 120 carries the

stop formations

121 and 122 on its respective axial faces and the spline pin 124, which is slidingly received in axial channel 126 of frame member 100. Stop pins 128 and 129 are fixed in the shaft and cooperate with the stop formations of the nut, in a manner already explained, to limit relative rotation of the shaft and frame to a predetermined range.

The brush carrier 130 of Fig. 6 has a sleeve portion 131 that is axially slidable on the crests of the threads 112 and is caused to rotate with the shaft, as by the axial slot 132 which receives stop pin 129. A compression spring 136 acts between the brush carrier and a disk 138 fixed on the rearward end of shaft 110. That spring urges the forward end of brush carrier sleeve 121 yieldingly against nut 120, causing the brush carrier to move axially with the nut.

The brush, as shown, comprises a resilient strip 140, one end of which is rigidly mounted in insulated relation on brush carrier 130 and which carries at its free end a contact formation 141. Electrical connection from the brush may be made via a light flexible wire 143 to a contact shoe 145 mounted in insulated relation in disk 138 and resiliently engaging the inner end of a fixed terminal post 146 on the axis of rotation of the shaft. Brush carrier 139 and disk 138 are preferably of insulating material.

The resistance winding 159 is carried in a thread-like helical channel 152 formed in the outer cylindrical surface of dielectric drum 154. The latter is mounted on frame 1S@ and forms with it a winding support of cylindrical shell formation. Winding channel 152 in the present instance Ihas a helical pitch substantially equal to the pitch of shaft threads 112. One turn of Winding 150 passes slidingly over contact formation 141 of the brush at 151 and is supported in spaced relation to the drum surface.

Rotation of shaft causes brush 140 to rotate with it, lifting the resistance winding progressively from its channel and permitting it to drop back into that channel after passage of the brush. At the same time, the brush is caused to move axially with nut 120, with the result that contact formation 141 follows the helical form of the winding. Contact is therefore always made at substantially the same point of the brush. The ends of the winding may be anchored in any suitable manner, for example as previously described.

A particularly advantageous feature of the embodiment of Fig. 6 is the dual function of nut 120. It will be noted that travel rof nut performs the two functions of actuating the stop mechanism, in a manner analagous to nut-

s

102 and 103 of the previously de scribed embodiment, and `of driving the axial movement of brush 140, a function not present in the previous embodiment.

Figs. 8 and 9 represent an illustrative type of brush structure, which can be employed, for example, in connection with the overall construction of either of the previously described embodiments, but which is here described typically as a modification of the embodiment 4of Figs. l-5. As in those figures, winding 60 is carried in helical channel 62 in the convex cylindrical surface of drum 42, and brush carrier 82 is mounted lon a shaft journaled on the axis of that drum. In the present modification the brush proper comprises a structure adapted to slide in, and be guided by, direct contact with helical channel 62 and to support the winding in spaced relation with the channel at substantially a single point. Structure 160, as shown, is formed from a single piece of sheet metal bent in U-form, with the bend of the U suliiciently pointed in section to form a V-edge 162 that will tit and ride positively in the thread channel 62. Each leg of the U is formed with two iiat shoulders 164 with a projecting tongue 165 between them, all four shoulders 164 lying in a common plane parallel to edge 162, and the two tongues being aligned substantially at right angles to that edge, allowance being preferably made for the helix angle of threads 62. The V-edge 162 is preferably curved to conform to the curvature of those threads, as shown best in Fig. 9. The interior of the U, opposite edge 162, is preferably more smoothly curved, as indicated at 163, providing a suitable surface over which the resistance wire may slide as the contact member moves under it.

The movement of brush 160 longitudinally of helical channel 62 is controlled by an arm 170, which is shown of fiat and somewhat resilient material having one end secured rigidly to brush carrier 82 and extending parallel to the axis of the instrument closely spaced radially :outward of the winding. A longitudinal slot 171 in arm freely receives the two tongues 165 of the brush proper, and the inner face of the arm rests flatly on the shoulders 164. Arm 170 is so formed as to normally lie closer to threads 62 than shown in Figs. 8 and 9, so that it urges brush 170 yieldingly into firm but light engagement with those threads. 'lio accommodate the axial movement of brush 170, and provide reliable electrical Contact, a flexible connection 172 is preferably provided between the brush proper and arm 170. The arm may be connected via screws 173 and a contact shoe 86a to an external terminal similar to 90 of Fig. 2, for example.

An advantage 'of the described structure is that it permits reduction to a very low value the distance that resistance winding 60 must be lifted from its normal position in channel 62. Particularly in view of that relatively short distance, the inherent resiliency of the resistance wire is sutiicient for effective operation, and

resilient structure in the brush itself is ordinarily not necessary.

I claim:

l. A variable electrical resistance structure, comprising a cylindrical shell, a resistance winding carried in insulated relation on the convex surface of the shell, a mounting plate at the forward end of the shell and normal to the axis thereof, a control shaft journaled on the axis of the shell and operable from forward of the mounting plate, brush means mounted `on the shaft and extending therefrom at the rear of the shell including a substantially linear contact formation extending forwardly substantially parallel to the convex surface of the shell and closely spaced radially outwardly thereof, a portion only lof the winding being -supported by the contact formation in spaced relation with the shell, said shaft having a thread within said shell, a nut engaging said thread, said nut being slidingly keyed with respect to said shell, and a stop formation arranged to engage said nut to limit the' rotation of said shaft to a predetermined limited range that includes more than one rotation.v

2. A variable electrical resistance'structure as defined in claim l, and in which the brush means includes a brush support slidingly mounted on the control shaft for axial movement thereof and slidingly keyed thereto, and resilient means urging the brush support toward the nut.

3. A variable electrical resistance structure, comprising a generally cylindrical support, a brush mounted for relative rotation with respect to the support about the axis thereof and spaced radially outward of the convex surface of the support, and a .resistance winding supported on the convex surface of the support, one turn of the winding passing radially outward of the brush t in electrical contact therewith, and means for adjustably varying the tension of the winding, said means comprising a slotted stud in said support secured to said winding at one end.

4. A variable electrical resistance structure, comprising a generally cylindrical support, a brush mounted for relative rotation with respect to the support about the axis thereof and radially outward of the convex surface of the. support, stops means acting between the brush and the support to limit movement of the brush with respect to Athe support to a predetermined range that includes more than one complete relative rotation, said stop means including a pair of engaged helical thread elements, one of said elements being coupled to said brush to prevent rotation thereof relative to the brush and the other being coupled to -said support to prevent rotation thereof relative to the support, and a resistance wire supported on the convex surface of the support, a portion only of the wire passing radially outward of the brush in electrical contact therewith, the wire being anchored at its ends with respect to the support at points spaced beyond the ends of the said range of movement of the brush with respect to the support.

5. A variable electrical resistance structure, comprising: a cylindrical support member, a control shaft member journaled on the axis of the support, threaded means coaxial with the shaft within the support, the said means being slidingly keyed with respect to one of those members and in threaded relation to the other, and stop formations`r carried Vby the said means and by the said other member, respectively, and cooperating to limit relative rotation of the shaft and the support to a predetermined limited range that includes-more than one revolution, brush means mounted for rotation with the control shaft and including a contact formation closely adjacent the convex surface of the support, a resistance winding carried in insulated relation on the convex surface of the support, a portion only of the winding being supported by the contact formation in spaced relationr to the support, the windingbeing anchoredwith respect to the support at points-.spaced beyond the ends of the range of movement-of the contact formation.

6. A variable electrical resistance structure, comprising a generally cylindrical support, a control shaft journaled on the axis of the support, two threaded elements coaxial with the shaft within the support, the said elements-being slidingly keyed to one of those members and in threaded relation to the other, the elements being 4spaced axially rof the threads, a stop formation carriedvby each of the elements, two stop formations carried by 'the said other member and cooperating with the respective first mentioned stop formations to limit relative rotation of the shaft and the support to a predetermined limited range that is variable by variation of vthe said axial spacing of the two threaded elements, brush means mounted for rotation with the control shaft and including a contact formation closely adjacent the surface of the support, a portion only of the winding being supported by the contact formation in spaced relation to the support, the winding being anchored with respect to the support at points spaced beyond the ends of the lrange of movement of the contact member.

7. A variable electr-ical resistance structure, comprising a cylindrical support, a control shaft journaled on the axis yof the support for relative rotation with respect thereto, a nut coaxial with the shaft within the support, the nut being slidingly keyed with respect to one of those members and in threaded relation to the other, stop formations carried by the nut and by the said other member, respectively, and cooperating to limit relative rotation of the shaft and the support to a predetermined range, brush means mounted for rotational movement with't'he shaft and for axial movement with the nut and including' a contact member movable by virtue of shaft rotation along a helical path of limited length closely spaced radially outward with respect to the convex surface of the support, the length of the said path being limited by virtue of the said limited range of relative rotation'of the shaft and the support, a resistance winding helically wound in insulated relation on the convex surface of the support with a pitch substantially equal to the pitch' of thev threads between the nut and the said other member, a portion only of the winding electrically contacting the contact member and being supported thereby in spaced relation to the support, and means anchoring the winding at points spaced beyond the respective limits of the said helical path of movement of the contact member.

8. A variable electrical resistance structure, comprising a cylindrical shell, a control shaft journaled on the axis 'of the shell, a nut in threaded relation to the shaft and slidingly keyed to the shell, a brush carrier axially movably mounted on the shaft for rotation therewith, means causing the brush carrier to move axially of the shaft with thenut, stop formations carried by the nut and by the shaft, respectively, and cooperating to limit the relative rotation of the shaft and the shell to a range that includes more than one full revolution, a brush carried by the brush carrier for axial and rotational movement therewith in a helical path closely adjacent the convex surface of the shell through a limited range that corresponds to the said range of relative rotation of the shaft with respect to the shell, a resistance winding helically woundy in insulated relation on the convex surface of the shell with a pitch substantially equal to the pitch of the threads between the nut and the shaft, a portion only of the winding electrically contacting the brush and being supported thereby in spaced relation with the shell, and means anchoring the winding at points spaced beyond the respective ends of the said range of movement of the brush.

9. A variable electrical resistance structure, comprising a cylindrical support having a generally helical channel in its convex surface, the walls of the channel being of I dielectric` material, a resistance winding normally lying freely in the channel, a contact shoe slidable in the channel, control means for the contact shoe comprising a member relatively rotatable with respect to the cylindrical support about the axis of the support said member being in radially spaced relation to the convex surface of the support and slidingly engaging the contact shoe, the contact shoe supporting a portion only of the resistance winding in spaced relation to the channel walls and electrically contacting the resistance winding within the said portion thereof.

10. A variable electrical resistance structure as defined in claim 9 and including a slotted stop formation projecting from the convex surface -of the support in position to limit movement of the shoe longitudinally of the channel, the resistance winding being received by the slot in the stop formation.

Y 1l. A varia-ble electrical resistance Structure, comprising a cylindrical support having a generally helical channel in its convex surface, the walls of lthe channel being of dielectric material, a shoe riding in the channel and having a portion extending radially Ifrom the body of the shoe, a shaft journaled 'on the axis of the support, a control member mounted on the shaft for rotation therewith and spaced radially from the convex surface of the support, the control member having an 'axially extending slot in which the radially extending portion of the shoe is slidingly received, a resistance wire vnormally lying freely in the channel, a portion only `of the resistance winding being electrically engaged by the shoe and supported thereby adjacent the point Iof engagement in spaced relation fto the channel walls.

12. A variable electrical resistance structure, comprising a cylindrical support, a control shaft journaled on the axis `of the support, stop means acting between the control shaft and the support to limit their relative rotation to a definite predetermined yangular range, a resistance wire helically wound on the convex surface of the support with its ends anchored thereto at points that are mutually spaced by more than the said angular range, brush means electrically contacting Iand supporting a portion only of the wire in spaced relation to the support, and means mounting the brush on the shaft for rotation therewith through a limited angular range corresponding to the said angular range of the shaft, the said mounting means normally defining the rotational position of the `brush on the shaft eectively rigidly and permitting manual relative rotational movement of the brush and shaft to center the angular range of movement of the brush between the anchored ends of the winding.

13. A variable electrical resistance lstruct-ure comprising a frame, a control shaft journaled on said frame and rotatable abou-t an axis, convex cylindrical support means on said frame and concentric with said axis, a brush means on said frame, one of said means being xed to said frame, the yother of said means being revolvable about said axis and being coupled to 'said shaft to be revolved thereby relative Ito said `one means, a pair of thread elements in engagement with each other and concentric about said shaft, one of said thread elements being movable in the direction o-f the length of lsaid axis, one of said lthread elements being coupled to said shaft -to be revolved thereby about said axis and the other being coupled to said frame to prevent its revolution, whereby said movable thread element is moved 'axially `as said shaft is rotated, a pair of cooperating stop formations to limit the relative revolution of said means to a predetermined limited range that includes more than one revolution, one of said stop formations being axially movable and connected to said axially movable thread element, one Iof said stop formations being coupled to said shaft to be revolved thereby about said axis and the other being coupled to said frame to prevent i-ts revolution, said brush means including a contact formation closely adjacent the convex support means, a resistance winding carried in insulating relation on said convex support means, a portion of the winding being supported by the contact formation in spaced relation to the support, the winding being anchored to said convex support means at points spaced therearound beyond the ends of said limited range of relative revolution.

14. A variable electrical resistance structure as claimed in claim 13 and in which said convex cylindrical 'support means defines a generally helical channel of uniform cross section having walls fof insulated material, said resistance winding normally lying freely in said channel, said Contact formation having an extremity shaped to fit in said channel, the pitch of said channel being substantially equal to the pitch of said thread elements.

15. A variable electrical resistance structure comprising a frame, a control -shaft j'ournaled on said frame and rotatable about an axis, convex cylindrical support means on said frame and concentric with said axis, a brush means `on said frame, one of said means being fixed to said frame, the other of -said means being revolvable about said axis `and |being coupled to said shaft to be revolved thereby relative to lsaid one means, at least a portion `of said shaft being threaded, a threaded element slidingly keyed to said frame lfor movement axially of said shaft 4and engaging the threaded shaft portion, a stop formation fixed to the shaf-t arranged to engage said threaded element to limit relative revolution of said means to a predetermined limited range that -includes more than yone relative revolution, and a resistance winding carried 4in insulating relation on said con-Vex support means, a portion of the winding -being engaged by said brush means, the winding 'being anchored to said convex support means `at points more widely spaced than the ends of said range of relative revolution.

16. A variable electrical resistance structure as claimed in claim 15 and ,wherein said Ithreaded element is a nut on said shaft, said nut having a plurality `of stop holding positions, and a stop removably secured to said nut at one Aof said positions, said stop yformation being arranged to engage said stop.

17. In a variable electrical resistance structure of the multi-turn type having a helical resistance element with a plurality of turns carried in insulated relationship on a supporting drum and a contact element engaging said resistance element, said resistance and contact elements being revolvable with respect to each other, and a drive shaft arranged to revolve said resistance and contact elements with respect to each other: positive-acting precise stop mechanism enabling said resistance and contact elements to revolve with respect to each other for more than one complete turn and yet positively and precisely stopping said revolution at a predetermined position comprising a rotatable feedscrew driven by said shaft in predetermined relationship with respect to the revolution of the resistance and contact elements with respect to each other, a travelling member engaging said feedscrew, said travelling member including means cooperating with said supporting drum to prevent rotation of said member on said feedscrew while allowing translation of said member along said feedscrew as said feedscrew rotates, a first stop element on said member and a second stop element driven by said shaft and revolvable with respect to said iirst stop element, said first and second stop elements missing each other during one turn of said feedscrew and said second stop element positively abutting against said first stop element during a subsequent turn of said feedscrew.

18. A variable electrical resistance structure comprising a generally cylindrical support having a convex surface with a helical channel therein, a brush mounted for relative rotation with respect to the support about the axis thereof and spaced radially outward of the convex surface of the support, a resistance winding supported on the convex surface of the support in said helical channel, one turn of the winding passing radially outward of the brush in electrical contact therewith, and means for adjust'- ably varying the tension of the winding comprising a rotatable lstud` secured to, saidw'indingat one endend-positioned within` a recess in said support below therlevel of said helical channel, and afterminal postelectrically connected to said rotatable stud; l

19. A variable electrical resistance structure compris.- ing a frame, a control shaft journaled onv said frame and rotatable about yan axis, convex cylindrical support means on said frame and concentric with said axis,l a brush means on said frame, one of said means being fixed to said frame,. and the other of said means being revolvable about said axis and being coupledto said shaft to be revolved thereby relative `to ysaid one means, apart on said shaft being threaded,- said support means having `a recess therein extending longitudinally thereofa threaded element engagingY the threaded part on the vshaft and having a projectingportion eiectively slidingly keyed in'to said recess-in the support means for movement axially of said shaft While preventing rotation of said threaded element with respect to said support means, a pair'of spaced stop formations fixed vto the shaft on eithersidevof said threaded element, said threaded element travellingnaxially with respect to said shaft betweenl said stop formations and arranged to engage said stop formations at leach end of its travel to limit relative revolution of said means to a predetermined limited range'that includes more-than one relative revolution, and a resistance winding carried in insulating relation on said convex support means', a portion of the winding being engaged by said brush means.

References Cited in the-le of this patent UNITED STATES PATENTS `1,918,013 v Bermann July 11, 1933 2,442,469 Palya June 1, 1948 2,495,321 Gibbs et al. Jan. 24, 1950 2,519,752 Fox` -Aug. 22,1950 2,533,656 Wills Dec. 12, 1950 2,662,150 Mars Dec. 8, 1953