US2706230A

US2706230A - Variable resistor

Info

Publication number: US2706230A
Application number: US292214A
Authority: US
Inventors: Marlan E Bourns; Marvin E Harrison; Edward J Goeppinger; Robert M Whitehorn; William M Hedges
Original assignee: Bourns Inc
Current assignee: Bourns Inc
Priority date: 1952-06-06
Filing date: 1952-06-06
Publication date: 1955-04-12
Anticipated expiration: 1972-04-12

Description

April 12, 1955 M. E. BOURNS ET AL VARIABLE RESISTOR 3 Sheets-Sheet 1 Filed June 6, 1952 lNVE/VTOQS MAELAN. E. BOUlZ/VS MARVIN E. HARE/JON EDWQQD J GOEPP/NGEQ 203527 M. WH/TEHOQN WILL/AM M. 1 150 55 A4414! 2'. W AGENT April 12, 1955 VARIABLE RESISTOR 3 Sheets-Sheet 2 Filed June 6, 1952 fit; I 5

BY A 11! 5 W AGENT April 12, 1955 M. E. BOURNS EIAL 2,706,230

VARIABLE RESISTOR Filed June 6, 1952 a Shets-Sheet s 2'1 .l'i; 155: a 2 I40 IN VENTOES [44 22 MAQLAN E. Bow/vs MARVIN E. HARE/SON EDWAQD J GOEPP/NGE ROBERT M. WHITEHO WILL/4M M HEOGES Y/WZW fiGENT United States Patent VARIABLE RESISTOR Marlan E. Boums, Marvin E. Harrison, Edward J. Goeppinger, Robert M. Whitehorn, and William M. Hedges, Riverside, Calif.; said Harrison, said Goeppinger, said Whitehorn, and said Hedges assignors to said Boums Application June 6, 1952, Serial No. 292,214 22 Claims. (Cl. 201-62) The present invention relates generally to variable resistors, and more particularly to potentiometers, although certain aspects of the invention are applicable to other types of electrical instruments.

The primary object of the invention is to provide a new and improved variable resistor that is extremely compact, light in weight, simple in construction, accurate in response, and easy to manufacture and service.

Another object is to provide a linear type variable resistor wherein the contact-carrying member is shifted linearly within its case by means of a pin or other lateral projection extending through a slot in the side of the case. In this same connection, a further object of the invention is to provide a new and extremely effective closure means for sealing the said slot against entrance of dirt, moisture, or other foreign matter into the interior of the case.

Another object is to provide a new type of conductor particularly adapted for use with a sliding contact, which utilizes the minimum amount of metal, and at the same time, provides a precise contact surface lying entirely within a single plane. This is an important factor, owing to the exceedingly high cost of the platinum alloy required for this type of service; the said alloy being characterized by a very low contact resistance. Because of its high cost, it is imperative that the amount of metal used in the conductor be held to the absolute minimum. However, as the cross section of the conductor is reduced, it becomes increasingly more ditiicult. to produce and maintain a perfectly level contact surface, and in the small size required for this type of instrument, it has long been a serious problem to obtain a contact surface of the necessary precision. The present invention supplies a novel and eminently satisfactory solution to the problem by providing a conductor formed of a thin strip of sheet metal of the desired alloy, bent to a substantially U-shaped cross section. The bend of the U presents a contact surface lying entirely within a single plane, and the arms of the U are preferably inserted down into a channel formed in a supporting part of dielectric material. The bend of the U projects above the surface of the supporting part, and the sliding contact rides on the crest of the bend.

A further object of the invention is to provide a new and improved arrangement for slidably supporting the contact-carrying member within the case and preventing the same from turning therein.

Another object of the invention, in one of its aspects, is to provide a linear motion variable resistor wherein the operating means for shifting the contact-carrying member is connected to the latter in a manner to prevent the transmission of torsional force to the said member. This object is accomplished by providing an operating rod that is rotatably connected to the contactcarrying member, so that if the rod is turned, the said member is unaflected.

A further object is to provide a novel spring means interposed between the rotatable operating rod and the contact-carrying member, so as to take up all endwise clearance between the rod and the member, thereby eliminating any back lash or shake between the parts.

Another object is to provide a new and unique arrangement for securing the contact elements to the sliding support member.

Another object is to provide a double contact arrangement, whereby the conductor is engaged at two 2,706,230 Patented Apr. 12, 1955 longitudinally spaced points thereon by two contacts mounted on separate spring fingers and individually pressed against said conductor by the spring force of their respective fingers. The engagement of either .of the contacts is thus independent of the other contact, and the likelihood of an open circuit in the instrument caused by failure of the sliding contact member to engage the conductor is greatly minimized.

Still another object of the invention is to provide a construction wherein guide rods for the sliding contact member are interlocked by a pin, thereby securing the rods against endwise movement with respect to the case, and the projecting end of the said pin is also seated in a hole in a cover plate carrying the resistance element and conductor engaged by the contact memher, so that the said cover plate is accurately positioned on the case, with the conductor and resistance element parallel to the axis of the guide rod. The pin also prevents lateral shifting of the cover relative to the case, which would cause misalignment of the conductor and resistance element with respect to the path of the contact member.

The foregoing and other objects and advantages of the present invention will become apparent to those skilled in the art, upon consideration of the following detailed description of two illustrative embodiments thereof, reference being had to the accompanying drawings, wherein:

Figure l is a variable resistor vention;

Figure 2 is an elevational view of the same, with the cover plate removed and turned inside face up, so as to show the internal arrangement of the instrument;

Figure 3 is a transverse section through the unit;

Figure 4 is a longitudinal vertical section through the unit, taken substantially at 44 in Figure 3;

Figure 5 is a perspective view of the U-shaped conductor formed of sheet metal, which is one of the more important aspects of the invention;

Figure 6 is a side elevational view of another embodiment of the invention;

Figure 7 is a top plane view of the same, with the cover plate removed and turned inside face up, so as to show the internal arrangement of this form;

Figure 8 is a transverse section through the instrument of Figure 6;

Figure 9 is an exploded perspective view of the contact-carrying member and end of the operating rod, showing the manner of attaching the contact elements to the member, as well as structural details of the rotatable connection between the operating rod and member; and

Figure 10 is a longitudinal vertical section through the contact-carrying member, taken at the median plane thereof.

Referring first to Figures 1 to 5, inclusive, we have designated this embodiment of the instrument in its entirety by the reference numeral 12, and it will be noted that the unit is contained within an elongated metal case 14 having a cavity 16 formed in one side thereof. A cover plate 18 of phenolic resin or other suitable dielectric material is secured to the side of the case 14, closing the cavity 16 to form a chamber 20, within which is disposed a linearly movable contactcarrying member 22.

Projecting upwardly from the top of the case 14 along the side opposite the cover plate 18 are two spaced mounting flanges 24, each of which is slotted at 26 to receive mounting screws. The side of the case below the flanges 24 is milled off to provide a shallow, fiat recess 26, and seated in said recess is a thin metal plate 28, which is secured to the case by rivets 30. The plate 28 has a longitudinally extending slot 32 provided therein, and projecting through the slot 32 and through a second slot 33 in the side of the case is a laterally extending pin 34, which is adapted for connection to the operating mechanism or instrument with which the unit 12 is to be used.

The inner end of the pin is cut down somewhat in diameter and extends through a hole in the member perspective view of a linear motion embodying the principles of our in- 22, being headed at 36 to secure the pin to the member. The pin 34 may either be fixed solidly to the member 22, or may be free to turn with respect thereto, depending upon whether the pin is apt to "be subjected to torsional forces, or to pure translational movement. Where torsional forces are anticipated, the pin 34 would be allowed to turn freely in the member 22, so that no torque could be transmitted to the member.

The slot 33 in the case is sealed against the entrance of dirt, moisture, or other foreign matter into the chamber by means of a sliding closure 38, which completely covers the slot at all positions of the pin 34. The closure 38 comprises a thin, fiat plate having a hole at 40, through which the pin 34 projects. The closure plate 38 is seated in a shallow groove 42 milled into the side of the case along both edges of the slot 33, and is slidably confined along its edges between the plate 28 and the groove 42. The length of the plate is such that the ends of the slot 32 remain covered when the pin 34 is at the opposite extremity of its travel, and clearances around the edges of the plate are held as close as is consistent with free movement of the member 22.

The member 22 may be in the form of a block of metal, as shown, which is slidably supported on a pair of vertically spaced,

parallel guide rods

44 and 46. The

rods

44, 46 extend longitudinally through the chamber 20 parallel to the

slots

32, 33, and their ends are received within holes 48 in the ends of the case 14. Each of the rods has a peripheral groove 50 cut in one end thereof. A pin 52 is inserted in a hole 54 in the case 14 and engages the groove 50 to secure the rod against endwise movement with respect to the case. The end of the pin 52 projects above the surface of the case, and is received within a hole 56 in the cover plate 18. By virtue of this arrangement, the cover plate is accurately positioned on the case 14 and is prevented from shifting laterally with respect thereto by the same pins that secure the

guide rods

44, 46 against endwise movement.

Mounted on the inner side of the cover plate 18 parallel to the line of travel of the member 22 is a conductor 58 and resistance element 60; although the instrument can be made up with a number of resistance elements and conductors, all arranged parallel to the line of travel of the member 22. The resistance element 60 may take any desired form, but is herein shown as a flattened coil of fine resistance wire wound on a core 62. The coil is seated in a channel 64 formed in the cover plate 18, and is cemented or otherwise secured therein, with one edge of the coil projecting slightly above the inner surface of the cover plate. The ends of the coil are connected by wires 65 and 66 to

terminals

67 and 68, the said wires extending through channels 69 and 70 in the cover plate. The

terminals

67, 68 are preferably stud inserts, and may conveniently be molded into the cover plate.

The conductor 58 .is a unique feature of theinvention, and comprises a thin strip of low contact resistance platinum alloy sheet metal bent into a substantially U-shaped cross section, presenting a fold line or bend at 72, which is perfectly straight and level from one end to the other, and which lies entirely within a single plane. By the term U-shaped we also include other similar shapes, such as cross sections resembling the letters C, J, V, and the like. The ends of the U are inserted down into a channel 74 in the cover plate 18 parallel to the channel 64, and the conductor is cemented or otherwise secured in place. A wire 75 is passed through holes in both arms of the U, as shown in Figure 5, and the end thereof is bent back against the side of the conductor and soldered thereto. The wire 75 passes along a channel 76 in the cover plate, and is connected to a terminal 78.

Running on both the conductor 58 and resistance coil 60 is a sliding contact 80, which is mounted on member 22. The contact 80 comprises a U-shaped strip of spring copper, the base of which is riveted to a plate 81 of dielectric material, such as phenolic resin, and the ends of which extend parallel to the conductor 58 and coil 60, in registration therewith. Plate 81 is secured by rivets 82 to member 22. The ends of the U-shaped copper strip are bent out at an angle to the surface of the plate 81, and form spring fingers 83 and 84. Contact points 85 of platinum alloy or other low resistance metal are attached to the ends of the fingers 83, 84, and these points ride on the conductor 58 and coil 60.

A second contact element 86 of L-shaped contour is secured to the member 22 on top of the U-shaped strip, and one arm of the L lies on top of and parallel to the arm 84, forming a spring finger 88. The contact point 85' on finger 88 engages the conductor 58 at a point thereon spaced longitudinally from the contact point of the underlying finger 84. Each of these two contact points is independently pressed against the conductor by the spring pressure of its respective finger 84 or 88, and the likelihood of an open circuit caused by lack of contact with the conductor 58 is thereby greatly reduced. A double contact of this kind can only be used, however, with a strip conductor wherein the position of the contact points along the length of the conductor has substantially no etiect on the values being measured. For resistance elements, it is necessary to limit the electrical connection to a single point of contact, owing to the fact that the values being measured vary with the position of the point of contact along the length of the element.

Used as a potentiometer, the instrument is connectedinto a bridge circuit to measure the ratio of voltages obtained across

terminals

67, 78 and across

terminals

68, 78. The voltage ratio is varied by sliding the member 22 along the

guide rods

44, 46; and the operating pin 34 can be connected to a movable member, such as an aircraft control surface, for example, to provide a signal that can be interpreted to give the precise position of the said movable member at all times. The instrument can also be connected into a circuit to provide a control action responsive to the position of the member 22 along the length of the resistance coil 60. Such uses of the instrument are well known in the art, and need not be described in more detail here.

As Will be noted in Figure 3, the member 22 is provided with a hole 90 in its upper portion, through which the guide rod 44 passes, and a channel 91 extending up into the member from the bottom surface thereof, within which the guide rod 46 is disposed. The guide rod 46 does not touch the bottom of the channel 91, but instead, rides on opposite side walls of the channel adjacent the bottom. This arrangement provides a low-friction sliding support for the member 22, and allows the member to accommodate a substantial amount of non-parallelism between the guide rods. Bosses 92 and 93 are provided at opposite ends of the cavity 16, and these are engageable by the ends of the member 22 to limit the travel thereof in either direction.

Referring now to Figures 6 to 10, inclusive, we have designated the second embodiment of our invention by the reference numeral 112. As in the other case, the instrument consists of a body or case 114 having a cavity 116 provided therein. The cavity 116 extends all the way through the body from top to bottom, and is closed on both sides by two cover plates 118, enclosing a chamber 120. Slidable longitudinally within the chamber 120 is a member 122 carrying contact assemblies 124 on the top and bottom sides thereof. Resistance coils 126 and U-shaped conductor strips 128 are mounted within channels in the cover plates 118, and are positioned parallel to the line of travel of the member 122. The ends of each resistance coil are connected to

terminals

138 and 132 on their respective cover plates, while the conductor strips are connected to terminals 134. The unit illustrated also has a center tap 136 to the mid-point of the coil 126, which is connected to a terminal 138.

The sliding member 122 is made of a dielectric material, and may conveniently be molded of phenolic resin to the block form shown in the drawings. Fastening means, preferably in the form of U-shaped staples 140, are molded into the block as inserts, with their ends projecting outwardly from the top and bottom surfaces thereof. Each of the contact assemblies 124 is like the construction described in the first embodiment, and comprises a U- shaped element 142, with an L-shaped element 144 disposed on top thereof. Both elements have holes 146 provided in their base portions, and the projecting ends of the staples are inserted through these holes and then bent down flat on top of the outer element 144, to secure the assemblies to the block 122. The bent over staples are then soldered in place.

The block 122 has a hole 148 formed therein, through which a stem 150 on the end of an operating rod 152 is inserted. The rod 152 is freely rotatable within the hole 148, and is secured in the block by an E-washer 154, which seats in a circumferential groove 156 in the stem 150.

projecting above the surface of On its opposite side, the block is counterbored at 158, and seated within this counterbore is a rippled spring washer 160, which bears on one side against the bottom of the counterbore, and on the other side against the shoulder 162 at the junction of the stem 150 with the rod 152. The spring washer 160 provides a spring means between the block 122 and operating rod 152, taking up all endwise clearance between them. The rippled contour of the washer provides a plurality (four, in the embodiment shown) of circumferentially spaced points of spring contact, which insures stability and prevents the block from rocking with respect to the rod 152.

The block 122 is held against turning with respect to the case 114 by the fiat, parallel inner surfaces 164 on the sides of the case, which are spaced apart to provide the minimum clearance between the said surfaces and the sides of the block for free movement of the latter. Any rotational tendency of the block 122 is prevented by the engagement of the sides of the block with the surfaces 164 of the case.

The operating rod 152 has its axis parallel to the line of travel of the member 122, and slidably projects through a hole in the end of the case. The outer end of the rod can be attached to the movable part with which the instrument is associated, and axial movement of the rod causes the contact-carrying member 122 to move linearly within the case. The freely rotatable connection between the rod 152 and member 122 prevents transmission of any torsional force to the latter.

While we have shown and described in considerable detail what we believe to be two preferred forms of our invention, it will be understood that the invention is not limited to these details, and that various changes may be made in the shape and arrangement of the several parts without departing from the broad scope of the invention, as defined in the following claims.

We claim:

1. A variable resistor comprising a case, a member supported on said case for movement relative thereto, a re sistance element and a conductor disposed parallel to the line of travel of said member, said conductor comprising a thin strip of electrically conductive sheet metal bent longitudinally to present a fold lying within a plane tangent to the curve of the fold, and a sliding contact on said member running on the crest of said fold and traveling longitudinally along the length of said conductor.

2. A linear motion variable resistor comprising a case, a member supported for linear movement with respect to said case, a resistance element and a conductor disposed parallel to the line of travel of said member, said conductor comprising a thin strip of electrically conductive sheet metal bent along a longitudinal line to present a crown of short radius, the crest of which lies entirely within a single plane, and a sliding contact on said member running on said crest and traveling longitudinally along the length of said conductor.

3. In an electrical element, a part of dielectric material having a channel formed in one surface thereof, a conductor seated within said channel, said conductor comprising a thin strip of electrically conductive sheet metal bent into a generally U-shaped cross section, the arms of said U extending down into said channel and the bend of said U projecting above the surface of said part, and a sliding contact engaging the crest of said bend.

4. In an electrical element, a part of dielectric material having a channel formed in one surface thereof, a conductor seated within said channel, said conductor comprising a thin strip of electrically conductive sheet metal bent into a generally U-shaped cross section, the arms of said U being sprung into said channel and engaging the side walls thereof with a resilient spring pressure, the bend of said U projecting above the surface of said dielectric part, and a sliding contact engaging the crest of said bend.

5. A variable resistor comprising a case, a member supported on said case for movement relative thereto, a part of dielectric material attached to said case, said part having a channel formed in one surface thereof, a conductor seated within said channel, said conductor comprising a thin strip of electrically conductive sheet metal bent into a generally U-shaped cross section, the arms of said U extending down into said channel and the bend of said U said part, and a sliding contact mounted on said member and running on the crest of said bend.

6. A linear motion resistor comprising a case having a cavity formed in one side thereof, a member movable linearly within said cavity, a cover plate of dielectric material mounted on said one side of said case to close said cavity, said cover plate having a channel formed in the inner surface thereof parallel to the line of travel of said member, a conductor seated within said channel, said conductor comprising a thin strip of electrically conductive sheet metal bent into a generally U-shaped cross section, the arms of said U extending down into said channel and the bend of said U projecting abovethe surface of said cover plate, a resistance element mounted on the inner surface of said cover plate parallel to said conductor, and sliding contact means on said member engaging both said resistance element and the crest of said conductor.

7. A linear motion variable resistor comprising a case, a member movable linearly with respect to said case, a resistance element and a conductor mounted on said case parallel to the line of travel of said member, sliding contact means on said member engaging bothsaid resistance element and said conductor, means constraining said member against turning relative to said case, an operating rod rotatably connected to said member and disposed parallel to the line of travel thereof, and spring means interposed between said rod and said member to exert a spring force between them for the purpose of taking up all endwise clearance.

8. A linear motion variable resistor comprising a case, a block of dielectric plastic material movable linearly with respect to said case, a U-shaped staple embedded in said block with its ends projecting from one surface thereof, a contact element having a pair of holes provided therein through which the projecting ends of said staple pass, said staple ends being bent over to secure said contact element to said block, and a resistance element and conductor mounted on said case and slidably engaged by said contact element.

9. In an instrument of the class described having an elongated conductor and a member movable along a path parallel to said conductor, a pair of spring fingers mounted on said member and electrically connected together, and an electrical contact on the end of each of said fingers, both of said contacts engaging the same conductor at longitudinally spaced points thereon and being individually pressed against said conductor by the spring force of their respective fingers, the engagement of either of said contacts with said conductor being independent of the other.

10. In an instrument of the class described, the combination of a case having a cavity formed therein, a cover mounted on said case and closing said cavity to form a chamber, a guide rod extending through said cavity and through holes in opposite ends of said case, a member slidably supported on said guide rod, a conductor mounted on said case, contact means on said member slidably engaging said conductor, and a pin extending through a hole in said case and interlocking with one end of said guide rod to secure the latter against endwise movement with respect to said case, said pin projecting above the surface of said case and seating within a hole in said cover to position the latter with said conductor parallel to the line of travel of said member, said pin serving to prevent said cover from shifting laterally with respect to said case.

11. A linear motion variable resistor comprising a case having a cavity formed in one side thereof, another side of said case having an elongated slot provided therein, a member movable linearly within said cavity, a lateral projection on said member extending through said slot to provide external actuating means by which said member can be shifted linearly within said case, a thin flat plate attached to said lateral projection and bearing on the outer surface of said case, said plate being of a length to cover both ends of said slot when said member is at either end of its travel, another plate attached to said case on the outside of said first-named plate and enclosing the latter, said last-named plate having a slot provided therein through which said lateral projection extends.

12. A linear motion variable resistor comprising a case,

an electrical element mounted on said case, a pair of guide rods mounted on said case generally parallel to said electrical element, a contact-carrying member slidably mounted on said guide rods and engaging said electrical element, said member having a hole formed therein to receive one of said rods and a channel to receive the other red, said channel opening in the direction opposite to said hole, and the bottom of said channel being spaced from the adjacent surface of said other rod to provide clearance, whereby said channel is adapted to accommodate a limited amount of non-parallelism of said rods, said member being guided on said one rod and being held against turning with respect to said case by the engagement of said other rod with the sides of said channel.

13. An electrical instrument comprising a case, a conductor mounted on said case and comprising a thin strip of sheet metal bent longitudinally to a generally U-shaped cross section, the bend of said U lying entirely within a plane, and a contact member supported on said case for movement along a path parallel to said conductor, said contact member slidably engaging the outer surface of said bend and moving longitudinally along the length thereof.

14. A linear motion variable resistor comprising a case, a member movable linearly with respect to said case, a part of dielectric material mounted on said case and having a channel formed therein parallel to the line of travel of said member, a conductor seated within said channel, said conductor comprising a thin strip of sheet metal bent into a generally U-shaped cross section, at least one arm of said U extending down into said channel, a resistance element mounted on said dielectric part parallel to said conductor, sliding contact means on said member engaging both said resistance element and the crest of said conductor, a pair of terminals connected to the ends of said resistance element, and another terminal connected to said conductor, said conductor serving as a return to transmit electricity from said resistance element and contact means to said last-named terminal.

15. A linear motion variable resistor comprising a case, a member movable linearly with respect to said case, a resistance element and a conductor mounted on said case parallel to the line of travel of said member, sliding contact means on said member engaging both said resistance element and said conductor, means constraining said member against turning relative to said case, an operating rod rotatably connected to said member and disposed parallel to the line of travel thereof, and a rippled washer spring interposed between said member and a shoulder on said rod to exert a spring force between them for the purpose of taking up all endwise clearance.

16. A linear motion variable resistor comprising a case means projecting through one of the walls of said housing for moving said carrier along said guide rods, and terminal means on said cover connected to said conductor strip and resistance element.

19. A variable resistor comprising a body, an elongate resistance element mounted on said body, a member of non-conductive material movable linearly with respect to said body along a path parallel to said resistance ele ment, a contact mounted on said member and integrally attached thereto, said contact wiping on said resistance element, means slidably engaging said member to restrain the same against turning relative to said body, an

operating rod rotatably connected to said member for moving the same linearly with respect to said body, electrical return means connected to said contact, and terminal means on said body connected to said resistance element.

20. A variable resistor comprising a body having sides and ends defining an enclosed cavity, an elongate resistance element mounted on one of said sides on the inside of said cavity, a member of electrically non-conductive material disposed within said cavity and movable linearly with respect thereto along a path parallel to said resistance element, a contact mounted on said member and integrally attached thereto, said contact having sliding engagement with said resistance element, means on said member slidably engaging an inside surface of one of said body sides parallel to said resistance element to restrain said member against turning relative to said body, an operating rod rotatably connected to one end of said member and extending parallel to the line of travel thereof, said operating rod projecting through an aperture in one end of said body, and terminal means connected to said contact and to opposite ends of said resistance element.

21. A variable resistor comprising a body having a portion of non-conductive material having at least one longitudinally extending groove provided therein, an elongate resistance element disposed within said groove, an elongate conductor strip mounted on said body alongside said resistance element and parallel thereto, a member a movable linearly with respect to said body along a path having a cavity formed therein, one side of said case having an opening provided therein, a contact carrier movable linearly within said cavity, a lateral projection on said carrier extending through said opening to provide external actuating means by which said carrier can be shifted linearly within said case, a slide member attached to said lateral projection and movable therewith, and a cover enclosing said slide member, said slide member being closely surrounded on all sides to prevent the entrance of dust or foreign material, said cover having a slot provided therein through which said lateral projection extends and said slide member being of a length to cover both ends of said slot when said carrier is at either end of its travel.

17. A linear motion variable resistor comprising a case, an electrical element mounted on said case, a pair of guide rods mounted on said case generally parallel to said electrical element, a contact-carrying member slidably mounted on said guide rods and engaging said electrical element, said member having'a hole formed therein to receive one of said rods and a slot to receive the other rod, said slot extending radially with respect to said one rod, said member being guided on said one rod and held against turning with respect to said case by engagement of said other rod with the sides of said slot.

18. A linear motion variable resistor comprising a housing having an elongated cavity formed in one side thereof, a pair of parallel guide rods disposed within said cavity and mounted at their ends in the ends of said housing, a carrier slidably supported on said guide rods and having a contact attached thereto, a cover plate attached to said one side of said housing and enclosing said cavity, a conductor strip and a resistance element mounted on said cover plate parallel to said guide rods, said contact wiping on said conductor strip and said resistance element,

parallel to said resistance element and said conductor strip, a contact mounted on said member and integrally attached thereto, said contact having sliding engagement with both said resistance element and said conductor strip, means slidably engaging said member to restrain the same against turning relative to said body, and an operating rod rotatably connected to said member and disposed parallel to the line of travel thereof for moving the same linearly with respect to said body.

22. A variable resistor comprising a body having a longitudinally extending cavity formed therein, said cavity having at least one fiat side, another side of said cavity having a pair of parallel longitudinally extending grooves provided therein, an elongate resistance element disposed within one of said grooves, an elongate conductor strip disposed within the other of said grooves, a member movable linearly within said cavity along a path parallel to said resistance element and said conductor strip, said member having a flat side slidably engaging said flat side of said cavity so as to prevent the member from turning relative to the body, a contact mounted on said member and integrally attached thereto, said contact having sliding engagement with both said resistance element and said conductor strip, and an operating rod rotatably connected to said member and disposed parallel to the line of travel thereof for moving the same linearly with respect to said body.

References Cited in the file of this patent UNITED STATES PATENTS 1,660,979 Thayer Feb. 28, 1928 1,697,412 Caruso Jan. 1, 1929 2,242,327 Rubinstein May 20, 1941 2,243,990 Thorn et al. June 3, 1941 2,561,535 Paulet et a1. July 24, 1951 FOREIGN PATENTS 28,792 Great Britain of 1902