US2629799A

US2629799A - Adjustable resistance device

Info

Publication number: US2629799A
Application number: US218431A
Authority: US
Inventors: Donald F Hastings
Original assignee: Bendix Aviation Corp
Current assignee: Bendix Aviation Corp
Priority date: 1951-03-30
Filing date: 1951-03-30
Publication date: 1953-02-24
Anticipated expiration: 1970-02-24

Description

Feb. 24, 1953 D. F. HASTINGS 2,629,799

ADJUSTABLE RESISTANCE DEVICE Filed March 50, 1951 2 SHEETS-SHEET l ml lIII llllllllllll m I 1 E 3 INVENTORJHI Q-N N DO/VALD E HAST/NGS I C. HM

Feb. 24, 1953 D. F. HASTINGS ADJUSTABLE RESISTANCE DEVICE 2 SHEETSSHEET 2 Filed March 30, 1951 I I 0K INVENTOR. DUN/1L0 F HA 5 T/NGS Patented Feb. 24, 1953 UNITED STATES PATENT OFFICE ADJUSTABLE RESISTANCE DEVICE Donald F. Hastings, Suffern, N. Y., assignor to Bendix Aviation Corporation, Teterboro, N. J a corporation of Delaware Application March 30, 1951, Serial No. 218,431

16 Claims. 1

This invention relates to control devices for electrical circuits and more particularly to an adjustable resistance device having means for compensating for non-linear changes in the resistance thereof.

Adjustable resistance devices, such as potentiometers and rheostats, are commonly employed in an electrical circuit or in a network for inserting a predetermined amount of resistance into the circuit. These devices in some cases take the form of a pair of drums about which a resistance element or wire may be wound. In this type of a device, one end of the resistance element is fastened to one drum while the other end is fastened to the other drum so that upon rotation of the drums the wire will be unwound from one drum and wound on the other drum. A stationary contact is positioned at a point midway between the drums to engage successive portions of the resistance element as it is passed from one drum to the other. The contact is connected in the circuit for inserting a predetermined value of resistance therein determined by the length of wire on one drum as compared to the length on the other drum. Since the cross section and length of the wire are known, the resistance desired to be inserted in the circuit may be obtained by rotating the drums a predetermined amount. In some devices, the contact is moved about a stationary drum to engage a resistance wound about the outer periphery of the drum. In both cases, a definite error may arise because the change in resistance along the wire is nonlinear due to the non-uniform physical structure of the wire. A non-linear change in resistance may occur for other reasons, such as, change in resistivity of the wire due to non-uniform grain structure as a result of improper heat treatment, or because the composition of the wire itself varies from a predetermined standard. Again, the error may arise from the fact that the drum may not be perfectly concentric with the driving shaft, or, the diameter of the threads on the drum may be difierent to provide in effect a greater length of wire in one thread as compared with the length of wire in another thread. It has been found in practice, that most of the commercial adjustable resistance devices, such as potentiometers, cannot furnish predetermined values of resistance having less than 0.10 percentage error. These potentiometers are not suitable for circuitry wherein linearity must be held to closer tolerances.

The present invention therefore, contemplates a precision adjustable resistance device which includes means for compensating for the nonlinear changes in resistance of the resistance element. In one embodiment of the invention, a rotatable drum has a resistance element in the form of a wire wound about the periphery thereof, the ends of the element being fastened to the drum. A pair of spaced insulated pulleys carry successive portions of the wire as the drum is rotated. A contact connected in the circuit is positioned midway between the pulleys for engaging the wire so as to insert a predetermined value of resistance in the circuit depending upon the amount of rotation of the drum. Means are provided to displace the contact from its normal midway position along the wire, the amount and direction of displacement depending upon the non-linear changes in resistance of the wire, to adjust the resistance of the circuit linearly. In a second embodiment of the present invention, the contact comprises a stationary deformed wire curved in accordance with the non-linear change in resistance of a resistance element wound about a single rotatable drum to ad ust the resistance of a circuit linearly.

Accordingly, an object of the present invention is to provide a precision adjustable resistance device for compensating for the non-linear change in resistance of the resistance element.

Another ob ect is to provide a novel adjustable resistance device wherein a resistance element is Wound on a rotatable drum, and a contact engages the element and is displaced relative thereto irom a reference position in accordance with the non-linear changes in resistance of the element.

A further object is to provide a novel precision ad ustable resistance device wherein a resistance element is wound on a rotatable drum and engages a movable contact WIllCil is moved along the resistance eiement oy cam means to compensate Ior non-iinear changes in resistance or the element.

A still further object is to provide a novel ad ustable resistance device wherein a resistance element wound on a rotataoie drum en ages a. SUZLUAOIIELI'Y contact preiormeu in accordance with the nun-anew change in resistance or the element to adjust the resistance or a. circuit linearly.

The ioregoing and other ob ects and advantages or the present invention will appear more may nereinaiter iroiii a consideration or the detailed description which loiiows, taken togetner with the accom anying drawings wherein two embodiments of the invention are illustrated by way of example. It is to be expressly understood, however, that the drawings are for the purposes of illustration only, and are not to be construed as defining the limits of the invention.

In the drawings wherein like reference numerals refer to like parts,

Fig. l is a side elevational view of one embodiment of the novel adjustable resistor of the present invention;

Fig. 2 is a rear elevation view of part of the adjustable resistance device of Fig. 1, taken along line 2-2;

Fig. 3 is a front elevation view of the adjustable resistance device of Fig. 1 showing the collector ring and brush arrangement for connecting the resistance element in a circuit;

Fig. 4 is a top plan view of a roller-cam arrangement utilized in the adjustable resistor of Fig. 1;

Fig. 5 is a top plan view of another form of the present invention; and

Fig. 6 is a front elevation view of the embodiment of Fig. 5.

Referring now to the drawings for a more detailed description of the present invention and more particularly to Fig. 1 thereof, the novel adjustable resistor hereof is shown as a potentiometer and is generally designated by the numeral II. Potentiometer ll comprises a rotatable drum I2 fixed to a driving shaft 13 which is journalled in a pair of spaced plates 14 and I5. Shaft l3 may be manually actuated, or may be driven by automatic means to suit the particular requirements of the system with which the potentiometer is to be employed. Drum [2 has fixed thereon an externally threaded sleeve l6 of insulating material which accommodates a predetermined length of a resistance element or wire I! positioned in the threads of the sleeve. The ends of wire I! are fixed to the drum [2 by metallic inserts HA and 11B and are electrically connected by the inserts to a pair of concentric collector rings I8 and I9 respectively (shown by broken lines in Fig. 3), disposed on one face of the drum. Brushes 20 and 2i (shown in broken lines in Fig. 3) engage the collector rings I8 and I8 respectively, and are mounted in an insulated terminal block 22 fixed to plate M. Connection of the resistance element ll to a circuit or electrical network may be made by conductors (not shown) connected to the brushes 2i) and 2 I.

Part of one turn of the wire I! is carried free of the threads of sleeve I6 by a pair of insulated pulleys 23 and 24 spaced from the drum. The pulleys are mounted on a carriage 25 which comprises a pair of spaced frame members 26 only one of which is shown in Fig. 2. The pulleys 23 and 24 are supported for movement axially of the drum by a pair of shafts 21 and 2B respectively, journalled in the spaced members 26. Shaft 21 is hollow and accommodates therethrough a shaft 27A the ends of which are seated in plates l4 and I5. Spaced members 26 are spring-held at the end adjacent pulley 24 by a coil spring 29 which is fixed at its other end to a spacer bar 30 supported by plates l4 and I5. In the instant arrangement, members 25 and pulley 24 pivot about shaft 21A. For purposes of clarity, shaft 21A and pulley 23 are not shown in Fig. 1. By reason of the foregoing arrangement, the wire I! is tensioned by the pulleys 23 and 24, and as the shaft I3 is actuated successive parts of the wire pass between the pulleys. The pulleys are free to axially slide along shafts 21 and 28 as well as rotate, and therefore, as the shaft 13 is rotated the pulleys take up a position directly over the points at which the wire meets the threads of sleeve 16.

A contact 3| comprising a pair of contact members 52 and 33 normally engages the wire I! at a point which is midway between the pulleys (Figs. 1 and 2) The contact members 32 and 33 are carried by a contact arm 34 which is supported for oscillation about a point 35 directly oelow drum I2. A pair of pins 36 (Fig. 2) fixed to the lower side portions of the arm are seated in bearings fixed to a substantially U-shaped bracket 31 to permit oscillation of arm 34 about point 35. An opening 38 formed in arm 34 permits the arm to be disposed between plates l4 and I5 and about shaft [3. A shaft 39 journalled in plates [4 and 15 carries bracket 31 and is adapted, by means hereinafter to be described, to angularly displace the bracket about the axis of the shaft. A conductor (not shown) may be connected to the U-shaped bracket 31 to connect the contact 3| in a circuit.

Fixed to one end of shaft l3 and disposed between plate 15 and a third plate 40 is a pinion 4| which meshes with a relatively large gear 42 supported on a shaft 43 journalled in plates [5 and 40. Gear 42 is fixed to a coupling sleeve 44 supported on shaft 43, the sleeve having fixed at the other end thereof a pinion 45 which moves with gear 42. A second relatively large gear 46 meshes with pinion 45 and is supported on a sleeve 48 disposed on a shaft 41 journalled in plates l5 and 40. Gear 46 has fixed thereto an annular face plate 49 which has a plurality of threaded openings formed at equal radial distances from the center of the plate. The open ings are spaced at predetermined and equal intervals from each other and are each adapted to received a cam adjusting screw 50, only two of which are shown in Fig. 1. The adjusting screws 50 have a kerf 5| cut in the head thereof for permitting the insertion of a suitable tool to axially displace the screws relative to face plate 4%]. A thin deformable cam ring 52 of spring material is spaced from the face plate 49 and fixed for rotation therewith by the screws. The end portions of screws 50 have smooth shank portions which fit into openings formed in cam ring 52 which are aligned with the threaded openings in the plate 49. Disposed on each end of the smooth shank portions of screws 50 and on both sides of ring 52 are buttons 53 which permit the screws to be rotated with respect to the ring but prevent axial movement thereof with respect 'to the ring. The contour of the cam ring 52 may by reason of the foregoing, be adjusted to form any desired shape simply by rotating screws 50 a predetermined amount in either direction.

A cam roller 54 is disposed between plates I5 and 40 for engagement with the surface of cam ring 52 (Fig. 4). Roller 54 is loosely mounted for rotation on a shaft 55 (Fig. 4) which is fixed perpendicularly to a rectangular shaped shaft 56 supported for angular displacement in a block 51 fixed to plate l5. A pin 55 is also secured to shaft 56 and at right angles to shaft 55 to engage a second pin 59 fastened to a collar which is fastened to shaft 39. A coil spring 6| biases the contact arm 34 to the right, as seen in Fig. 2, to hold

pins

58 and 59 in contact with each other to take up backlash in the arrangement described. It is apparent from the foregoing structure that cam roller 54 and shaft 55 are moved by the contoured surface of cam ring 52 to displace shaft 56 and rock pins 58 and 59, thereby oscillating contact arm 34 about shaft 39. In this manner, the contact 3| is displaced from a point midway between the pulleys an amount depending upon the contour of cam ring 52, the contact 3| engaging the Wire H as it is displaced.

Gears 4|, 42, 45 and 46 provide a gear reduction arrangement between drum l2 and cam ring 52, and is such that the cam makes less than one full revolution for the required number of revolutions of the drum to pass the entire active length of the wire I! over the space between pulleys 23 and 24. In the instant arrangement, one revolution of drum l2 passes one turn of wire past the contact 3|, and cam ring 52 is rotated an angular distance equal to one half the distance between two successive screws 50. Thus, the contoured surface of the cam ring between two successive screws is directly related to two turns of the resistance element on the drum. In other words, the displacement of contact 3| with respect to two turns of wire is controlled by a single screw of the adjustable cam 52.

The value of resistance to 'be inserted in the circuit in which the resistance element I! is included depends upon the point of engagement of contact 3| along the length of the resistance wire. Thus, when the contact 3| is positioned directly over the first thread of sleeve |6, looking from left to right in Fig. l, the length of wire I! between the insert |'|A connected to collector ring l8, and the contact is a minimum and, therefore, the resistance inserted in the circuit is a minimum. As drum I2 is rotated in a direction to increase the effective length of wire between contact 3| and insert |'|A the resistance inserted in the circuit likewise increases, and if the drum is rotated until a minimum length of wire remains between contact 3| and insert ||B connected to collector ring l9, then the resistance in the circuit is the greatest. Since the length and cross-sectional area of the wire are known, the linear change in resistance of the wire as the drum is rotated may be computed. Therefore, if it is desired to insert a certain value of resistance in the circuit, the shaft I3 is rotated until the contact 3| engages a point on the wire which will give the desired resistance. Normally, contact 3| is at a point midway between the pulleys 23 and 24 when the change in resistance is linear. However, since the cross-sectional area of wire I! cannot be made perfectly uniform in practice, and the wire may vary in resistivity, and/or there is a variation of the thread diameter of the drum, a non-linear change in resistance results as the wire is moved past the contact 3|.

To overcome this non-linear change in resistance, contact 3| is displaced from the midway point, and along the wire, so as to insert more or less resistance in the circuit depending upon whether the resistance of the wire between the insert |'|A connected to collector ring l8 and the midway position of the contact is greater or less than its calculated value. If the resistance is greater than its calculated value, then the contact 3| will be displaced from its midpoint position to decrease the length of wire between collector ring l8 and the contact, and, conversely, if the resistance is less than its predetermined value then the contact is displaced so as to increase the effective length of wire in the circuit. In this manner, the non-linear change of re- 6 sistance in wire H as it is driven by drum |2 past the contact 3| is corrected automatically to effect a linear change in resistance.

Considering the operation of the arrangement described, if it is desired to insert a resistance into a circuit which requires the drum to make two complete revolutions, shaft I3 is rotated two revolutions. The threads of sleeve l6 are arranged to advance in one direction so that the wire, as it leaves the sleeve, moves the pulleys 23 and 24 axially of the drum to the right, as seen in Fig. 1, to position the pulleys directly over the points at which the wire meets the drum. The contact 3| also moves to the right about its pivot 35 due to the movement of the pulleys and the wire thereon. Motion of shaft I3 is taken up through reduction gears 4|, 42, 45 and 46 to displace cam ring 52 an angular amount equal to the distance between two successive threaded members on the cam. If it is assumed that the linear change in resistance of the two turns of wire on drum |2 is not uniform, then the portion of the cam ring between the two threaded members 50 under consideration is contoured by the screws in accordance with a previous determination of non-linearity of change in resistance of the particular two turns of wire, to move cam roller 54 outwardly or inwardly with respect to the cam ring. This movement is transmitted to pin 58 which rocks pin 59 to angularly displace shaft 39 and U-shaped bracket 31 to thereby displace contact 3| from the midway position. The contact 3| will be moved either to the left or right of the midway position depending upon whether the amount of resistance is to be decreased or increased in conformance with the previous determination of non-linearity of wire An improved embodiment of the present invention is illustrated in Fig. 5 of the drawings, wherein a rotatable drum 10 having a sleeve H of suitable insulating material formed thereon is rotated by a shaft 12 fixed to the drum. Sleeve H is threaded externally-thereof and accommodates a predetermined length of a resistance element or wire 13 wrapped about the sleeve. Each end of the wire is fastened to the drum and connected to its respective collector ring and brush, in a manner similar to the adjustable resistor of Fig. 1, so as to connect the wire in an electrical circuit or network. The collector ring and brush structure is identical with the corresponding structure disclosed in Fig. 1. A pair of insulated pulleys I4 and 15 are supported for rotation, and movement axially of the drum, by shafts l6 and I1, respectively, the latter being journalled in a pair of spaced plates not shown. Successive portions of resistance element 13 are carried and tensioned by the pulleys, and as the drum I0 is rotated the pulleys are free axially to slide along the shafts 16 and TI and thus take up a position directly over the points at which the wire meets the threads of the drum.

Coming now to the novel feature of the improved embodiment, a stationary block 18 of insulating material is positioned between the pulleys l4 and 15, by means not shown, and has cemented or otherwise attached thereto, a contact member 19 consisting of a curved wire. Physically connected to one end of contact member 19 is a terminal element which connects the contact member in the electrical circuit by way of a conductor 8|. The contact member 13 is so positioned with respect to the wire 13 carmade in the design and arrangement rled by the pulleys, that the contact member is engaged by the resistance element as the pulleys move axially of the drum in the direction of the arrows. The pulleys 14 and '15 tension the resistance element 13 to provide a positive contact pressure between the contact member and resistance element. Contact member 19 is deformed or curved in accordance with the change in resistance of each turn of resistance wire 13 on the drum to compensate for non-linear change in resistance. The deformation of contact member 18 may be easily computed before it is attached to the stationary block 18 by positioning a straight contact wire on the block midway between the

pulleys

14 and 15. Since the length and cross-sectional area of the resistance elementare known as well as variations in resistivity, the change in resistance as the element is moved past the contact wire may be found by connecting the resistance element and contact wire to a suitable indicating instrument. 13y moving the resistance element past the contact wire, the indicating instrument will efiect indications of the non-linear change in resistance of the resistance element. When the resistance is less than the predetermined value for the length of resistance element between the contact wire and the end connected to the collector ring the contact point is advanced by deforming the contact wire so as to include the additional resistance required, and co ersely, if the resistance is less than the predetermined value the con tact point is moved back to decrease the amount of resistance. In this manner, the non-linear change in resistance of the resistance element is compensated and the curvature of the contact wire 19 may be determined. It is readily apparent that deformed contact member 19 accomplishes the same results as effected by the gear reduction, cam and contact arrangement of the first embodiment previously described and, as such, is an improvement thereover.

It may be readily understood from the foregoing that the present invention provides an adjustable resistance device which is capable of compensating for non-linear changes in resistance of its resistance element so as to insert in a circuit a linear change in resistance. The adjustable resistance device may be used interchangeably as a potentiometer, rheostat and voltage divider by connecting the resistance element and contact member in a circuit in a manner apparent to those skilled in the art.

Although two embodimentsof the invention havebeen illustrated and described in detail, it is to beexpressly understood that the invention is not limited thereto. Various changes may be of parts Without departing from the spirit and scope of the invention as will now be understood by those skilled in the art.

I claim:

1. A device for preselecting theresistance to be inserted into an electrical circuit, comprising a resistor element having non-linear resistance characteristics, resistance selecting means for displacing said element along a predetermined path so as to select the value of resistance to be inserted into said circuit, and contact means adapted for connection with said electrical circuit for engaging said resistor element as it is displaced in said predetermined path, said contact means being calibrated in accordance with the non-linear characteristics of said resistor element for compensating for non-linear-changes in l the resistance so that a linear change in resist ance will be efiected in said electrical circuit.

2. A device for preselecting the resistance to be inserted into an electrical circuit, comprising a resistor element having non-linear resistance characteristics, a, rotary member having said resistor element wound thereon, the ends of said element being adapted for connection in said circuit, selecting means adapted to drive said rotary member to select the value of resistance to be inserted in said circuit, and contact means adapted for connection in said circuit and engaging said resistor element, said contact means being calibrated in accordance with the nonlinear resistance characteristics of said resistor element for compensating for non-linear changes in the resistance of said element so that a linear change in resistance is effected in said circuit by said selecting means.

3. A device for preselecting the amount of resistance to be inserted into an electrical circuit, comprising a resistor element having nonlinear resistance characteristics, means for displacing said element along a predetermined path so as to select the value of resistance to be inserted in said circuit, a movable contact adapted for connection in said circuit engaging the resistor element as the latter is displaced along said path, and means calibrated in accordance with the non-linear resistance characteristics of said resistor element for displacing said contact from a, reference position and along said element to compensate for the non-linear resistance characteristics of said resistor element.

4. A device for preselecting the amount of resistance to be inserted into an electrical circuit, comprising a resistor element having nonlinear resistance characteristics, a drum having said resistor element wound thereon, the ends of said resistor element being fastened to said drum, means for driving said drum to select the value of resistance to be inserted in said circuit, a movable contact adapted for connection in said circuit and engaging said resistor element, and means calibrated in accordance with the non-linear resistance characteristics of said element and connected for displacing said movable contact from a reference position and along said element to compensate for the non-linear resistance characteristics of said resistor element.

5. A device for preselecting the amount of resistance to be inserted into an electrical circuit, comprising a resistor element having nonlinear resistance characteristics, a shaft, a drum mounted for rotation by said shaft, said drum having said resistor element wound thereon, means connected for actuation by said shaft and calibrated in accordance with the non-linear resistance characteristics of said resistor element, a movable contact adapted for connection in said circuit and engaging said resistor element to adjust the value of the resistance to be inserted into said circuit, said contactb'eing displaced from a reference position and along said resistor element by said calibrated means to compensate for the non-linear resistance characteristics of said resistor element.

6. 'A device for preselecting the amount of resistance to be inserted into an electrical circuit, comprising a resistance wire havinga, nonlinear resistance characteristic, a drum having threads formed thereon, said drum having said resistance wire wound thereon and lying in said threads, a, contact adapted for connection in said circuit and positioned to engage said resistance wire, a shaft for rotating said drum, and means calibrated in accordance with the non-linear resistance characteristic of said wire, said lastmentioned means connected for actuation by said shaft and adapted to displace said contact along said wire to compensate for the non-linear resistance characteristic of said wire.

'7. A device for selecting the value of a resistance to be inserted into an electrical circuit, comprising a resistance wire having a non-linear resistance characteristic, a movable contact adapted for connection in said circuit, a drum having sa d resistance wire wound along the periphery thereof, a shaft for rotating said drum, means for lifting a portion of said resistance wire from said drum so as to bring successive portions of said wire into engagement with said contact, and means calibrated in accordance with the non-linear changes in resistance of each turn of wire on said drum, said last-mentioned means being connected for actuation by said shaft for displacing said contact along said wire to com ensate for the non-linear resistance characteristic of said wire.

8. A device for selecting the value of a resistance to be inserted into an electrical circuit, comprising a resistance wire having non-linear resistance characteristics, a drum having threads formed thereon, said drum having wound thereon and in said threads said resistance wire, a shaft connected for rotating said drum, a cam follower arrangement calibrated in accordance with the non-linear resistance characteristics of each turn of said wire on said drum and connected for actuation by said shaft, a movable contact adapted for connection in said circuit and engaging said resistance wire to select the value of resistance to be inserted into said circuit upon rotation of said drum, said movable contact being displaced from a reference position and along said wire by said cam follower arrangement to compensate for the non-linear resistance characteristics of said wire in order to insert the correct resistance in said circuit.

9. A device for compensating for non-linear changes in the resistance of a resistor element adapted for connection into an electrical circuit, comprising a drum mounting said resistor element along the outer surface thereof to constitute a plurality of turns of the resistance element, a shaft connected for rotating said drum, a deformable cam ring connected for angular displacement by said shaft from a reference position, said cam ring being deformed in accordance with the non-linear changes in resistance of said element and having marked ofi thereon positions corresponding to each turn of said element on the drum, a cam follower adapted to engage said cam ring, a movable contact adapted for connection in said circuit and positioned to engage successive turns of said element to select the value of resistance to be inserted into said circuit, said movable contact being connected for displacement along said element by said cam follower to adjust the value of the resistance to be inserted into said circuit linearly.

10. A device for selecting the value of resistance to be inserted into an electrical circuit, comprising a resistor element having non-linear resistance characteristics, a drum mounting said resistor element, means for rotating said drum, a contact adapted for connection in said circuit and positioned to engage said resistor element so as to select the value of resistance to be inserted into said circuit, said contact being curved in accordance with the non-linear resistance characteristic of said resistor element for compensating for the non-linear resistance characteristics of said element.

11. A device for selecting the value of resistance to be inserted into an electrical circuit, comprising a resistor element having a non-linear resistance characteristic, a drum having said element wound thereon, means for driving said drum, and a stationary preformed contact adapted for connection in said circuit and formed in accordance with the non-linear resistance characteristic of said element, said contact engaging successive portions of said resistor element and positioned with respect to the latter so as to vary the point of engagement therebetween to compensate for the non-linear resistance characteristic of said resistor element.

12. A device for selecting the value of resistance to be inserted into a circuit, comprising a resistor element having a non-linear resistance characteristic, a drum having threads formed thereon for accommodating said resistor element, means for rotating said drum, tensioning means spaced from said drum and adapted to move in a direction axially of the drum upon rotation of the latter, said tensioning means being adapted to carry successive portions of said resistor element as the drum is rotated, and a contact member preformed in accordance with the non-linear resistance characteristic of said resistor element and adapted for connection in said circuit, said contact member being positioned so as to engage successive portions of said element to provide a linear change in the value of the resistance inserted into the circuit.

13. A device for selecting the value of resistance to be inserted into an electrical circuit, comprising a resistor element having a nonlinear resistance characteristic, a drum having a plurality of threads formed thereon, said drum having said resistor element wound in said threads, means for rotating said drum, tensioning means spaced from said drum and adapted to move in a direction axially of the drum, said tensioning means being adapted to carry successive portions of said resistor element, and contact means adapted for connection in said circuit and engaging said resistor element, said contact means including a contact wire spaced from said drum and arranged longitudinally thereof so that successive portions of said resistor element move along the length of said contact wire upon rotation of said drum, said contact wire being deformed in accordance with the nonlinear resistance characteristic of said resistor element.

14. An electrical device comprising a predetermined amount of a resistor element adapted for insertion into an electrical circuit, the element being of a nature such that as a varying amount thereof is inserted into the circuit the change in the resistance produced thereby is other than linear, contact means engaging said element, and means for displacing said element relative to said contact means whereby for a given displacement thereof a given amount of the element is inserted into the circuit, said contact means being constructed to vary the amount of the element inserted into the circuit by the displacing means to thereby compensate for the non-linear resistance characteristic of said element.

15. An electrical device comprising a predetermined amount of a resistor element adapted for insertion into an electrical circuit, the element being of a nature such that as a varying amount thereof is inserted into the circuit the change in the resistance produced thereby is other than linear, contact means engaging said element, means for displacing said element relative to said contact means whereby for a given displacement thereof a given amount of the element is inserted into the circuit, and means for varying the point of engagement of said contact means with said element whereby the amount of the element inserted into the circuit by the displacing means is varied to thereby compensate for the non-linear resistance characteristic 01' said element.

16. A variable resistance device comprising a resistor element having a non-linear resistance characteristic, a contact engaging the element, means providing relative displacement between REFERENCES CITED The following references areoi record in the file of this patent:

UNITED STATES PATENTS Number Name Date 1,213,723 Zeisel Jan. 23, 1917 FOREIGN PATENTS Number Country Date 111,700 Austria July 15, 1928 819,411 France July 5, 1937