US2125954A

US2125954A - Variable resistor

Info

Publication number: US2125954A
Application number: US734810A
Authority: US
Inventors: Henry G Richter
Original assignee: Individual
Current assignee: Individual
Priority date: 1934-07-12
Filing date: 1934-07-12
Publication date: 1938-08-09
Anticipated expiration: 1955-08-09

Description

Aug. 9, 1938. H. e. RICHTER I VARIABLE RESISTOR Fiied July 12, 1934 2 Shets-Sheet 1 .86 n w 2 5 w n m INVEN:I'OR Henr G. Richler BY f 9 A'ITQBNEYS Aug. 9, 1938. H. G. RICHTER 2,125,954

I VARIABLE RESISTOR Filed Ju1 12, 1954 2 Sheets-Sheet 2 INVENTOR BY 5 z I a ATTORNEYS Patented Aug. 9, 1938 UNITED, STATES,

- PATENT OFFICE- 27 Claims.

This invention relates to variable resistance elements, and more particularly to high resistance elements commonly employed for volume control purposes.

The object of the present invention is to generally improve variable resistance units, especially such units in which the resistance element is a thin film of carbonaceous, graphitic, or other suitable resistance material. More particular objects of my invention reside in the provision of a unit of this class which will be compact yet utilize all of the resistance material at the maximum radius permitted by the over-all dimension of the unit; which will contact the full width of the film by a line contact, thereby avoiding short-circuiting of substantial film area; which will be characterized by parallel resistance paths of equal length, thereby avoiding unequal loading of parts of the film; and the calibration or resistance value of which will not be modified in use due to play or wobble of the control shaft of the unit. These objects I fulfil, generally, by

coating the resistance filmon a ring-like insulation base having a cylindrical periphery, which construction results in further important manufacturing advantages, such as the ease of uniformly or similarly coating a large quantity of resistance elements, and the ease of calibrating the elements before inserting the same in the complete units. I

A further object of my invention is to so construct the unit that itmay be made up of simple elements economical to manufacture and assemble, and resulting in a rugged durable unit. Still further objects center about the soldering lugs for connecting the unit in circuit, and are to support the lugs in desired position and to connect the same to the resistance element and to the movable contact arm without appreciably increasing the bulk, complexity, or expense of the unit; and to make possible the addition, when desired, of a lug connected to or tapping the resistance film at any desired point while positioning said lug at a fixed point on the unit.

Still another object of my invention resides in the provision of a standard unit having a casing open at one. end with which there may be interchangeably used, at the option of the manufacturer, either a simple closure plate or a power switch automatically operated by the resistor, or one or more tandem units all operated in unison with the main unit. Considerable difllculty has heretofore been caused the manufacturer by tandem units, especially in taking care of replacement orders, because the tandem units are bought in accordance with a variety of specifications, and the types which must be kept on hand for replacement may run to extremely large numbers.

In accordance with my invention, only the individual. units need be standardized and kept on 5 hand, for the varied combinations of difierent units may be assembled as desired when ordered. The construction is valuable in the field, because a service man may assemble a single needed unit for a repair job, and is valuableeven for pro- 10 duction purposes because separate standardized units may be assembled at the factory in accordance with the orders received. The tandem units are themselves open at one end and may be closed by a closure plate or a power switch or 16 another tandem unit, just as in the case of the main unit, thus producing an assembly which is exceedingly flexible in meeting varied commercial requirements.

To the accomplishment of the foregoing and such other objects as will hereinafter appear, my invention consists in the variable resistor elements and their relation one to the other, as hereinafter are more particularly described in the specification and sought to be defined in the claims. The specification is accompanied by 1 drawings, in which:

Fig. 1 is a rear elevation of a variable resistor embodying features of my invention, the interior of the resistor being exposed by removal of the closure plate;

. Fig. 2 is a section taken in the plane oi the line 2--2 of Fig. 1;

Fig. 3 is a section taken in the plane of the line 3'-3 of Fig. 1'; v v

Fig. 4 is a section through a soldering lug, and is taken in the plane of the line 4-4 of Fig. 1;

Fig. 5 is a section illustrating the location of an additional soldering lug for intermediately tapping the resistor, and is taken in the plane of the line 55 of Fig. 1;

Fig. 6 is a similar section in place;

Fig. 7 shows the resistance element;

Fig. 8 shows a resistance element provided with an intermediate tap;

Fig. 9 shows a power switch assembly which may be applied to the resistance unit; v

Fig. 10 is a side elevation of the resistor and switch assembly;

Fig. 11 is a side elevation of a resistor and tandem resistor preparatory to assembly of the same;

Fig. 12 is a front sistor;

with the added lug elevation of the tandem re- Fig. 13 shows the contact shoe of the resistor; and

Fig. 14 shows a modified thrust bearing for the control shaft.

Referring to the drawings and more particularly to Fig. '7 thereof, the resistance element comprises an insulation ring or base 12 the outer periphery of which is coated with a resistance film M. The ring i2 may be made of any suitable insulation material, preferably a molded synthetic resin or condensation product. The periphery of the ring is preferably cylindrical, and the ring may most simply be made rectangular in cross-section. The resistance film l4 may be made of known resistance paints preferably of the carbonaceous or graphitic type. As is more particularly described in my copending application Serial No. 734,811, filed concurrently herewith, a large number of the rings l2 may be stacked on a mandrel and simultaneously and uniformly sprayed with the paint. In the said application. I further disclose how the element may be given a uniform or a tapered resistance variation, as may be required for any particular purpose. The film is interrupted or ground away at l8, and the breach I8 is preferably aligned with a keywav i8 on the inner wall of the ring. The ring is preferably made with sufficient radial thickness to make the same rigid and to permit mounting holes 20 and an assembly hole 22 to be drilled therethrough in an axial direction. At the same time there is a large hole left at the center which is utilized as is subsequently described.

Referring now to Figs. 1 through 5, the resistance element I2 is mounted within and housed by a casing 24. This casing is preferably a single piece of molded insulation and comprises a cylindrical wall 25 closed at one end by a front wall 28, the other or rear end of the casing being left open. A threaded bushing 80 of conventional type is secured to casing 28 in any desired manner. In the present case the end 32 of'the bushing is fluted and driven into the casing with a force fit to prevent rotation of the bushing, and lips 88 are thereafter expanded or staked outwardly.

The resistance element I2 is seated around and located by a ledge 36 arranged concentrically with bushing 80, and is held in place by screws 38 passing through the element and threaded into the casing.

The resistance film is yieldably engaged by a slidable contact oscillated by a control shaft 48 carried in bushing 30. Control shaft 48 is squared at its inner end 2, and the squared end has permanently moullte :raereon a metallic thrust washer ll. an insulatim washer 46, the dished hub 48 of radial contact arm 50, an insulation washer '52, and, finally, a stop arm 54. The shaft end 82 is riveted. or may be staked outwardly, as by the four punches 58 indicated in Fig. 1. Shaft with its contact arm assembly is slid throughbushing 80 and is held in place by a split ring 88 set'into a suitably located groove 80 on the control shaft.

The contact arm of the resistor comprises not only the hub portion 48 and radial portion 58 previously referred to, but also a yieldable arcuate portion 62 best shown in Fig. 1. Radial portion 80 overlies the face of the resistance element 12, and its outer end 64 is bent inwardly or forwardly in generally parallel relation to but spaced fr m resistance film i4. Arc-date portion 82 is unmrm -':*:;1; .2: ."n-retai curved to conform to the curvature of the resistance element. It is secured at one end to the radial contact arm by a rivet B6 and by a mating indentation and recess at 68 taking the place of a second rivet. Strap I32 is preferably substantial in length and in the present case far exceeds a quadrant, thereby affording a yieldable and non-chattering engagement between the contact shoe l0 and the resistance element.

Contact shoe 78 is exceedingly simple in form, as is best shown in Fig. 13, it comprising merely a rectangular surface having at each end upwardly bent ears 12 which are curved in outline to act as fulcrum pins for the shoe. The surface of the shoe may be plane, thus affording tangential or line contact with the resistance film. It is preferably made of a non-oxidizable metal. It is supported between the resistance element and the contact arm and is moved with the contact arm by reason of seats or recesses 14 which receive the ears I2. It will be manifest. from inspection of Fig. 2, that the contact shoe 10 may oscillate sidewardly to seat itself across the full width of the film regardless of play or Wobbllng of the control shaft. Also, that the shoe may have a width equal to the full width of the resistance film, the front wall of the casing being recessed outside the ring i 2, as is clearly shown in Fig. 3.

Appropriate soldering lugs are provided for connecting the resistor in circuit. In the present :r.

case, lugs 16 and 18 are provided for connection to the ends of the resistance film, although it will be understood that either lug may be used alone when desired.

Lugs

16 and 18 pass through the .outer wall 28 of the casing and are clamped between the resistance element i2 and the end wall 28 of the casing, as is best shown in Fig. 4.

The inner end of the lug is preferably reversely bent, as shown at 80, and wall 28 is preferably cut away to receive the. lug and to prevent lateral displacement thereof. Referring now to Fig. 7, a highly conductive paint is applied from the resistance film around the face of the element, as is shown at 82, for contact with the soldering lugs 18 and 18. Reverting to Fig. 4, the end 80 of lug I6 is recessed slightly'less than its thickness so as to insure good contact between the conductive paint 82 and the lug. To further promote good contact at this point, the screws 38 which secure the resistance element within the casing, are not located diametrically opposite one another, but instead are preferably disposed nearer the soldering lugs, as is clearly evident from inspection of Fig. 1. The contact between the resistance element and the soldering lugs causes a slight bias of the resistance element relative to the control shaft, as is indicated in Fig. 2, but this is no disadvantage, and, on the other hand, is actually of advantage because it results in a slight oscillation of the contact shoe relative to the contact arm as the unit is operated, and this oscillation prevents the accretion of undisturbed dirt or oxide between the contact shoe and the contact arm.

Connection to the contact arm is made by the center lug which passes through the casing and beneath the resistance element, as is best shown in Fig. 2. This lug does not, however. contact the resistance fllm and is accordingly preferably located at the breech I6 between the ends of the resistance film. The lug is bent upwardly or rearwardly at 88 within the keyway l8 of the resistance element. The inner end of the lug bears directly against the contact arm and is preferably formed into a ring 88, best shown in y 2,120,904 Figs. 2 and 3, which ring surrounds insulation washer 40 and bears against the forward side of the hub 40 of the contact arm. If desired, teats 00 may be formed on ring 00 to improve the electrical contact. The lug is. of course, made of material suiliciently resilient to provide a yieldable or spring engagement with the hub of the contact arm. The contact arm is. of course, preferably insulated from the control shaft, as has heretofore been described. The connection between the lug 04 and the contact arm 00 does not add to the axial dimension of the casing, because it takes place inside the ring I2.

The unit shown in Figs. 1 through 5 is arranged to receive an additional soldering lug for intermediately tapping the resistance element. For this purpose the casing 24 is apertured and provided with a recess 92. best shown in Figs. 1 and 5 and corresponding to those provided at the lugs I8 and I8. but the recess 92 is disposed diametrically opposite the lug 84. When an intermediate tap is sought it is merely necessary to preliminarily insert a soldering lug 04 in recess 92 before securing the resistance element I2 in place, as is shown in Fi 6. While the tap is disposed at the center of the resistance element it is not necessary that the electrical connection be made at the same point. and it is possible to change the tapping point without shifting the physical location of the lu This is accomplished as shown in Fig. 8. in which it will be noted that conductive paint is extended from the resistance film at the tapping point around to the face of the element and is then continued along the faceof the element to a point. opposite the keyway I8 or registering with the fourth lug 94.

The open or rear end of casing 24 is closed by a heavy gauge sheet metal closure disc I00. This disc has a diameter larger than the inner but smaller than the outer diameter of the casing, and the edge of the casing is stepped or recessed at I02 to receive the disc I00. The disc is provided with a locatin projection I04, and the .edge of the casing is cut away to receive the projection I04. This insures registration of the disc with respect to the casing. The casing is held in place by a single assembly screw I06 which passes through the front wall 28 of the casing. then through the hole 22 of the resistance element. and is threaded in a mating hole I00 in the closure disc. This screw is so placed on the unit that it does not interfere with the oscilla tion of the contact arm. which possibility is an advantage of the illustrated contact arm, while the use of a single through screw for a strong, rigid assembly is itself an improvement over prior constructions.

The closure disc is struck inwardly to provide a motion limiting stop lug IIO (see Figs. 3 and 5) a which cooperates with the stop arm 04 heretofore referred to as secured to the inner end of the control shaft 40. The end II2 of stop arm 54 is stepped rearwardly. as is best shown in Fig. 2, in order to brin the same close to the disc I00 I and in alignment with the stop lug IIO while at the same time affording ample clearance between the stop lug H0 and the remaining mechanism at the end of the control shaft. A locating lug H4 is bent inwardly at the opposite end of stop arm 54 and is received in a mating recess in the insulation washer 52. It will be understood that with the herein-described construction the various parts are necessarily kept in correct registration and relation during assembly of the unit.

In many cases the resistor, particularly when used as a volume control, is combined with a power s witch for controlling the energization of the complete radio receiver, phonograph, or other device in which the resistor is used. The present unit is readily modified by substituting a power switch assembly for the closure plate I00. Thus, referring to Figs. 9 and 10, I provide an insulation switch casing I20 closed at its forward face by a metal disc I22 dimensioned like closure disc I00 and similarly provided with a projection I04,1a threaded opening I00, and an appropriate stop lug "0'. Connections are made to the switch through lugs I24. The switch is preferably of the toggle variety and is controlled by a rocker I25 to which are secured bearing surfaces I20 which project through a window I00 in disc I22. The bearing fingers I20 lie in the path of movement of stop arm 54, and the parts are so arranged that when the stop arm reaches its end position the switch is turned oil, this being the condition of the parts shown in Fig. 9 in which stop arm 54 is indicated in broken lines. It will be understood that when the resistor is turned from its end position the switch is tripped during the first part of the movement. The switch is held in assembly with casing I02 (or casing 20) by means of assembly screw I00, received in threaded holes I00. The screw does not interfere with the switch mechanism, not only because of its location, but also because of its shortness, for it terminates just inside the plate I22.

For some purposes it is necessary to provide Such units have heretofore proved troublesome to stock because of the variety of combinations of resistance and taper values for the individual units in addition to the variations commonly met with for single units. These problems are minimized by a further feature of my invention in accordance with which multiple resistor units are provided by adding one or more tandem units to a standard resistor. Thus, referring to Figs. 11 and 12,.the resistor I32 is a standard unit such as has heretofore been described. Instead of closing the back of the unit with-either a closure disc or a power switch, I may add a tandem unit I04. This in turn may be open at its back face and adapted to be closed by a closure disc or a power switch unit. The back edge of the tandem unit I04 is preferably made exactly like the back edge of the standard unit I02, so that the usual closure disc I00, or the switch unit of Figs. 9 and 10, or

still another tandem unit may be used to close like the standard casing except that its forward edge is stepped or shouldered at I00, and provided with a locating projection I00, thus adapting the unit to mate snugly with the unit I02. In other words, the front end of unit I04 is shaped like and replaces the closure plate I00. The assembly screw I40 is approximately twice as long as before, and passes through the pair of units from end to end. i

The internal mechanism of the tandem unit I04 is exactly like the standard unit I02. The mechanisms differ slightly externally due to the fact that whereas the control shaft 40 of the standard unit projects well out of the casing, the

. is taken in section like Fig. 3.

a drive disc I48. Disc- I46 is slotted at 8, the waste metal beingbent outwardly to form bearing-surfaces I50 so located and dimensioned as to receive the stop arm 54 of the main-unit I32.

It will thus be understood that when the units are. nested together, stop arm. is no longer used as a stop arm but merely as vcoupling mechanism to rotate the succeeding unit. However, the stop arm of the tandem unit i then assumes the motion limiting function or/and the] power switch operating function. It will also be under,-- stood that -as many tandem units =may be added as desired, and in each case the unitis rotated by the stoparm of the preceding unit while the stop arm of the final unit is used for motion limiting or/and switch control functions.

In accordance with a further feature of my invention, I make possible the-elimination of'a split ring for holding the control shaft in place. Referring to Fig. 3, it is customary to use a split ring 58 in a groove 60 to hold the control shaft against axial movement. This arrangement is simple and convenient except for the tendency on the part of some manufacturers using these units to specify a preferred length for the threaded bushing 30. in many cases differ only by'a small'fraction of an inch, yet it is necessary for the, manufacturer of the resistor to provide different shafts havin the groove 60 locateddilferently by a corresponding amount.

I overcome this cilllculty and at the same time provide a more frictionless type of hearing by an arrangement illustrated in Fig. 14, which figure Referring thereto, the shaft 10' is like shaft ill but is devoid of the groove to and split ring 58. The 'closure plate I is like closure plate I00 but is provided with a hole or seat M0 for a single ball'bearing I62 which is interposed between theclosure' plate and the end of the control shaft. The end of the control shaft is preferably not recessed to receive ball 162, but instead is left fiat. This is done to dispense with the" necessity for accurate alignment of the seat I60 with the shaft axis, or,

in other words, to greatly increase the-manufacturing tolerances involved. Incidentally, the squared end 42' of shaft 40f may be.secured to the stop arm 54' slightly differently than heretofore-described, as by using a round end for the shaft which is passed through a square hole in the stop arm and which end projects beyond the stop arm and is then riveted downwardly by a high-speed riveter which causes the metal to flow outwardly into the square hole. To assemble the unit-it is simply necessary to place the ball I62 on closure plate I00 while holding the plate horizontally, and to then place'the unit downwardly over the closure plate, after which the assembly screw I is screwed in place.

It is believed that the mode of constructing and using, as well as the many advantages of my improved variable resistor, will be apparent from the foregoing detailed description thereof. A

high resistance value of uniform or tapered characteristic is readily obtained because the resistance is merely a film. This film is applied to and backed by a sturdy rigid insulation base. The use of a cylindrical ring facilitates manufacture and calibration of the units and is of advantage in operation because the resistance paths are all of equal length-and therefore uniformly loaded. Variations from concentricity in the unit, or wear and consequent play of the control shaft do not affect the resistance and calibration- These. preferred lengths j 'of the unit. The contact shoe makes a straight line contact with the resistance film, thereby avoiding short-'circuiting' of substantial film area, which is of particular value in tapered resistance units. The .unit is compactin' dimension because all of the resistance film is used at nearly the maximum radius of the unit. A smooth, uniformly light, and non-chattering contact is provided between the wiper shoe and the resistance film, and the shoe is freeto seat itself across the width of the film. The unit is readily adapted ,to'meet varied requirements because of the interchangeability of the closure disc, the power switch, and the tandem unit. An intermediately tapped soldering lug 'may be provided when desired, and the tapping point may be varied without physically shifting the lug. More than one intermediate lug may be used if the casing is modified to receive the same. The unit is simple, compact, inexpensive, and sturdy.

A metal casing may be used if lined with sheet lnsulationi The soldering lugs may be secured directly to the ring, as by riveting through the ring. It will therefore be apparent that while I have shown and described nu; invention in preferred forms, many changes and modifications may be made inthe structures disclosed, without departing from the spiritof the invention, defined-in thefollowlng claims. -I claim:

1. A'variabie resistor comprising a rigid preformed closed ring-of insulation material having a cylindrical outer surface, the radial thickness of said ring being largenenough to-give the ring structural rigidity yet small enough to leave a substantial space at the center of the ring, a film of resistance material coated directly on said cylindrical outer surface. a control shaft, and

slidable contact means insulatedly secured to said shaft and slidably bearing directly against the resistance him. and connection means to said contact means, said connection means being disposed within said ring-and including a metallic ring and a wiper in contact therewith, said ring and wiper being insulated from the control shaft.

2. A variable resistor comprising a rigid pre-.

formed closed ring of insulation material having a cylindrical outer surface, the radial thickness of said ring being large enough to give the ring structural rigidity yet small enough to leave a substantial space at the center of the ring, a film of resistance material coated directly on said cylindrical outer surface, a control shaft concentric with the ring, a yieldable contact arm insulatedly secured to said shaft, and connection means to said arm, said connection means being disposed within said ring and including a metallic ring and a wiper in contact therewith, said ring and wiper being insulated from the control shaft. and a contact shoe self-adjustably mounted on said arm and slidable directly on the resistance film.

3. A variable resistor comprising a rigid preformed closed ring of insulation material, a film of resistance material coated directly on the outer peripheryof the ring, the radial thickness of said ring being large enough to give the rin structural rigidity yet small enough to leave a substantial space at the center of the ring, a control shaft concentric with the ring, a contact arm insulatedly secured to said shaft, and connection means to said arm,'said connection means being disposed within said ring and including a metallic ring and a wiper in contact therewith, said ring and wiper being insulated from the control shaft, said contact arm comprising a radial portion at one face of the ring, and a resilient arouate portion secured to said radial portionand lying outside of the ring, and a contact shoe'disposed between said arm and the resistance film and slidable on the latter.

4. Avariable resistor comprising a rigid pre formed closed ring of insulation material having a cylindrical outer surface, the radial thickness of said ring being large enough to give the ring structural rigidity yet small enough to leave a substantial space at the center of the ring, an interrupted film of resistance material coated directly on said cylindrical surface, a control shaft concentric with the ring, a contact arm insulatedly secured to said shaft, said contact arm comprising a radial portion at one face of the ring, and a resilient arcuate portion secured to said radial portion and lying outside of the ring, a contact or wiper shoe disposed between said arm and the resistance film, and bearing tangentially against the cylindrical surface in order to provide line contact therewith, said shoe being pivoted for transverse oscillation relative to the arm, and

connection means to said arm, said connection means being disposed within said ring and including a metallic ring and a wiper ,in contact therewith, said ring and wiper being insulated from the control shaft.

5. A variable resistor comprising a cylindrical casing with an end wall made of insulation, a ring of insulation material, said ring having its peripheral surface coated with a film of resistance material, means securing the ring with its side face tightly against said side wall, soldering lugs having their outer endsproiecting outwardly from the cylindrical wall of the casing and having their inner ends interposed between the ring and insulation surface and held in place by the pressure therebetween, and conductive paint "extending from the resistance film around the face of the ring to the lugs.

6. A variable resistor comprising a ring of insulation material, the radial thickness of said ring being small enough to leave a substantial space at the center of the ring, said ring having a cylindrical outer surface coated with a film of resistance material, means securing the ring to a suitable insulation surface, soldering lugs interposed between the ring and'insulation surface, means connecting the resistance film to one of the lugs, a control shaft, a contact arm insulatedly mounted on said shaft and supporting a wiper shoe in contact with the resistance film, and means within said ring connecting the contact arm to another of the lugs.

'7. A variable resistor comprising a ring of insulation material, said ring having acylindrical outer surface coated with an interrupted film of resistance material, said ring being relatively thick in radial direction and perforated to receive screws passing therethrough in an axial direction and securing the ring to a suitable insulation surface, the radial thickness of said ring being small enough to leave a substantial space at the center of the ring, three soldering lugs interposed between the ring and insulation surface and held in place by the ring, conductive paint extending from the ends of the resistance film around the face of the ring for connecting.

the ends of the resistance film to the outer two lugs, a control shaft concentric with the ring,

and a contact arm insulatedly mounted on said" shaft and supporting a wiper shoe in yieldable sliding contact with the resistance film, the center lug within the ring and bearing against the contact arm in the space within the ring.

8. A variable resistor comprising a hollow casing open at one end, a bushing secured to' the closed end, a control shaft passing therethrough,

a ring of insulation material having a cylindrical outer surface coated with an interrupted film of resistance material, means securing the same to the closed end of the casing in concentric relation with the shaft and bushing, soldering lugs passing through the casing and having their inner ends interposed between the ring and casing, means connecting the resistance film to one or more lugs, a contact arm mounted on the inner end of the control shaft and supporting a wiper shoe in contact with the resistance film, means within the ring connecting the contact arm to another lug, a stop arm mounted on the inner end of the control shaft and holding the contact arm in place, a closure plate outside the stop arm for closing the open end of the casing, and a stop lug on the inner face of said closure plate cooperating with the stop arm.

9. A variable resistor comprising a hollow casing open at one end, a bushing secured to the closed end, a control shaft passing therethrough, a ring of insulation material having a cylindrical outer surface coated with an interrupted film ing lugs passing radially through the casing and having their inner ends interposed between the ring and casing and held in place thereby, conductive paint extending from the ends of the resistance film around the face of the ring to two of the lugs, a contact arm mounted on the inner end of the control shaft and supporting a wiper shoe in yieldable contact with the resistance film, the third lug coming within the ring and bearing yieldably against a part of the contact arm, a stop arm mounted on the end of the control shaft and holding the contact arm in place, a closure plate outside the stop arm for closing the open end of the casing, and a stop lug on the inner face of said closure plate cooperating with the stop arm. g

10. A variable resistor comprising a ring of insulation material havinga cylindrical surface and a side face adjoining said cylindrical surface, said cylindrical surface being coated with a film of resistance material, a soldering lug bearing against said side face, conductive paint extending from an intermediate point on the resistance film to the face of the ring and along the face of the ring to the lug, whereby the resistance may be tapp d at a desired intermediate point even if displaced from the lug.

11. A variable resistor comprising a ring of insulation material having a cylindrical outer surface and a side face, said cylindrical outer surface being coated with an interrupted film of resistance material, means securing the same to an insulation surface with the side face lying against said surface, soldering lugs having their inner ends interposed between thering and surface, conductive paint extending from one end of the resistance film around the side face of the ring to one of the lugs, conductive paint extending from an intermediate point on the film to the side face of the ring and along the face of the ring to another lug, whereby the resistance may be tapped at a desired intermediate point even if displaced from the lug.

12. A variable resistor comprising a ring of insulation material having a cylindrical outer surface and a side face, said cylindrical outer surface being coated with an interrupted film of resistance material, means securing the same to an insulation surface with the side face lying against said surface, four soldering lugs passing radially through the casing and having their inner ends interposed between the ring and surface, conductive paint extending from the ends of the resistance film around the side face of the ring to two of the lugs, a contact arm mounted on a control shaft and supporting a wiper shoe in yieldable contact with the resistance film, a third lug being arranged to bear against the contact arm, and conductive paint extending from an intermediate point on the resistance film to the face of the ring and along the face of the ring to the fourth lug, whereby the resistance may be tapped at a desired intermediate point by the lug without changing the position of the lug.

13. A variable resistor comprising a casing, a. bushing secured to one end of said casing, a control shaft passing therethrough, a ring of insulation material, said ring having a cylindrical outer surface coated with an interrupted film of resistance material, a contact arm on said control shaft, said arm comprising an annular hub portion, a radial portion alongside the ring, and an arcuate portion secured to said radial portion and lying outside of the periphery of the ring, a contact shoe interposed between the end of the arcuate portion and the resistance film, and a soldering lug having a resilient portion bearing against the hub portion of the contact arm.

14. A variable resistor comprising a casing, a bushing secured to one end of said casing, a control shaft passing therethrough, a ring of insulation material, said ring having a cylindrical outer surface and a side face, said cylindrical outer surface being coated with an interrupted film of resistance material, means securing the ring to the casing, soldering lugs passing through the casing and having their inner ends interposed between the ring and casing, conductive paint extending from the resistance film around the side face of the ring to one or more lugs, a contact arm on said control shaft, said arm comprising an annular hub portion, a radial portion, and an arcuate portion secured to said radial portion and lying outside of the periphery of the ring, a contact shoe interposed between the end of the arcuate portion and the resistance film, said shoe contacting tangentially with the fdin for line contact and being oscillatable on the arm, one lug passing within the ring and having a resilient annular portion bearing against the hub portion of the contact arm.

15. A variable resistor comprising a hollow casing open at one end, a bushing secured to the closed end, a control shaft passing therethrough, a ring of insulation material, said ring having a cylindrical outer surface coated with an interrupted film of resistance material, means securing the same to the closed end of the casing in concentric relation with the shaft and bushing, soldering lugs passing through the casing and having their inner ends interposed between the ring and casing and held in place thereby, a contact arm on said control shaft, said arm comprising an annular hub portion, a radial portion arranged at the face of the ring, and an arcuate portion secured to said radial portion and lying outside of the periphery of the ring, a contact shoe interposed between the end of the arcuate portion andthe resistance film, a third lug having a resilient annular portion bearing against the hub portion of the contact arm, a stop arm on said shaft, a closure plate for the open end of the casing, and a stop lugon said closure plate cooperating with the stop arm.

16. A variable resistor comprising a casing open at one end, a bushing secured to the closed end, a control shaft passing therethrough, a circular resistance element, a contact arm mounted on the control shaft for contact with the resistance element, a stop arm mounted on the control shaft, a power switch assembly, a closure for the resistor casing forming a part of the power switch assembly, a stop lug on the closure for cooperating with the stop arm to limit the movement in either direction of the control shaft, and operating means on said power switch projecting through the closure into the resistor casing for cooperation with the stop arm of the resistor.

17. A variable resistor comprising a casing open at one end, a bushing secured to the closed end, a control shaft passing therethrough, a ring of insulation material having a cylindrical outer surface coated with an interrupted film of resistance material, means securing the same to the closed end of the casing, a contact arm mounted on the control shaft and SllpIJOltln a wiper shoe in yieldable contact with the resisiance film, a stop arm mounted at the end of the control shaft, a power switch, a closure or the resistor casing forming a part of the power switcl, assembly, a stop lug on the closure i'J-r cooperaiing with the stop arm to limit th :l' the control shaft, and means: switch projecting through the closure into the resistor casing for cooperation with the stop arm of the resistor.

18. A variable resistor comprising a hollow cas ing open at one end, a bushing secured to the closed end, a control shaft passing thercthrough a ring of insulation material having a cylindrical outer surface coated with an interrupted film resistance material, means securing the same t i the closed end of the casing in concentric relation with the shaft and bushing, soldering lugs passing radially through the casing and havim: their inner ends interposed between the ring and casing and held in place thereby, conductive ."vment of said power paint extending from the resistance film around the face of the ring to one of the lugs, a contact arm mounted on the control shaft and suppor ing a wiper shoe in yieldable contact with the re sistance film, one of the lugs being arranged to bear yieldahly against a part of the contact arm. a stop arm mounted on the control shaft, a power switch assembly, a closure disc for the resistor casing forming a part of the power switch assem bly, a stop lug on the closure for cooperating with the stop arm to limit the movement of the control shaft, and means on said power switch projecting through the closure disc into the resistor casing for cooperation with the stop arm of the resistor,

' 19. A tandem variable resistance unit comprising a main single resistance unit of standard type mounted within a casing having an open rear end adapted to detachably receive a closure plate, a

control shaft for varying the resistance of the received within the open end of the first unit, a bushing on said forward end supporting a control shaft for the second unit, and means for coupling the control shaft of the second unit to the control shaft of the first unit when the units are connected in tandem, the part of said means on the first unit being standard to the first unit even when no second unit is added.

20. A tandem resistance unit comprising a main resistance unit of standard type mounted within a casing having an open rear end adapted to detachably receive a closure plate having a stop lug, a second resistance unit mounted within a casing having a similar open rear end adapted to receive said closure plate, the shafts of each of said units having a stop arm for cooperation with the stop lug, the forward end of said second unit being dimensioned like the closure plate and adapted to be received within the open end of the first unit, and means on the control shaft of the second unit mating with the regulation stop arm of the first unit when the units are connected in tandem.

21. A tandem resistance unit comprising first and second resistance units arranged end to end in coaxial relation, each of said units comprising a hollow casing open at one end, a bushing secured to the closed end, a shaft passing therethrough, a resistance element, a contact arm mounted on the shaft and supporting a wiper shoe in contact with the resistance element, and a stop arm mounted on the shaft, the shaft of the first unit projecting well out of the unit for manual operation, a closure plate adapted to close the open end of either casing, a stop lug on said closure plate cooperating with the-stop arm, the closed end of the casing of the second unit being dimensioned like the closure plate and adapting the same to be received at the open end of the first unit, the shaft of the second unit having at its forward end a coupling member cooperating with the regular stop arm of the first unit, so (that both units are operated: in unison.

22. A tandem resistance unit comprising first and second resistance units arranged end to end in coaxial relation, each of said units comprising a hollow casing open at one end, a bushing secured to the closed end, a shaft passing therethrough, a resistance means securing the same to the closed end of the casing, a contact arm mounted on the shaft and supporting a wiper shoe in contact with the resistance film, and a stop arm mounted directly on the end of the shaft, the shaft of the first unit projecting well out of the closed end of the unit for manual operation, aclosure plate adapted to close the open end of either casing, a stop lug on said closure plate cooperating with the stop arm, the forward end of the casing of the second unit being dimensioned like the closure plate and adapting the same to be received at the open end of the first unit, the shaft of the second unit having at its forward end a coupling member cooperating with the regular stop arm of the first unit,

'so that both units are operated in unison.

23. A variable resistor comprising a hollow casing open at one end, a bushing secured to the closed end, a control shaft passing therethrough, an insulation ring smaller in diameter than the casing and coated with a resistance film on its outer cylindrical wall, a contact arm mounted on the control shaft and having a part lying between the exterior of the ring and the interior of the casing, said part extending in a peripheral direction and being relatively long and yieldably contacting with the resistance ring, a closure I plate closing the open end of the casing, a power switch on said closure plate, means on said shaft to actuate said switch, and a single screw for detachably holding the closure plate on the casing, said screw passing through the casing and ring and being threaded into said closure plate at a point substantially inwardly of the casing wall and displaced from the center but so positioned as not to interfere with the oscillation of the contact arm over the effective portion of the resistance ring, nor with the said switch actuating means.

24. A variable resistor comprising a hollow cup shaped casing open at one end and closed at the other, a bushing permanently secured to the closed end, a control shaft passing therethrough, the outer end of the shaft projecting from the closed end of the casing, and the inner end of the shaft terminating inside the casing, a ring of insulation material coated with a film of resistance material, means securing the same to the closed end of the casing, a contact arm mounted on the control shaft inside the casing and supporting a wiper shoe in yieldable contact with the resistance film, a stop arm mounted directly on the inner end of the control shaft within the casing, a thin fiat closure plate fully closing the open end of the casing beyond the stop arm, and a stop lug projecting inwardly into the casing directly from the inner face of said closure plate, said stop lug cooperating with the stop arm to limit the movement of the shaft in both directions.

25. A tandem variable resistance comprising a main resistance unit including a casing having an open rear end adapted to detachably receive a closure plate, a second resistance unit including a casing having a similar open rear end adapted to receive said closure plate, aresistance element and a cooperating control shaft and contact arm in each of said casings, the forward end of said second unit being closed and dimensioned like the closure plate and adapted to be received within the open end of the first unit, means for coupling the control shaft of the second unit to i the control shaft of the first unit when the.

the effective portions of the resistance elements.

26. A variable resistor comprising a stationary approximately cylindrical casing, a stationary ring formed of insulating material disposed concentrically within said casing and having a cylindrical outer surface, the ring having a radius smaller than the casing to provide an annular space between the periphery of the ring and the interior of the casing, said ring being secured to one end of the casing, a film of graphitic, carbonaceous, or like resistance material coated on the periphery of said ring and extending from side edge to side edge of the ring, the band of resistance material being equal in width to the thickness of the ring, a control shaft disposed axially of and rotatable relative to said ring and casing, an arm extending radially from said shaft outside of that face of the ring opposite the face secured to one end of the casing, a metallic contact shoe engaging said film of resistance material and adapted to slide thereover, and a resilient readily yieldable arcuate arm partially surrounding said ring and fixedly secured at one end to the radial arm, said contact shoe being secured at the other end of the yieldable contact; arm, said arcuate arm and contact shoe being received in the aforesaid annular space between the ring and casing.

27. A variable resistor comprising a hollow casing open at one end, a bushing secured to the closed end, a control shaft passing therethrough, a ring of insulation material coated with a film of resistance material, means securing the same to the closed end of the casing, a contact arm mounted on the control shaft and supporting a wiper shoe in yieldable contact with the resistance film, a stop arm mounted directly on the end of the control shaft within the casing, and a closure for the open end of the casing, said closure having a stop lug projecting, inwardly directly from the inner face of the closure and cooperating with the stop arm, the open end of the casing being adapted to interchangeably detachably receive said closure in either of two forms, one of said closures being in the form of a plain plate and the other being in the form of a power switch, the aforesaid stop lug having an effective location best suited to the partioular'form of closure on which it is mounted.

HENRY G. RICHTER.