US2101441A - Rheostat or potentiometer - Google Patents

Description

Dec. 7, 1937. 1 MRSTEN 2,101,441

RHEOSTAT OR POTENTIOMETER y 'Filed Jan. 12, A1934 2 sheets-sheet 1 TF1-'127062Z 5203/ I Tf1 INVENTOR YJesse Marsen,

Dec. 7, 1937. J. MARSTEN 2,101,441

RHEOSTAT OR POTENTIOMETER Filed Jan. 12, 1934 2 Sheets-Sheet 2 68` Y 32 4/ MJ 31 4951/48 ,5f

INVENTOR .fesse Mars en ATTORNEYS Patented Dec. *7, 1937l PATENT OFFICE RHEOSTAT 0R POTENTIOMETEB Jesse Max-sten, Philadelphia, Pa., asllxnor: to International Resistance Company, Philadelphia, Pa., a corporation o! Delaware Application January 12, 1934, Serial No. 706,348

27 Claims.-

The invention relates to rheostats, and/or potentiometers, and, .with respect to its more specic features, to rheostats and/or potentiometers for use in connection with audio and radio frequency apparatus to control the intensity of the sound waves.

One object of the invention is to provide, in a variable resistance device, a contact which will short-circuit a minimum portion of the resistance element or in which the sum of resistance on both `sides of the variable contact shall be, as nearly as possible, equal to the total resistance of the resistance element.

Another object of the invention is to provide a variable resistance device giving a smooth uniform change of resistance as the movable .contact is moved. Another object of the invention is to provide a variable resistance device which will give a minimum amount of noise in an audio circuit. Another object of the invention is to pro. vide a type of contact area which consistently makes contact at the same point or points of the area and maintains absolute uniformity of change of resistance in accordance with the resistivity at each part of the resistance device.

' l Another object of the invention is to provide a device of the character indicated, of extremely simple and durable construction. Another object of the invention is to provide a device of this class subject to a minimum amount of wear of the re- 0 sistance `element in use in order that the device u susceptible to variation in embodiment so far as the resistance curves are concerned to accommodate the varying needs ol purchasers thereof. Another object of the invention is to provide an effective and reliable manner of making the electrical connection between xed terminals and the resistance element. Another object of the invention is to provide'a means for introducing a tap at any point along the resistance element between the end terminals without interfering with the smooth movement of the movable contact element or in any manner destroying the symmetry of surface of the resistance element. Another object of the invention is to provide a resistance element and a contact adapted to form a good path for the current at the contact point subject to a minimum of wear while in use and a minimum of change due to passage of time or conditions of the atmosphere. Another object of the invention is to provide a rheostat and/or potentiometer and switch readilyadaptable for instal;

lation in commercial radio sets and which may be easily connected in circuit therein as desired.

The invention accordingly consists in the features of construction, combinations o! elements and arrangement of parts and in the several steps and relation and order oi each oi said steps to one or more of the others thereof, all as will be illustratively described herein, and the scope of the application of which will be indicated in the following claims.

In the accompanying drawings, in which is shown one of various possible embodiments of the mechanical features of this invention,

Figure 1 is a rear elevation of the rheostat;

Figure 2 is a cross-sectional view taken on the line 2-2 of Figure l; l

Figure 3 is a plan view of the resistance element; I

Figure 4 is a developed View of the periphery of the insulating disc and resistance element with connections;

Figure 5 is a perspective view of a contact brush;

Figure 6 is a front elevation of the insulating base;

Figure '7 is a rear elevation of th'e insulating base;

Figure 81s an elevation of the commutator disc and contact lingers;

Figure 9 is a side elevation of the commutator and contact fingers and operating shaft;

Figure 10 is a sectional view through a radio panel showing the rheostat mounted in elevation;

Figure 11 is a sectional view through the metal casing showing a snap switch device'in elevation;

Figure 12 is a sectional view on the line |2-l2 of Figure l1 (with the parts in a different position) showing the switch actuator in elevation;

Figure 13 is'a diagrammatic view illustrating the construction of the resistance element and its relation to the various contacts;

Figure 14 is a view similar to Figure 13, being however a cross-sectional view taken on the line ld--N of Figure 13;

Figure l5 is a plan view of a terminal;

Figure 16 is a diagrammatic view of the resistance element and certain contact making devices, viewed as from the front of a radio set in which the apparatus is installed.

Similar reference characters refer to similar parts throughout the several views of the drawings. A

Referring ilrst to Figure 2, I provide an insulating support or plate i0 for mounting the electrical and mechanical elements of the rheostat. This support or plate i0 may be made of any suitable insulating substance; however I prefer to make it of moldable material in order that the several parts thereof, hereinafter pointed out, may be made in a single piece reducing cost and eliminating the possibility of mechanical vibration. Among substances suitable for this purpose are molded hardrubber and artificial tially a disc having a hole i i therethrough at` the center. 'As shown by a comparison of Figures 2, 6, and 7, the hole il, which is round, 'merges at the rear of the support i@ with a hexagonal recess i2 the bottom of whichforms a shoulder i3. I provide a brass bushing ill which fits in the hole ii and which has a hexagonal head i5 tting in the recess l2. The bushing it has a cylindrical portion i@ of substantially the same length as the hole ii, this cylindrical portion i6 being bounded by a shoulder Il. I secure theplate l@ and bushing ifi together by inserting the bushing in the plate, as shown in Figure 2, and forcing the metal of the shoulder il outwardly with a suitable tool; thus the bushing and the plate are permanently locked together.

The bushing i12 constitutes a journal for a shaft i8 as shown in Figure 2. Angular movement of this shaft it? causes the resistance in circuit to be varied as will hereinafter appear; it is the control shaft for controlling the intensity or volume of sound of the radio or otherA audio frequency apparatus, and in this particular embodiment of the invention it also controls the main switch of the apparatus. Still referring to Figure 2, one end of the shaft it has a portion- I9 of reduced diameter leaving a shoulder 20. An insulating disc 2i, preferably of canvas phenol or other strong insulating substance, is attached to the end of the shaft it, resting against the shoulder 2li and securely fastened in place by heading over the portion i9, sothat the disc 2l rotates with the shaft it.

Referring now particularly to Figure 8,'I pro-` vide a commutator and movable contact member 22 made out of a single piece of metal having contact arms 23, 23, 23 as an integral part. thereof. The member 22 is of metal so that it shall be conductive, and of sheet metal in order that it may be readily and cheaply manufactured and in order that the contact arms-23 may incorporate means for' securing spring pressure against the resistance without the introduction of further separate members involving a movable contact. It will be readily appreciated that it is desirable to keep the number of separate electrical elements as low as possible consistentwith other features lin order that the device shall not be `rnicrophonic in an audio circuit. Furthermore, the member 22 is preferably made of va nonoxidizable metal and for reasons which will hereinafter appear, a metal which shall not release an ox'idizable deposit. It is also to be noted that the-conductivity of the member 22 should bev good, and that expensive metals are desirably not employed. I prefer to use' Monel metal for the member 22, being an alloy of copper and nickel, and possessing all the above desired characteristics, which has, furtherfore, proved satisfactory in use. I have found that nickel is also satisfactory.

Referring again to Figure 8, the member 22 is roughly a disc having an outward projection 24 from which the three contact arms 23 extend inl a substantially circumferential direction. Comparing Figure 8 with Figure 9 it will be noted that the arms 23 are-inclined with respect to the plane of the member 22.

The member 22 is securely fastened to the insulating disc 2i by means of a plurality of hollow rivets 25, located in holes 25 (see Figure 8). The member 22 is insulated from the shaft iS by reason of the fact thatI the head of the portion i9 of reduced diameter is spaced from the member 22 which has a central hole 2 therein. Comparing Figures 8 and 2, there is a projection 28 at right angles to the plane of the member 22A which is formed by pressing out the metal of the member 22 from a rectangular part E@ thereof which merges with the hole 2l. This projection 23 constitutes a stop to limit the movement of the shaft it and all parts carried thereby in either direction, contacting with a stationary stop which will be presently described. As best shown in Figure 2 one of the rivets 25 constitutes the end of a short rod @il which pro-s4 jects at right angles to the surface of the member 22 on the side opposite that from which projects the projection or stop 28 and the arms 2t, and

I peint out that it preferably consists of a strip of impervious insulating substance unaffected by moisture, and I have found phenol to be very satisfactory. The surface of this insulating substance is covered with a conductive resistance material preferably of a carbonaceous nature;

For the attachment .of the resistance @l and the electrical connection of its two ends in circuit I provide combined mounting members and terminals consisting of rivets 32 having a broad under surface engaging the top of the resistance 3i, and extending through the insulating support or plate i@ and also holding in position bendable sheet metal projecting terminals 33 for the connection in circuit of the device. These terminals- 33 are best shown in Figure 16 and comprise an enlarged portion 3d having a hole therein and a long shank 35 havinga hole at the end Athereof. Referring now to Figure 6 the enlarged portions 3d of the terminals 33 t in recesses :it in the insulating substance with their Shanks 35 extending through grooves 3`i that merge with the recesses and thus they are held from angular motion.

A third terminal 38 like the terminals 33 is located between them and positioned in a recess 3Q like the recess 36 which has merging with it a erably made of phosphor bronze, which is shown in perspective in Figure 5 and has contact surfaces I3 contacting the under portion of the commutator and movable contact member 22 to connect it and its arms 23 in circuit. The phosphor bronze Y-shaped brush 42 is not only a good conductor and makes an excellent nonmicrophonic contact with the movable commutator member 22, but it also is resilient and constitutes a spring to keep the contact surfaces 43 pressed against the bottom of the member 22.

In many apparatus it is desirable to use a single variable resistor in two circuits, or tozuse portions of the, variable resistor element in diiferent circuits, and therefore to establish an electrical connection with the resistance element of the rheostat somewhere between its end terminals. To that end I provide means to tap into the resistance element 3| at any desircdpoint, comprising a terminal 43 (Figure 15) having projections, ridges, upset portions or the like, which is held in place by a rivet Il like the rivets 32,` only shorter, and which. as shown in Figure 2, establishes contact with a piece of sheet Ametal 43 which in turn makes contact with the resistance!! in a manner which will be more fully pointed out.

hereinafter. The ridges in the terminal l cause it to grip the insulating base i0 so that it isv as firmly held in place as the other terminals, but it can be located anywhere.

From the foregoing description it will be seen that rotation of the shaft I l the contact arms23 to travel over the surface of the resistance 3|. 'I'he contact arms 23 can be connected to receive current, positive, negative or alternating, by means of the terminal Iland connections described including the Y-shaped brush I2 and commutator and Vmovable contact member 22. One or the other or both ends of the resistance 3l can be connected in circuit, as desired, through the terminals 33 and rivets 32, and current can valso be taken oil' the resistance element 2l at any desired place by means of .the terminal I8 and piece of sheet metal 4B. A

Referring to Figure 7, on the back of the insulating support or .plate l0 I provide a trapeaoidal projection 30 molded integral with the plate IB, and this is contacted by the projection 28 to form a mechanical stop limiting movement of the parts in each angular direction. y

Referring now to Figures 10 andll, I preferably provide a sheet metal casing 3i covering the operating parts of the rheostat and also serving to support a switch which will be presently described. The sheet metal casing Bi is in the form of a cylindrical cup as shown by Figures 10 and 1l, and, referring to Figure 10 in connection with Figure 6, is provided with three integral bendable metal tabs 52 received in three corresponding recesses 53 on the front of the plate I0, the tabs being bent over in position in the recesses to secure the parts together, and the lip of the cup 5I being received against an annular shoulder 54 in the plate lll which is best shown in Figure 2.

The entire device is adapted for attachment to a radio panel with the rheostat and easing 3i on the inside of the radio receiving apparatus andthe shaft I3 projecting outside y thereof..

These panels of radio apparatus, represented by the numeral 55 in yFigure 10, are at present 'generally made out of sheet steel or the like for the sake of economy.A Accordingly all'terminals of the rheostat should be insulated therefrom. The bushing I4, as shown in Figure l0, extends through a hole formed in the panel Si, the bushopposite the terminal 33 in case it is desired to ground this terminal by connecting it to the bushing il. I likewise 'provide two conical projections 58 which seat in holes in the panel 55 to prevent relative angular movement of theplate l0. When the nut 56 is tightened the plate i0 and all parts of the rheostat are firmly secured in position. The shaft I3 is prevented from movement inwardly of the rheostat by a groove 53 therein in which ilts a split spring washer 60. It will be seen that shaft i8 and all parts carried by'it can be readily inserted in the bushing il' for assembly of the device prior to insertion -of this washer 60. The front of the shaft IB has ailixed to it in any usual or suitable way a. knob or handle il by means of which the rheostat is controlled. It will be seen that notwithstanding the grounding of certain metal parts of the device, the movable commutator and contact member 22 is insulated from all ground connections unless a deliberate ground connection is made to the bushing by a wire or other means fromthe terminal 38.

Considering now the construction of the resistance 2l, and referring particularly to Figures 4, 13, and 14, the base or insulating part 62 is, as already stated, preferably of phenol or' similar articial resin or the like. Upon one surface of this support 62 I apply, as with a brush or otherwise, a basic layer of carbonaaeous substance 63. For the substance 63 I prefer a carbon or graphite suspension in phenol varnish. I may use graphite and phenol varnish together, or I may use graphite and lamp black and phenol varnish, and in some cases I use a water solution of one of these carbonaceous substances, with varnish.

After the coating 63 of carbonaceous substance together withsome binder is applied to the strip of resin 62, I bake the coated strip at a relatively high temperature for a time long enough to stabilize the resistivity of the coating 53 so that it shall not be subject to any substantial subsequent variation through aging. The baking process also hardns the coating $3, thus protecting against abrasion from the movable contact, renders it impervious to moisture, and the phenol varnish therein to a certain extent unites with the phenol of the base 62 so that practically an integral piece results in which the top layer thereof contains the desired amount of conductive carbonaceous substance.

' Considering now the contact arms 23 which,

, as heretofore stated, are preferably made of microscopic irregularities or other variations in thickness ofthe resistance material do not cause sudden. shifting of the point at which the major portion of the current between contact and' resistance material flows, as in the case of many former constructions utilizing a flat conwill presently be pointed out.

tact shoe or one having contact with the resist- 1 ance element over a wide area. At the same time by the provision of three arms the resistance at the contact and through the contact itself is kept at a low value as compared with the total resistance which is a desideratum in this class of devices. It should be noted, moreover, that the three arms 2 3 are resiliently urged into engagement with the resistance 3ipand conseis the lowest point' of its nger (eliminating the necessity of accurate alignment), and variations in the resistancev element surface do not cause a shift in the locus of contact nor change the contact resistance. Owing to the resiliency of the contact ngers the pluralityv of contacts is maintained at alltimes.

I have found that a very satisfactory variable contact is achieved between a rounded surface of Monel metal and a baked graphitic substance including resinous binder. The graphite acts to a certain extent as a lubricant but does not show a detrimental amount of wearvover a long period of time and a great many thousand oscillations of the arms 23. Furthermore, it will be observed that the path ,traced by the projections @6, being relatively narrow as compared with the total width of the resistance element 3i, could not cause resistance thereof to be greatly increased even if some of the graphite were eventually worn away, and this is a particular advantage resulting from the use of ball-like projections b on resilient contact arms.

Considering,r now Figure 4, this shows in a developed view an edgeelevation of the rheostat disc it, as though the rheostat were rotated for the-draitsman so that the left' andv right hand ends of the iigure represent the same straight line. The split is made at the center terminal 38 which is shown half on the left hand side of Figure and half on the right hand side thereof. One of the rivets 32 has in contact therewith a piece Vof sheet metal @i like the piece of 4sheet metal it already referred to. This piece Si is a long arcuate strip having bent-up ears t8 and having a hole for the passage of the rivet 32, and the metal around the hole is pressed into the hole through the disc iii so that direct contact shall be made between the rivet 32 and the piece of metal tl. The ears t3 are bent over and contact with the sides of the resistance element .tl and with the top thereof for a purpose which The sheet metal piece d8 -has corresponding ears 69 which are likewise bent up and over onto the resistance element 3i. It is noted that the contact fingers 23 Y clear all ears @il and 63S.

- Referring now more particularly to Figure 13 it will be noted that it is a diagrammatic view, but the general relationship of the parts is similarto Figure 4. Under the head of each rivet 32 I apply a coating lil of carbonaceous material of relatively goodY conductivity, and I apply'sucho a. coating also under the ears 68 and 49. Distinguishing these coatings by the numerals 10a,

. conductive material.

'10b and 10c, I note that I apply under the ears S8 tothe otherwise completed resistance strip Si a preliminary coating "li of relatively highly This coating, as shown by the diagrammatic cross-section of Figure 14, extends down the sides of the resistance element 3i as well as across the top thereof. It may be very conductive carbonaceous substance, such as a coating of the nature hereinbefore described for the surface S3 but with a very high proportion of graphite or the like, or it may be a metallic spray or paint. The bent-up portions t@ contact this coating li along the sides thus establishing direct low resistance contact between the film ofresistance material @3 at the cross-section tl-it and the corresponding terminal 32.

As heretofore indicated the coating 'ita is placed over the coating il. This may be a baked coating of low resistance material of Athe nature already referred to, but having a resistance somewhat higher than the coating il. Under the ears .lit and under the'coat'ing 'iib is another coating ii of the nature already described, the result in this case being to establish direct electrical con- -tact between the rivet di and the carbonaceous substance 83 at the ears lit.

lThe purpose of applying the coating il to the resistance device ti is to establish a low resistance contact between the ears tt and le of the sheet metal strips ti and l, and the resistance element at these particular points. VThis is established both along the sides and along the marginal edges of the strip 52. p `Each of the low resistance coatings l@ serves a further purpose in making a good contact between the rivets 32 and the resistance t3 as the contact resistance between high resistance material and a metal is higher than between a metal and a low resistance, and it is desirable that contact resistance be reduced to a iow value. It will be readily appreciated that provision of several distinct means for insuring excellence of contact between the several terminals and the resistance device 3i increases the eciency of the product considered in bulk. 'in other words the chances oi an individual rheostat and/or potentiometer failing to live up to specications are much reduced. it will be seen that I may vary the construction of the resistance element and its connection to terminals as desired in order to create a diversity of product for varying uses, and one or more of the means for eecting good-contact may, in particular instances, be dispensed with.

Considering now Figures l0, ll, and i2, I mount a snap switch device i5 on the sheet metal casing 5i. This snap switch may have terminals l, and the casing thereof, indicated by the nu- I meral iii may be of phenol or the like. I shall not herein describe the details of this snap switch as it maybe of any usual or desired form, but considering the internal view of Figure 12, it has a pair of ears Tl between which is adapted to enter the rod 3d. Figure l2 shows the casing with the rod just leaving the space between the ears il; upon reverse rotation lof the rheostat the rod enters between the ears again and snaps the member it supporting ears 'it to the dotted line position of Figure i2. A spring le is provided to snap the member it to its extreme positions after the rod Sti has started the motion.

Inside the 'casing v75 are knife switch elements casing 15 may be rigidly secured to the casing 5| by rivets 80 (Figures 10 and 12) and the movable parts of the switch project through a hole 8| in the casing 5l. Thus preliminary motion of the knob BI operates the snap switch 15, which serves the purpose of supplying current to some device, for example a radio set. A certain angular motion of the contact arm is required for this operation and the contact fingers 23, when the switch is closed, are therefore in a position along the resistance element 3l spaced a certain distance XX (see now Figure 16) from the terminal 32. If it were not for the provision of one or more of the several short circuitlng devices herein described, the resistance introduced in circuit at this momentwould be the resistance of the coating 63 along the arc It is frequently desirable that the resistance at this moment between the variable contactv and the nearest terminal (32) be extremely low, for example of the order of l to 10 ohms. While the highly conductive coating 10a results in a low resistance, nevertheless it is not always possible thereby to secure a suiliciently low resistance economically. Therefore, desirably I incorporate the strip 61 in my apparatus, vlocating the ears 68 at the locus of the contacting points, of the arms 23 at the moment that'the switch makes the radio set operative. It will be seen that the current path at that moment is from the ilngers 23 to the ears 68 radially, or if a metal coating 1| is used, merely depthwise through the superimposed coating 10, the remainder of the current path being, in either event, substantially a short circuit. In this manner an exceedingly low resistance is secured which could not otherwise be obtained.

- It will thus be seen that there has been provided by this invention a method and apparatus in which the various objects hereinabove set forth, together with many thoroughly practical advantages, are successfully achieved. It will be further seen that I have provided a switch and rheostat or potentiometer particularly applicable for use as a volume control in'a radio apparatus, and that the construction shown can be connected in circuit in a. great variety of ways to suit the requirements of particular radio manufacturers, and thus the product has a high degree of universality and may be a standard product for use in a great number of diiferent radio circuits or electrical apparatus.

As various possible embodiments may be made of the above invention, and as many changes might be made in the embodiment above set forth, it is to be understood that all matter hereinbefore set forthor shown in the accompanying drawings is to be interpreted as illustrative and not in a limiting sense.

I claim: s

l. In apparatus of the class described, au insulating base, a bushing in said basava shaft mounted in said bushing, a disc of insulating material mounted on said shaft, a metallic disc mounted on said insulating disc, a plurality of fingers integral with said metal disc, a horseshoe shaped piece of resistance material mounted on said base contacted by said plurality of ngers, a terminal and lead connected near one end of the horseshoe to said resistance material, and a lead connected at an intermediate portion of the horseshoe and comprising a metallic plate under said resistance material having ears extending therefrom and in contact with marginal edge portions of said resistance material, whereby a smooth unbroken path is provided for the contacting ends ofl the plurality of fingers despite the connection of the lead at an intermediate portion of the horseshoe.

2. In apparatus of the class described, a base, a rotatable shaft mounted in said base, a plurality of metal fingers carried by said shaft, a horseshoe shaped piece of resinous insulating material 'mounted on said base, a baked coating of carbonaceous substance onv said horseshoe shaped piece, and spherical. shaped projectionsl formed on said metal fingers to form point contacts with said baked coating, the points of contact of said projections and said coating being in a radial line with respect to the axis of said shaft.

3. In apparatus of the class described in combination, a resistance strip, means forming a contacting part engaging said strip and adapted to move therealong, a terminal connection substantially at one end portion of said strip including a pair of elements engaging the sides of said strip and spaced from said last mentioned end thereof, a switch in the circuit of said terminal, and means associated with said switch and said part adapted to close said switch when the contacting portion of said part is substantially adjacent said elements.

4. In apparatus of the class described in cornbination, a resistance strip, a contacting part, means adapted to move said part along said strip and in engagement with said strip, a Iterminal structure connected to said strip and having elements contacting said strip at points spaced from the ends thereof, the portion of said strip adjacent said elements being of much greater electrical conductivity than the remaining portion of said strip, a switch in the circuit of said terminal structure,.and means associatedwith said part and adapted to actuate said switch when the contacting portion of said part engages said last mentioned portion of said strip.

5. In apparatus of the class described, in combination, an insulating base, a substantially circular shaped iiat strip having electrical conductivity of high resistance value mounted on said base, a shaft rotatably mounted in said base so that its axis-of rotation passes through the center point of said strip, an armature arm connected to said shaft and shaped to engage said strip so that rotation of said shaft moves the contacting portion of said arm along said strip, means forming a terminal connection at one end portion of said strip including an ear engaging a side of said strip at a. point spaced from the end to which said terminal is connected, 'a snap switch connected to said base, and means associated with said arm and said switch adapted to move said switch to its on position as the contacting portion of said arm reaches a point adjacent said ear in its path of travel from said last-mentioned end of said strip.

' 6. In apparatus of the class described, an insulating strip, a coating of high resistance matevrial baked upon said strip, a narrow band of low resistance material painted across said strip,.a piece of sheet metal on the side of said strip opposite said coating, and ears extending up around the edges of said strip and in contact with said narrow band.

7. In apparatus as claimed in claim 6, the combination with the elements therein specified of a sheet metal and said high resistance through said coating on the edges.

8. -In apparatus of the class described, in combination, a strip having a iiat surface of substantial length, said strip being an electrical conductor ofhigh resistance, an armature arm adapted to engage said strip, means adapted to move said arm along said strip, a terminal connection at one end portion of said strip including a pair of parts engaging the sides of said strip and spaced from said last-mentioned end thereof, a switch adapted to be placed in the circuit o f said terminal, and means associated with said switch and said arm adapted to close said switch when the contacting portion of said arm is in substantial alinement with said parts.

9. In apparatus of the class described, a horseshoe shaped strip, said strip being a conductor having a high resistance, an arm, means lrotatably mounting said arm substantially at the `airis of said strip, and means connected to said arm and contacting said strip at minute spaced points along a radial line of said axis.

l0. In apparatus oi the class described, a horseshoe shaped strip, said strip being a conductor having a high resistance, an arm, means rotatably mounting said arm substantially at the axis of said strip, and means forminga plurality of lingers connected to said arm and engaging said strip at minute spaced points, said points being in .a radial line of said axis.

ii. In apparatus of the class described, a horseshoe shaped strip, said strip 4being a conductor having a high resistance, anarm, means rotatably mounting said arm substantially at the axis of said strip, and means including a plurality of resilient iingers having semi-spherical projections engaging said strip at minute spaced points ina radial line of said axis.

i2. in apparatus of the class described, a circular strip having a iiat surface of substantial length, said strip being an electrical conductor of high resistance, a plurality of contact parts engaging said strip at minute spaced points in a line substantially at right angles to the center line of said strip, and means connected to said contact parts adapted to hold them in said relationship with said strip and move them from end to end.

i3.` in apparatus oi the class described, a circular strip having a iiatsurface of substantial length, said strip being an electrical conductor of high resistance, a plurality of contact parts engaging said strip at minute spaced points in a line substantially at right angles to the center line of said strip, an arm connected to said contact parts, and means holding said arm in xed relationship to said strip, said arm being capable of movement along said strip whereby said contact points are maintained in said relationship during movement between the ends of said strip.

i4. In apparatus of the classv described, a strip having a at `surface o substantial length, said strip being an electrical conductor of high resistance, a plurality of contact parts vengaging said strip at minute spaced points in a line substantially at right angles to the center line of said strip, means connected to said contact parts adapted to hold them in said relationship with said strip and move them from end to end thereof, and means forming a terminal connection intermediate the ends of said strip including a part engaging the-side of said strip and spaced from the ends thereof to form an electrical connection therewith, said means leaving l an unobstructed surface in the coating path of travel of said contact points. '4

15. In apparatus of the class described, a strip having a fiat surface of substantial length, said vstrip being an electrical conductor. of high resistance, a plurality of contact parts engaging said strip at minute spaced points in .a line substantially at right angles to the center line of said strip,l means connected to said contact parts adapted to hold them in said relationship with said strip and move them from 'end to end, means l forming a terminal" connection intermediate the mutator arm adapted to engage said surface and g travel therealong from end to end, a terminal member disposed intermediate the ends of said strip and engaging a side thereof to leave said surface unobstructed, and a substance interposed between'said terminal and the side of said strip of greater conductivity than said strip.

i7. in apparatus of the class described, a strip having an elongated dat surface, said strip being an electrical conductor of high resistance, a commutator arm adapted to engage said surface and travel therealong from end to end, a terminal member disposed intermediate the ends of said strip and engaging a side thereof to leave said surface unobstructed, and means forming separate electrical connections with the opposite ends l of said strip.

i8. in apparatus of the class described, a strip having an' elongated dat surface, said strip being an electrical conductor of high resistance, a commutator arm adapted to engage said\` surface and travel therealong from end to end, a terminal'member disposed intermediate the ends of said strip and engaging a side thereof to leave said surface unobstructed, and means forming separate electrical connections with the opposite ends of said strip, a section of the strip adjacent one end thereof being of greater conductivity than the remaining portion of said strip'.

i9. in apparatus of the class described, a strip having a top surface of substantial length, said strip being an electrical conductor of high resistance, means forming a terminal connection with oneend of said strip and comprising a at piece of metal extending beneath Asaid strip throughout a portion of the length thereof, an ear in engagement with the side of said strip to leave said top surface unobstructed, and means forming a terminal connection intermediate the ends thereof including a part engaging the side of said strip to leave said top surface unob- I structed.

20. In apparatus of the class described, in combination, an insulating base, a at substantially circular strip secured to said base, a conducting coating on the exposed :dat surface of ends thereof, a switch in the circuit of said terminal, and means forming a mechanical connection between said switch and said arm adapted to close said switch when the contacting points of said fingers are substantially adjacent said ears in their movement from the end of said strip to which said terminal structure is connected.

2l. In apparatus of the class described, an insulating strip, a coating of high resistance material upon said strip, a band of low resistance material deposited across said strip, a piece of sheet metal on the side of said strip opposite said coating, and ears extending up around the edges of said strip and in contact with said band.

22. In a variable resistance device, a resistance element, a contacter, means for moving said contactor over the surface o'f said resistance element in a predetermined path, a terminal extending around the greater portion of the resistance element in a plane transverse to the line of movement of said'contactor and a gap in said terminal through which the contactor is adapted to be moved, said terminal having intimate contact with said resistance element adjacent the path of the contactor.

23. A variable resistance device comprising a resistance element, a contacter movable along the contact surface of said resistance element, and a terminal having a portion extending transversely partially around said resistance element and having its ends bent over into engagement with the contact surface of said resistance element'and spaced apart to permit the passage of said contactor between said ends.

24. In apparatus .of the class described, a strip having an elongated at surface, said strip being an electrical conductor of high resistance, a commutator arm adapted to engage said surface and travel therealong from end to end, a terminal structure disposed intermediate the ends of said strip and engaging a side thereof to leave said surface unobstructed, said structure-including a metallic part and a highly conductive substance Y CERTIFICATE 'Patent No. 2,1o1,irln.

JESSE It is hereby' certified that error interposed between said part and said strip, and means forming separate electrical `connections with the opposite ends of said strip.

25. In apparatus of the class described, a strip having an elongated flat surface, said strip being an electrical conductor of high resistance, a commutator arm adapted to engage said surface and travel therealong from end to end, a terminal structure disposed intermediate the ends of said strip and engaging a side thereof to leave said surface unobstructed, said structure including a. metallic part and a highly conductive material contacting said part and extending transversely across said strip, and means forming separate electrical connections with the opposite ends of said strip.

26. In apparatus of the class described, a strip having an elongated flat surface, said strip being an electrical conductor of high resistance, a commutator arm adapted to engage said surface and travel therealong from end to end, a terminal structure disposed intermediate the ends of said strip and engaging a side 'thereof to leave said surface unobstructed, said structure including a metallic part and a highly conductive coating comprising a carbonaceous material and a binder contacting said part and said strip, and means forming separate electrical connections with the opposite ends of said strip.

27. In apparatus of the class described, a strip having an elongated flat surface, said strip having a coating of high resistance material on one surface thereof, a commutator armadapted to engage said coating and travel therealong from end to end, a terminal structure disposed inter- 'mediate the ends of said strip and engaging a side thereof to leave said surface unobstructed, said structure including a metallic part and a highly conductive substance interposed between said part and said coating, and means forming separate electrical connections with the opposite ends of said strip.

JESSE MARS'I'EN.

OF CORRECTION Y December 7, 1957.

MARSTEN.

appears 1n the printed specification of the above numbered patent requiring correction asfollows: Page 2, second column, line 5, .for "furtherfore" read furthermorejpage 6, second column, line 1l, claim l5, after "end" second occurrence, and before thevcomma, insert thereof; line 1h., same claim, vafter "strip" insert and spaced from the ends thereof; line 50, c1aim,18', for "end" read of said connections; and that the said Lettera Patent should be read with these corrections therein that the same may Office.

conform to the record of the case in the Patent Signed and sealed this `8th day ofxFebruary, A. D. 1958.V

(Seal) .Henry Van Andale,- Acting -Connnissioner of' Patente.

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