US2572545A - Variable impedance device - Google Patents

Description

Oct. 23, 195] D. F. WALKER VARIABLE IMPEDANCE DEVICE 4 Sheets-Sheet 1 Filed Nov. 26, 1948 Oct. 23, 1951 D. F. WALKER 2,572,545

VARIABLE IMPEDANCE DEVICE Filed Nov. 26, 1948 4 Sheets-Sheet 2 Oct. 23, 1951 D. F. WALKER VARIABLE IMPEDANCE DEVICE 4 Sheets-Sheet 5 Filed Nov. 26, 1948 FIG. 60.

Oct. 23, 1951 WALKER 2,572,545

VARIABLE IMPEDANCE DEVICE Filed Nov. 26, 1948 '4 Sheets-Sheet 4 Patented Oct. 23, 1951 UNITED STATES PATENT OFFICE VARIABLE IMPEDANCE DEVICE Donald Ferguson Walker, Edinburgh, Scotland Application November 26, 1948, Serial No. 62,169 In Great Britain November 28, 1947 11 Claims. 1

This invention relates to variable impedance devices of the type comprising a closed or open chain of at least three serially-connected impedance elements energized by an input voltage applied across appropriate points of the chain'lf closed or across the ends of the chain if open, the chain being designed to yield a variation of output voltage which is related to the movement of a control in accordance with some given law.

Impedances of this type are frequently usedfor example in electrical computers-but are liable to be rendered inaccurate by inaccuracies in the mechanical control of the sliders (or like tapping devices) and, in the case of wire-wound potentiometers, by the fact that as a slider usually makes contact with only one point on each turn of wire the output voltage does not change smoothlybut in a series of steps each of which represents the voltage drop across one turn. It is true that the height of each step may be reduced-4hr example by increasing the number of turns of the windingbut such procedure is often inconvenient to apply. These inaccuracies may be accentuated by the various expedients usually resorted to in order to ensure approximate conformity with the given law, such as winding the wire on a specially shaped former, or winding it linearly and moving the slider in accordance with the law by means of some mechanical contrivance.

The object of the invention is to provide a variable impedance device of the type stated of simple construction in which the law relating the variation of output voltage to the movement of the control is observed to a high degree of accuracy.

In accordance with the present invention in a variable impedance device of the type stated the impedances of the said elements are such that the several total impedances between a datum point in said chain and successive common points between said elements and between said datum point and the ends, if any, of said chain are in accordance with said law, there being provided impedance components the impedance of each of which is high compared with the impedance of any of said elements and the number of which is greater than unity and less than the number of said elements, tapping means for connecting one of said components across any element and the remaining component or components across a like number of successive elements and/or off positions each to each, disposed on one side of said element, a common variable tapping means adapted to engage two or more adjacent components successively, means for deriving said output voltage between said common variable tapping means and said datum point in said chain, and means actuated by said control (a) for transferring one of said remaining components to connection across the adjacent element on the other side of said element, and (b) for traversing said common variable tapping means along said one of said components whilst said component remains connected across said element, then, without substantial change of said output voltage, onto said one of said remaining components after transference thereof, and then along said last-mentioned component whilst it remains connected across said adjacent element.

Also in accordance with the invention, in a variable impedance device of the type stated said elements comprise resistors, the resistances of which are such that the several total resistances between a datum point in said chain and successive common points between said elements and between said datum point and the ends, if any, of said chain are in accordance with said law, there being provided two resistive potentiometers the resistance of each of which is high compared with the resistance of any of said elements, tapping means for connecting one of said potentiometers across any element and the other potentiometer across an adjacent element or offposition, disposed on one side of said element, a common variable tapping adapted to engage said potentiometers successively, means for deriving said output voltage between said common variable tapping and said datum point in said chain, and means actuated by said control (a) for transferring said other potentiometer to connection across the adjacent element on the other side of said element, and (b) for traversing a common variable tapping along said one of said potentiometers whilst said potentiometer remains connected across said element, then, without substantial change of said output voltage, onto said other potentiometer after transference thereof, and then along said other potentiometer whilst it remains connected across said adjacent element.

Compensation means may be provided for increasing the accuracy of said device when said common variable tapping means is situated between the ends of any one of said components or when said common variable tapping is situated between the ends of either of said resistive potentiometers, as the case may be.

In the accompanying drawings,

Figures '1 and 2a to 2e illustrate the principle of operation of the invention,

Figure 3 is a curve showing a typical law observed by apparatus in accordance with the invention,

Figure 4 shows diagrammatically compensation means,

Figure 5 shows an embodiment of the invention'in mechanically simplified form,

Figures 6a to illustrates the operation of the embodiment shown in Figure 5,

A simplified form of the invention will now be described in order to make clear the principle of operation. The variable impedance device chosen in this example follows a cosine'law between the angles and 90, so that in response to movements of the control spindle, e. g. oi; "a predictor, in dependence on some variable angle the output voltage between a slider and a datum point is always proportional to cos 0. In this ev ce-ri e Fi u e-17mm. mped e l m in the form oi resistors I to 9 are connected in series to form an open chain between end points Aand J, across which points is applied an input voltage V. End point J, which is earthed, is the datum point 'referred to above. The eight common points between the resistors are B C, D resistor is thus connected between points A and B, resistor 2 between pointsB and C, and so on, resistor}; being connected between I and J The resistances of the resistors are such that the voltages between the end point or common points and earthed end point J follow a cosine law with end point A representing zero degrees, common point B representing C representing and. so on, with earthed end point J representing 90f. The actual resistance of each resistor is determined from the fact that the ratio of the total resistance between B and J to that between A and J is cos 10, or 0.9848; the ratio of thetotal resistance between C and J to that between A and J is cos 20, or 0.9397, and so on. V

4 v The end points and common points are brought out to tapping means in the form of studs similarly designated A to J across certain pairs of which may be connected the ends P and Q of a nrstresistive potentiometer II] the resistance of which is very high compared with that of any of the chain resistors.

Potenticmeter I0 is shown in Figure 1 connected across resistor 3, with the ends P and Q of the potentiometer respectively engaging studs C and D It will be clear that when slider ll .of'potentiometer I0 is at the end P the potential of the slider, is the same as that of the point C so that the output voltage V developed between the slider and the datum or earthed point J represents cos 20. When the slider is at the end Q the output voltage represents cos 30. And when the slider occupies some intermediate posipotentiometer I9 is moved to another pair of studs. To avoid this interruption and ensure a smooth change of potential throughout the sca e a second component, in the form of a second potentiometer I2, is provided, exactly similar to the first potentiometer; the ends of the second potentiometer are R and S, and in Figure 1 it is shown connected across resistor 4 with ends R and S engaging studs D and E Instead of each having a slider the two potentiometers share 4 the same common variable tapping or slider H. By a mechanical arrangement, hereinafter described, tliese "two potentiometers always occupy adjacent pair of studs'that is, they are always connected to three consecutive studs in the centre "one of which is shared by both potentiometers.

In Figure 1 they are shown occupying consecutive studs C to E and sharing stud D control .effectinglthe movement of slider II in dependence on the angle 0 automatically ensures thatwhentheslider reaches an end of a mqtentiomete l s potentiometer the other potentiometer is already in position across the next vacant resistor element (if any) to receive it so that the slider can move .smoothlyfrom the one potentiometer to thefotl'ifwi'thout 'siibsta tiereaah'g'e 6f hiitput i e e 'Ui l sf i i f wh t 3 d i l s o d l qi i fi3" fl ie 1 r @l {6r th first -potentiomter to b trans erred to thene git vacant resistor iel ei'neiit beyond the isec; .bjii'd potentiometer and this the peten iqmetets and the slider continue their travel along the l enge .t p The a ove angement will'be made'cleai'er .fri al i le me n re i e to P g a th l j oii e i nm i erai e' h w h the full range startihg from 0 and finishing at 9Q", is tray The two potentiomete'fs areito sen c Figure v "d n jte' p esie first t l o lresistol s i| and 2; Yet the end A of the th 9 matine d 2 l ia sirl z is a e 99?- ie i t studs Ah-B5 aedjB. Q e peei relymeted, te'n i i q et r smear n a e n era i lfas hel e 'a g .i'li o sie flri is gene l es-th t i ete mfen P i -B Th commonslider I I ;is, atthe end 1?; its put wa e wa er e n itepv-, lie nsaqw iioiedjbr' fqfihi qlii a ds th en th' r le qver i sthsiae' le be wee Wand 1 -i ,reaclies this'end Q its; potential is that Dream 3 i iFii i f T e, Slider r i fi t sf i tallies p 'n mi' 'l ls fiur i-ie i s,pet ptiel nia iii i fi .et hat, iiiep im B during this process o'wing to the'close prozgmity of -the end turns of thejtwo jpotentiometers.

gometer lfl is imposition-to receivelit, -As the slid r, c t nuin i srtrav rse, n ars th rend n ler y an e r d an over handf tothe studs D1,'E1. In'thiswaythe Towards the end of the scale represents cos or'zero. a Thefsystemworks infthe reverse'direc-tion in an e a s m la man r, 7

The precise moment in each hand-over-hand operation when a transference of a-potentiometer is effected is unimportant but itshould for convenience'take place whilst the slider is moving iromthe midpoint of theother potentiometertodsqne nd o t- A, s l

It willbe noticed that-the resistor elements may be divided into two groupsthose across which The V potentiometer l0 may be connected and those across which potentiometer 12 may be connected. Potentiometer l0 may be connected across the odd numbered resistors I, 3, 5, I, 9 but not across the even numbered resistors 2, 4, 6, 8, whilst the reverse applies to potentiometer 12.

By lengthening the chain by another nine resistors beyond the earthed point J and by connecting the source or constant voltage across the ends of the lengthened chain the equipment may be adapted to cover the range 0180 instead of 090. In this case the output voltage is of course that between the slider and earthed point J, which remains the datum point, this voltage being negative when the slider is operating in the 90-180 half of the chain.

In practice there are many more resistors in the chain than the nine which in order to simplify the explanation were assumedabove. The series of potential steps between successive common points are therefore many and small, whilst the effect of a high resistance potentiometer connected across a pair of adjacent common points is to smooth out the step concerned into almost exact coincidence with the corresponding section of the cosine curve relating output potential to slider displacement. Moreover, no difiiculty is experienced in providing the series of separate resistors each of which has a resistance that can be readily determined from tables of cosines, instead of a resistor that varies along its length as in the prior art.

It Will be appreciated that the potential of the slider does not adhere to the cosine law with strict accuracy when the slider is at any position on a potentiometer other than at its ends. The potential between these ends in fact follows a linear law and may be represented by a straight line joining the two adjacent plots on the cosine curve that represent the potentials of the two common points across which the potentiometer is connected. This is illustrated in Figure 3 which shows a cosine curve between 0 and 90 referenced in correspondence with Figure l, with the ends P, Q of potentiometer 10 connected to the points C, D. Where there are many resistors these plots lie close together and the straight lines connecting the plots therefore lie very close to the curve. Should however exceptional accuracy be required some form of error compensation may be necessary. The design of such compensation must allow for the facts that the extent of divergence of the straight line from the curve varies with the position on the curve of the plots representing the two common points concerned, being (in the case of a cosine curve) most at 0 and least at 90, and that the error is such as to render the slider potential too low when at any position except the ends. Any compensating potential must therefore vary with the position of the resistor in the chain, must be zero when the slider is at either end of the potentiometer across it, and be additive when the slider is between those ends.

One example of a compensating circuit for a cosine potentiometer as above described, the supply voltage of which is alternating, is shown in Figure 4. This incorporates a transformer 20 the primary 2| of which is connected between earth and the higher potential end of that potentiometer with which the slider l l at any given moment is engaged, in this case potentiometer Ill. The secondary 22 of the transformer is connected, in series with a very high resistance 23, between slider II and the higher potential end of potentiometer II). If theresistance of the resistor to which the potentiometer is connected (resistor 3 in this case) is assumed zero in comparison with the resistance of potentiometer 10 it will be seen that the secondary current creates a compensating potential difierence in the potentiometer only when slider I l is-not at one of the ends; when the slider is at the end P the secondary current does not flow in potentiometer In at all and when the slider is at the end Q the secondary current is almost entirely diverted by resistor 3. Secondary 22 is connected in such sense that the compensating potential increases the potential of slider II in all other positions. It will also be seen that the value of the compensating potential difference varies with the position of the resistor in the chain, since primary 2| is connected to the potentiometer across this resistor.

In the above description it has been assumed that the equipment operates in accordance with a cosine law. It may however operate in accordance with other laws, for example a sine law or a square law, the necessary alteration of the resistor values or of the compensating circuit will be obvious to those skilled in the art.

An embodiment of the invention will now be described by Way of example as applied to the cosine potentiometer form first described with reference to Figures 1 to 3, having as before an open chain of nine resistors and two potentiometers to cover the range 090. Thi is illustrated by Figure 5, in which details of mechanical construction have been much simplified in order to clarify the principle of operation. Reference letters and numerals corresponding to those of Figures 1 to 3 are used where applicable.

A first contact bank is formed by mounting on the face of a first insulating disc 30 near the periphery five metallic contacts or sectors 3| insulated from each other and equally spaced round the disc. They correspond to alternate studs A C E G and 1 stops (not shown) are provided between adjacent sectors A and I. A second contact bank is formed by mounting on a second disc 32, similar to the first disc, five contacts or sectors 33 corresponding to the remaining studs-B D F H and J the stops (not shown) being between sectors B and J Discs 30 and 32 are mounted in fixed relative coaxial position with the sector sides of the discs facing each other. The sectors of the two discs are interconnected exteriorly by the appropriate resistors, e. g. sector A on disc 33 is connected to sector 13 on disc 32 by resistor i, sector B to sector C (on disc 30) by resistor 2 and so on. The remaining resistors are omitted from the drawing for simplicity. It will be seen that as the total number of resistors is odd, the chain begins at the end contact or sector A of the first bank and ends on end contact or sector 5 of the second bank, the supply being applied as before across these end sectors, of which sector J is earthed.

In between and coaxial with the discs is a ring 34 of insulating material bearing the toroidal windings of the two potentiometers l3 and I2; half the ring, circumferentially, comprises potentiometer Ill and the other half potentiometer l2. Rotatably coaxial with the equipment so far described is a single slider [I designed to traverse round the two potentiometer windings. The ends P and Q of potentiometer ID are adjacent to the ends S and R respectively of potentiometer l2, the last turn at each end of one potentiometer being so close to the last turn of the other potentiometer as to allow the slider to touch both together and thus to successive sectors.

allowthe slider'to pass from one pot nti meter tothe other without a sudden change of poten tial, as described above. These turns are shown less closely spaced to avoid confusing the draw ing. Theends P and s of the potentiometer are led to two brushes P and S that engage the sec: tors of. disc 30 and the ends Q and R are ledto two brushes Q and R, engaging the sectors of disc 32. The brushes are fixed relative to the potentiometer ring by means not shown androtate with it. The positions of thehrliShcSrelative to each other and to the sectors Will appear later. The brushes are not wide enough to bridge Slider H and otentiometer ring .34 are retated relative to the fixed discs by the following system of gearing, in which provision ismade for the introduction of a fixed angle sif desired, so that the output may be proportional to cos (E ia). insteadof to cos 0. Ahollow control shaft 40 the angular position of which represents-the angle .0 passes freely through the centre of disc and operates afirst inputmember (not shown) of a differential gear 4| thesecond input member of which ,(also not shown) is operated :by a solid shaft d2 coaxially within shaft 4,3; the angular position of shaft '42 represents the fixed an le ,a. The output from gear 4! is the shaft :43; this drives slider ll direct and, through 4:11 reductiontgear 4d, the potentiometer ring 3d, a direction opposite to slider .II by means of radial spokes .45. Shaft 42 may of course be set to zero if only .00 dis required. .It will be seen that as the dive sectors are equally spaced round each disc the potentiometer ring 34 rotates oneeiifth of a revolution :whilst each brush moves .fro its position on one sector to the corresponding position on the next sector. Owing ,to the reduction gear 4-4 the slider makes ,iourtths of arrevolution during this period and will accord.- ingly make .one complete revolutionrelative to .the potentiometer ring, thus traversing both .potentiometers.

In order to understand the operation of :this equipment, reference should be made .to Figurefi in which the electrical part of the equipment is shown in developed form with thesectors straight 7 instead of curved and referenced %to correspond to Figure 1. It is convenient itO consider each sector as being hypothetically divided circumferentially into three parts (as sshown bv dotted lines) 'labelledX, Y, andiZ in the directionfof the earthed end of the chain. The -.circumferential length of the insulated space between adjacent sectors is the same as that ofone part. :Each part secured to shaft 49, whichis then rotated inaccordance with the'variable input anglefi. :I;h,e

potential of slider H thus-varies in accordance with 605 (01a). In traversing theiull range,-as before, from (0: ;a)=ll to (-0 a);=90f,- the position at the commencement is-shown in Fig ure oa.

.Slider H is at the end P of potentiometer v,lil, .which is engaging resistor l .by way-. of,brushes .P Q and sectors A B The positions "of .the brushes are, as follows: {P :and Q between parts X and Y of sectorsA andBrespectively; R and S enteringxspaces J, B .and A C fr0m-=$ectors J te se n eiivel :T Pos t v s lv hi R on se or J 'iwhiob sh wn a ai seemed the fi ure or onv n ence b ush S1 i -5W tor A at t is sta e ha 1 elec ic ig cance; thei nos. en me ely o re po d to t p s tion o poten i m ter J2- The arm 1101; sho n? that carries brushes 5.1 and Q1 is now against the stop, above referred to; between sectors B1 and J he sim a a m c y g th other rushe P and 1 now ain h s b w e sector A and IA Whilst he slider is tra e s n o enti met r ill from to Q a h b u h me a d ta e l to the len th o two pa t v n hi oe -i991 thebn sh s R1 an S mov f 011 setter J nd A o eoiorsB and C espect l u ransferring potentiometer 82 from the off position between sectors J a d bet en Which is no resister) into position across resistor 2 in the hand-over-hand manner above described; When therefore the slider leaves the end Q of potentiometerlll for the end B of potentiometer l2.,se,e Fig-ure;6b.the brushes have reached the fol-lowing positions: P and Q on parts Z of sectors A and B respectively'but about to leave them; and R and S on sectors B and ,C' re.- spectively, between parts X and Y. Potentiometer I2 is thus engaging resistor 2 when the slider moves onto it. This is the position occupied bv the brushes in Figure 5 Whilst potentiometerfl is engaged by the slider and its brushes are movingaalong sectors B and C the brushes P and Q of potentiometer It first traverse spaces AC and B D respectively and then reach parts X of sectors 0 and D (see Figure 6c), thus transferring potentiometer iii to the next resistor 3 in the hand-over-hand manner. It will be seen that, as shown with reference .to Figure 1, pctentiometer I0 may ,be'connected across onlythe odd-numbered resis torsi. .e. those disposed horizontally in Figure 6whereas potentiometer I2 may beconnected acrossonly the even-numbered ;resistors,i, e. those disposedobliquely.

The rest of the operation follows closely that already described, until the end of the scale is reached with the slider at the end Q of potentiometer 10, connected across resistor 9, andwith potentiometer l2 in an on position with its brushes onsectors J and A The essential requirements are that the brushes change see,- tors only when their potentiometer is not errgaged by the s1ider,-and, conversely, that as long as apotentiometer is so engaged its brushes remain on their respective sectors. .So long as one potentiometer is in position across .the next resistor before the slider reaches the end of the other potentiometer .the precise relative positions of thebrushesisnot of importance.

The compensation circuit previously described withreference to Figure 4 may easilybe applied to this embodiment, the only adaptation necessary beingth e provision of meansfor effecting a connection toithe higher potential end of whichever potentiometer isengaged by the slider. A simple way of doing this is to'provide a commutatorifl (see Figure '7) havingtwo semiecircular segments ..,5l and 52 connected respectively to ends P and Rof potentiometers In and l,.2'; the associatedbrush-rfic is ganged with slider lll of potentiometers lfland 1-2 soas to rotate in synchronismwith it. The two gaps 5t, 55 between the'segmentsare so located relative to brush 5 3 that whilst slider H is engagingpotentio neter -Il I,-brush.,53 v isv engaging, segment 5 so that the higher potential end .P of this potentiometer is connected ,to;brush,.53.by-wayof this segment.

Similarly whilst slider l i is engagin potential end I2, brush 53 is engaging segment 52, so that the higher potentiometer R of this potentiometer is connected to brush 53. The connections from slider H and brush 53 to the compensating transformer are as described with reference to Figure 4, the lead to the end P of potentiometer I0 in that figure being now connected instead to brush 53. The operation is the same as before.

The above-described embodiments may be modified to cover the range 0-360 instead of This may be efiected by employing only double the number of resistors required for the 0-90 range and addin suflicient additional sectors to raise the total number of sectors to equality with the total number of resistors. This is shown in single diagrammatic form in Figure 8, in which for simplicity the range 0 to 90 is covered by only three resistors-referenced I, 2, and 3-instead of by the nine resistors of the above embodiments. The part of the arrang ment covering this range is otherw se similar to Figure 1 and is correspondingly referenced. To cover the range 90 to 180 anoth r three resi tors are added. each being referenced by the prim d number of the corresponding resist r in the 0 to 90 range. To cover the range 180 to 360 (or 180 back to 0) no more resistors are r ouired: the sectors of this range are cros -connected as shown to the sectors of the 0 to 180 ran e. The chain is now a closed one. The in t volta e is applied to the s ct rs at the 0 and 180 points whilst the 90 and 270 points are earth d and are the datum points. The application oi this arrangement to the above-descri ed embodiment is quite strai ht orward: alt rnate sectors are located on disc 30 the remainder on d c 32. In t is arran ement continuous rotation of the otentiometer ring is of co rse permis i le and the stops and of! positions referred to above are not required.

The arrangement descr bed in the fore oing paragraph may be modifi d ve y sim ly to o erate in accordance with a sine r ther than a cos ne law. This may be effect d merely by rotating the calibrations (see Figure 8) clockwise through 90, leaving the connections to the s p ly and to earth as they were before. The e rthed datum points of zero output now correspond to 0 and. 180.

The above embodiments are describ d in order to illustrate the in ention and are not intended to limit its scone, which embraces various modifications of the eq i ment that sho ld be readily apparent. The resistors may for instance be replaced by other forms of im edance elements, for exam le chokes: the impedance components or potentiom'eters may be in the form of tap ed autotransformers; there may be more than two com onents, though of course their n mber must be at least one less than the number of impedance elements so as to leave at least one unbridged element for one of the components to be transferred to, this component being any of the ones not engaged by the common variable tapping at the time. The tapping means may be in the form of a common variable tapping traversing all components as described above, or may take the form of a separate tapping for each component combined with a simple selector switch that transfers the output lead from connection to the tapping of one component at the end of the traverse of that tapping along that component, to connection to the tapping of the next component. The compensation circuit described for application to a variable impedance device operates under a sine or cosine law may be modified in detail; for example the connection from the unearthed end of the primary of the transformer may be made to the lower potential end of the potentiometer engaged by the slider rather than to the higher potential end, and the connection from the secondary may be made to the end of this potentiometer that is not connected to the primary. Moreover the mechanical part of the control may be of a different type from that described.

What I claim is:

1. A variable impedance device comprising in combination, at least three impedance elements connected in series and having impedances satisfying a predetermined law; two impedance components each having a high impedance in comparison with the impedance of any of said elements; tapping means for connecting said components in shunt to any two consecutive ones of said elements, respectively; a tapping member adapted to engage either of said components; means for changing the shunt connection of said component not engaged by said tapping member from one element adjacent the other shunted element to the other element adjacent thereto; and means for traversing said tapping member along any of said components while said component is connected in shunt to one of said elements.

2. A variable impedance device comprising in combination, at least three impedance elements connected in series and having impedances satisfying a predetermined law; means for applying a voltage to the series connection of said elements; two impedance components each having a high impedance in comparison with the impedance of any of said elements; tapping means for connecting said components in shunt to anytwo consecutive ones of said elements, respect vely; a tapping member adapted to engage either of said components; means for changing the shunt connection of said component not engaged by said tapping member from one element adjacent the other shunted element to the other element ad acent thereto; means for traversing said tapping member along any of said components while said component is connected in shunt to one of said elements; means connected to sa d tapping member for compensating the error of the device when said tapping member engages a point between the ends of any of said components; and means for deriving a voltage between said tapping member and a predetermined point of the series connection of said elements.

3. A variable impedance device comprising in combination, at least three resistor elements connected in series and having resistances satisfying a predetermined law; two resistive potentiometers each having a high resistance in comparison with the resistance of any of said elements; tapping means for connecting said potentiometers in shunt to any two consecutive ones of said elements, respectively; a tapping member adapted to engage either of said potentiometers; means for changing the shunt connection of said potentiometer not engaged by said tapping member from one element adjacent the other shunted element to the other element adjacent thereto; and means for traversing said tapping member along any of said potentiometers while said potentiometer is connected in shunt to one of said elements.

' 4. A variable impedance device comprising in 7 Combin o t least three resistor elements-conping member adapted to engage either of said notentiometers; means for chan in the shunt connection of said potentiometer not. en a ed b said tapping member from one element adjacent to the other shunted element to the other element adjacent thereto; means for traversing said tapping member along any of said potentiometers while said potentiometer is connectedjin shunt to one of said elements; and means for doriving a voltage between said tapping member and a predetermined point of the series connection of said elements.

5. A variable impedance device comprising in combination, at least three resistor elements connected in series and having resistaneessatisfying a predetermined law; means for applying a voltage to the series connection of said elements; two resistive potentiometers each having a high resistance in comparison with the resistance of any of said elements; tapping means for connecting said potentiometers in shunt to any two consecutive ones of said elements, respectively;

a tapping member adapted to engage either of said potentiometers; means for changing the shunt connection of said potentiometer not encaged by said tapping member from one element adjacent the other shunted element. to the other element adjacent thereto; means for traversing said tapping member along any of; said potentiometers while said potentiometer is connected in shunt to one of said elements; means connected to said tapping member for compensating the error of the device when said tapping member engages a point between the ends of any of said potentiometers; and means for derivinga voltage between said tapping member and a predetermined pointof the series connection of said elements.

6. A variable impedance device comprising in combination, a luralityof resistance elements connected in series and including odd numbered elements and even numbered elements, said elements'having resistances satisfying a predetermined law; means for applying a. voltage to the series connection of, said elements; two banks of contacts, the successive junctions of said re: sistor elements being connected to successive contacts in each of said banks alternately; two pairs of brushes each being arranged for cooperation with oneof said banks, respeotivelyr Said brushes'being adapted to engage the contacts of said banks without bridging any two contacts; a first resistive potentiometer; a second resistive potentiometer, said potentiometers each having a high resistance in comparison with the resistance of'any of said elements; a, connection between one of said brushes cooperating with one of said banks and one end of said first potentiometer; a connection with one of said brushes cooperating with the. other of said banks and the i numbered elements and said even numbered-elements; a tapping member adapted to engage either. of said potentiometers; means for changing the shunt connection of said potentiometer not engaged by said tapping member from one element adjacent the other shunted element to the other element adjacent thereto; means for traversing said tapping member along any ofsaid potentiometers while said potentiometer is connected in shunt to one of said elements; and means for deriving a voltage between said tapping member and a predetermined-point of the series connection of said elements '7, A variable'impedance device comprising in combination, a plurality of resistance elements connected in series and including odd numbered elements and even numbered elements, said elements having resistances satisfying a prcdeter mined law; means for applying a voltage to the series connection of said elements; two banks of contacts, the successive junctions of said resistor elements being connected to successive contacts in each of said banks alternately; two pairs of brushes each being arranged for cooperationwith one of said banks, respectively, said brushes be ng p ed to eng ge the contacts of said banks without bridging any two contacts; a first resistive potentiometer; a second-resistive potentiometer, said potentiometers being disposed end-to-end on a circle and each, having a high resistance in comparison with the resistance of any of said elements; a connection between one of saidbrushes cooperating with one of said banks and one end of said first potentiometer;ia connection with one, of said brushes cooperatin with the other of; said banks and the other end of i t p tentiometer; connectionsbetween the ends of said second potentiometer and the remaining ones of said brushes, said brushes being disposed in relation to said banks tor said potentiometers being adapted to bridge, respe.o tively, consecutive ones of said odd numbered elements and said even numbered elements,- each pair of adjacent ends of said potentiometers being connected to those of said brushes which engage contacts of the same of said banks; a tapping member being adapted to rotate about the center of the circle and to engage either of said potentiometers; means. for changing the shunt connection of said potentiometer not en gaged by said tapping member from-one element adjacent the other shunted element to the other element adjacent thereto; means for traversing said tapping member along any of said potentiometers while said potentiometer connected in shunt to oneof said elements, whereby said-tape ing member upon leaving one of said potenti ometers, engages the other of said potentiometers; and means for deriving a voltage between said tapping'member and a predetermined point of the series connection of said elements,

8. A Variable impedance device comprising in combination, a plurality of resistance elements connected in seriesaand including odd numbered elements and even. numbered elements, said e1E: ments having resistances satisfying a predetermined law; means for applying a voltage to the series connection of said elements; two banks; of contacts, the successivejunotions of said reasistor-elements being connected to successive c cts in. ach of aid b ks. a ternately; two pairs of brushes each being arranged for cooperation with onset said banks, respectively; said brushes being adapted to engage the contacts of said ksw thout brid in y two c tacts; a i s 13 resistive potentiometer; a second resistive potentiometer, said potentiometers being disposed end-to-end on a circle and each having a high resistance in comparison with the resistance of any of said elements; a connection between one of said brushes cooperating with one of said banks and one end of said first potentiometer; a con- ,nection with one of said brushes cooperating with the other of said banks and the other end of said first potentiometer; connections between the ends of said second potentiometer and the remaining ones of said brushes,. said brushes being disposed in relation to said banks for said potentiometers being adapted to bridge, respectively, consecutive ones of said odd numbered elements and said even numbered elements; a tapping member being adapted to rotate about the center of the circle and to engage either of said potentiometers; gearing means associated with said tapping member and said brushes and ensuring that said tapping member traverses the entire length of one of said potentiometers while said brushes connected to said tapping member remain on the same of said contacts of said banks interconnected by a resistor element, and prior to the transference of said tapping member to the other of said potentiometers said brushes connected to the other of said potentiometer-s are transferred to the pair of contacts interconnected by the adjacent resistor element; and means for deriving a voltage between said tapping member and a predetermined point of the series connection of said elements.

9. A variable impedance device comprising in combination, a plurality of resistance elements connected in series and including odd numbered elements and even numbered elements, said elements having resistances satisfying a predetermined law; means for applying an alternating voltage to the series connection of said elements; two banks of contacts, the successive junctions of said resistor elements being connected to successive contacts in each of said banks alternately; two pairs of brushes each being arranged for cooperation with one of said banks, respectively, said brushes being adapted to engage the contacts of said banks without bridging any two contacts; a first resistive potentiometer; a second resistive potentiometer, said potentiometers being disposed end-to-end on a circle and each having a high resistance in comparison with the resistance of any of said elements; a connection between one of said brushes cooperating with one of said banks and one end of said first potentiometer; a connection with one of said brushes cooperating with the other of said banks and the other end of said first potentiometer; connections between the ends of said second potentiometer and the remaining ones of said brushes, said brushes being disposed in relation to said banks for said potentiometers being adapted to bridge, respectively, consecutive ones of said odd numbered elements and said even numbered elements; a tapping member being adapted to rotate about the center of the circle and to engage either of said potentiometers; gearing means associated with said tapping member and said brushes and ensuring that said tapping member traverses the entire length of one of said potentiometers while said brushes connected to said tapping member remain on the same of said contacts of said banks interconnected by a resistor element, and prior to the transference of said tapping member to the other of said potentiometer-s said brushes connected to the other of said potentiometers are transferred to the pair of contacts interconnected by the adjacent resistor element; a transformer including a primary and a secondary, said primar being connected between a predetermined point of the series connection of said resistor elements and either end of said potentiometer being traversed by said tapping member, said secondary being connected between said tapping member and either end of said potentiometer being traversed by said tapping member; and means for deriving a voltage between said tapping member and an alternating predetermined point of the series connection of said elements, whereby the voltage derived between said tapping member and the predetermined point of the series connection of said elements is corrected when said tapping member is located between the ends of said potentiometer.

10. A variable impedance device as claimed in claim 9, a commutator having a first semicircular segment and a second semi-circular segment, said first semi-circular segment being connected between said primary and said potentiometer being traversed by said tapping member, said other semi-circular segment being connected to the other potentiometer; and a brush connected to said primary and traversing said commutator in synchronism with said tapping member whereby said brush always engages said segment connected to said potentiometer engaged by said tapping member.

11. A variable impedance device as claimed in claim 10, the connection between said secondary and said potentiometer being completed by way of said commutator.

DONALD FERGUSON WALKER.

REFERENCES CITED The following references are of record in the me of this patent:

UNITED STATES PATENTS

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