US2498967A - Potentiometer - Google Patents

Description

Feb. 28, 1950 c. F SCHAEFER 2,498,967

POTENTIOMETER Filed Dec. 10, 1949 2 Sheets-Sheet 1 INVENTOR. CaeL F Scans/ 52 u xgnm HTTOQA/ Y Feb. 28, 1950 c. F. SCHAEFER POTENTIOMETER 2 Sheets-Sheet 2 Filed D60. 10, 1949 1 INVEN TOR. C421. F 55%45 5? A'TmR/VE Y Patented Feb. 28, 1950 POTENTIOMETER {la-l F. Schaeier, Port Washington, N. Y., assignor to The Nor-den Laboratories Corporation, New York, N. Y., a corporation of Connecticut Application December 1 it claims. 1

My invention relates to potentiometers or voltage dividers, and more particularly to an improved potentiometer in which voltage may be divided with improved accuracy.

A potentiometer depends for its accuracy upon the resistance element along which a brush is adapted to move. It is in essence a resistor con nected across a potential with an adjustable arm adapted to adiustably contact the resistor. Many resistance elements for potentiometers comprise a winding of resistance wire such as nickel chrome alloy or the like upon the insulating base or core with which a movable brush is adapted to cooperate. The brush divides the resistance element into two segments and the lengths of the segments with respect to the terminals determine the manner in which the voltage is divided. It has been very diflicult in prac= tice to produce a potentiometer having linearity throughout and a desired overall voltage drop. It is possible to adjust fixed resistors to iv a desired overall resistance within very narrow limits. A resistor, however, varies from segment to segment so that the resistance will not necessarily vary as a linear function of the movement of the brush over the resistance element. "in many applications, such as in phase shifters and electronic computing instruments, it is extremely desirable to have a potentiometer which will have not only the desired overall resistance but in which resistance of the segments can be varied with greater accuracy in order to produce a linear function.

One object of my invention is to provide a potentiometer having the desired overall resist once within narrow limits and in which a predetermined movement of the variable brush will produce a corresponding change in resistance within very narrow limits.

Another object of my invention is to provide a potentiometer comprising a resistance element connected across a. voltage source with a movable brush across which and one end terminal of the resistance element the output is removed in which a linear motion of the brush produces a corresponding linear change in potential, whereby voltage may be divided with great accuracy.

Other and further objects of my invention will appear from the following description.

In general my invention contemplates the division of the overall resistance of the potentiometer into a plurality of increments composed of fixed resistors divided into two groups. Each of the increments is adjusted to have the desired resistance within close limits. A pair of variable 0, 1948, Serial No. l32,235

lei. cor-4e) resistors each having a pair oi hrushcc adapted,

to embrace a resistance equal to ens-mil oi the increments, is provided. The two pairs of brushes are moved in synchronism with each other and with a witch apparatus adapted to switch the fixed resistors from one group to the other at "predetermined points. lit the variable resistors are rheostats having circularly disposed. resistors the crushes will be 18d" apart and the switch will be switched irom one group to the other alter each or movement of the pairs oi brushes. The motion oi the rheostat brushes is transmitted a rotor adapted to com meet the lined resistance increments in series through a gear reduction having a predeter mined ratio, so that connecting elements on the rotor advance to connect one adjacent oi fixed resistance increments every ltd of move-- ment of the rheostat brushes. The two romrs are connected to move together.

The arrangement is such that need to accept only the non-=linearity of a small fraction or the overa resistance. and in this way I am enabled to produce a potentiometer in which a voltage may be divided over a wide range with a high degree of accuracy.

In the accompanying drawings which :form part of the instant specification and which are to be read in conjunction therewith, and in which like reference numerals are used to indicate like parts lathe various views:

Figure l is a diagrammatic view showing a potentiometer containing one embodiment of my invention with one of the output segments being very small and the other being large, that is, with the parts in position near one extremity of adiustment.

Figure 2 is a diagrammatic view similar to Figure 1 showing the parts in position near the other extremity of adjustment.

Referring now to Figure 1, let us assume that the value of the resistance across the terminals of the potentiometer is represented by the expression The terminals oi the potentiometer are represented by binding posts 4 and l across which the potential is placed. The variable brush output is taken from binding post 8 and across either binding post 4 or binding post i or both. Expressed in terms of resistance, the resistance across 4 and 8 is constant while the resistance between 4 and 8 may be varied from zero to the full value of the resistance across the posts 4 and r access? A first rheostat is provided with a circularly disposed resistance winding is adapted to be swept by a pair or brushes l2 and is mounted on a suitable insulating arm it. The brushes l2 and it are 180 apart and are adapted to embrace the resistor l ll so that the value of the resistance between the brushes is R. In my copending application Serial No. lll'i'fikii filed July 30, 1949, I have shown a variable resistor in which a. pair of brushes sweeps a circularly disposed resistance element in combination with switching apparatus adapted to cumulatively place fixed resistors in series. In that application I have shown how the circularly disposed resistance element is may be displaced relative to the axis of rotation of the brushes to produce an overall resistance oi it. A similar arrangement can be employed in the instant case. This adjustment will insure that the resistance along the resistor it between brushes l 2 and it will have the pie-determined value R. A second circularly disposed resistor lit is adapted to be swept by a pair of brushes Eli and 22 mounted upon an arm 24 connected to a rotatably mounted shaft 26 to which a knurled knob is attached. The shaft 2t carries the insulating arm it for rotation therewith so that brushes it; and ill will rotate in unison with brushes 22 and The value of the'resistance along resistor it embraced by the brushes 2d and 22 is equal to R. Rotation of the arm 28 will rotate a shaft illlwhlch carries a gear 32 meshing with a larger gear which in turn meshes with and drives a larger gear which gear carries a shaft 238 secured to a drum to. The arrangement is such that the drum lit will rotate as a function of the rotation of the brush bearing arm it, but at a much reduced speed due to the reduction gearing comprising gears 32, ill, and The drum to is connected to a similar drum 52 by means of a shaft tit so that drums to and 152 will rotate in unison. The rotation of shaft 28 will rotate a cam shaft it by any suitable drive (not shown) so that a cam llti carried by the cam shaft at will rotate in phase with the brush arms it and 2t and at the same angular velocity. Mounted adjacent drum it 1 provide a group of fixed resistors til, 52, Elli, lid, and 53. Similarly adjacent drum 32 I mount a group of fixed resistors St. 52, 641, St? and 58. Each of the resistors t, 52, 5Q, 56, 5t, 6h, 62, 3, .56, and 58 is accurately adjusted to have a resistance value of 2B. While I have shown five resistors associated with each or the drums, it is to be understood that any desired number of fixed resistors may be employed. Mounted upon the drum 46 for rotation therewith and spaced to connect the resistors in series, I provide ,a pinrality of conducting bars l8, i2, 14, I6, and 78. A corresponding group of conducting bars at, 82, 84, 88, and 88 are mounted on the drum Ml forming a second group of conducting bars separated from the first group of conducting bars by a pair of brushes 90 and 92. The brushes 9t and 92 are adapted to connect adjacent resistors of the resistors 50, 52, 54, 56, and 58 in series, depending upon the position of the brushes, in a manner similar to the connecting of the resistors in series by the conducting bars 10, 12, M, l6, 18, 80, 82, 84, 88, and 88. The brushes, however, have a greater radius than the connecting bars and are adapted to contact contact points 94 and 96, the circuit across which is never completed by the conducting bars, due to the fact that the contact points 94 and 96 are placed out of. the locus of the travel of the conducting bars.

The drum s2 is provided with a first group of conducting bars 98, ltd, l lit, ltd and ltd. A second group of conducting bars loll, lid, lit, ll and iii; are disposed upon the drum d2 separated by a pair of brushes lift and 82d. The brushes lid and l at or" drum d2 correspond with brushes till and as of drum 38. In Figure l the brushes lit and its are shown contacting contact points lit and ltcl which correspond to contact points lid and The contact points m2 and lid are placed outside the locus of movement or the conducting bars as, its, this, its, act, lllil, llli, llil, lit and lit. The conducting bars, however, are adapted to contact contact points ltd and l28 which correspond to contact points ts and ti associated with drum lll. The brush it is connected to a switch point l til by conductor i232 and the brush a2 is connected to a switch point i3 3 by a conductor lBtl. The brush lid is connected to a switch point E38 by conductor ltd and the brush ltd is connected to a switch point M2 by a conductor l i l. A pair of switch arms use and let are pivotally mounted to rotate around pins llit and i512. A link ltd is pivotally connected to arm ltd at point its and to arm t lt at point ltd. The link ltd carries a cam follower E66 adapted to coact with the cam (it. A spring it? urges the arms Hit and M8 to move to the right as viewed in Figure to bring the cam follower Mill in contact with the cam. surface. The arm ldli carries a brush ltt which is connected by conductor lt i to a brush let; carried by the arm ltd. A conductor ltll connects the brush ted to the binding post 8. A conductor llll connects the binding post 3 to one end of the resistance winding ill. A conductor ill: connects the brush it to conductor lit which is connected to the contact point tl. Conductor W2 is also connected to binding post i222. The brush to in the position shown in Figure 1 is connected by conductor lid to the contact point Contact point 9% is connected by conductor lit to contact point l2?! and contact point its is connected by conductor l'lt to contact point if The brush 22 is connected to contact point llit by conductor lit. Contact point ltd associated with drum 32 is connected to brush it by conductor l8? and a fixed resistor let, the value of resistance of which is R. Similarly, the brush 2t is connected to contact point ltd by conductor lfilt and fixed resistor ltd having a resistance value of R.

In operation, let us assume the parts are in the position shown in Figure i. Let us trace ill ' the value of the resistance across binding posts 6 and 6. Starting from binding post 5 we follow conductor llll through a segment of the resistance K to the brush it. The path around the resistor it to the brush it through the re sistor it through conductor ass through the resistors dd, 52, 64, st and 88 through the conductor H8 is open at contact point 95. Following conductor lit and H3 to contact point tl, thence through brush 92 through conductor lit to switch point lit brush it? conductor I64 brush l 66 switch point ltd conductor l 32 brush 96, we arrive at resistor Ell. It will be noted that resistor 50 is connected in series with resistor 52 by conducting bar it and the resistor 52 is con- 70 nected in series with resistor 56 by conducting bar 12. Resistor 54 is in series with resistor 56 through conducting bar 74. Resistor 56 is in series with resistor 58 by conducting bar 16 and contact point N5 is in series with the resistors by conductig bar 18. Since each of the resistors IO, II, II, It, and II has a mwehaveinadditiontothellalreadynoteda resistance value of 103. Following conductor its we pass through the resistor I" having a value oi R and thence to brush 2. and along the rheostat resistance ill to brush It. The value 01' the resistance embraced between brushes ii and 22 is R. From brush 2! we pass through the segment K of the resistor I! and thence through conductor Ill to the binding post i.

The resistance we have just traced can be expressed as 12R+2K which is the value of the resistance indicated above when n equals live. The resistance between binding post and binding post I furthermore will be K which is the only resistance embraced between these two points. It will be noted that the path from brush 22 to contact point I28 through conductor H8 is open and that the path through conductor ill to contact point 08 is open. Let us now rotate the knob 2' in the direction of the arrow. As the resistance swept by brush i1 increases the resistance of rheostat l8 between brush and point 2! on resistor It decreases a corresponding amount. The overall resistance, therefore, between points i and G is not varied. The resistance, however, between binding post Q and binding post 8 is increased agreeable to the motion of the brush l! in the clockwise direction. As the resistance increases both drum It and drum 42 will rotate at a reduced speed and in phase with the motion oi the rheostat brushes. when brush l2 occupies the position of brush it in Figure 1, that is, when 180 of rotation of the brush bearing arms l8 and 24 has taken place,

the cam 58 will have rotated from the position shown in Figure 1 to the position shown in Figure 2, and brush 22 will occupy the position cc cupied by brush :0 in Figure 1.

when this point arrives drum 52 will have re tated so that brush in will be in contact with contact point I80, brush H8 wili be in contact with the end of resistor 60, conducting bar as will be in a position to connect resistors 80 and $2 in series, conducting bar iill will be in a position to connect resistors 82 and it in series, conducting bar ill! will be in a position to connect resistors 84 and 88 in series, conducting bar It" will be in a position to connect resistors 68 and 88 in series, and conducting bar we will be in a position connecting contact points lit and H8. The contact made by the conducting elements carried by the drum in Figure i will be near the point of being brokeru It makes little diilerence, however, whether they are broken or not since the switch brushes "2 and lit will disconnect these from the circuit on the operation of the cam. Inasmuch as both rotors and their associated rheostats will always register the same resistance and the brushes will always be located at the same relative point in the overall resistance, it is not necessary that the switching take place at any critically exact moment. The exact relationship between the position of the brushes and the point of switching is dilhcult to control in operation, but in the arrangement I have shown the correct resistance will always be included between the terminals 4 and 6 and the correct resistance will always be registered irrespective of diilerences in the path of travel of the brush arms I! and 24 with respect to the point at which switching takes placer At points of maximum and minimum adjustment or the variable potentiometer arm, it will resistencevalueei be necessary to register more and maximum on both rotors independent or whether the switch connects one rotor or the other to the terminal 8. I will point out how I have avoided a critical point of switching at the terminal points of adiustment hereinafter.

Let us now trace the overall resistance between binding posts 4 and 8. From bindin post l we follow conductor I10 throu h the segment of resistor III of the value of K to brush ll through resistance ill or the value 0! R. through conductor "2 to contact point I" to brush I, which is now in contact with this contact point, through conductor I to contact point I through brush I86, which is now in contact with this contact point, through conductor I through brush I82, which is now in contact with contact point use, through conductor I to brush H8, which is now in contact with the end of resistor 80, thence through conducting bar 88 through resistor 82 through conducting bar it through resistor 64 through conducting bar it! through resistor 66 through conducting bar it through resistor 68 through conducting bar MB to contact point I28, thence through conductor M8 to brush 22 through the resistance is through conductor Ill to binding post I. It will be seen that the overall resistance, therefore, has not varied. The resistance between binding post i and binding post 8, however, is now R-i-K, the it being introduced by the resistor Hi4, and the K between binding post 939 and the position of the brush it.

As motion is continued in the direction of the arrow in Figure l, the value of the resistance between brush is and binding post 8 will continue to decrease while the value of the resistance between brush i4 and resistance terminal its will continue to increase by a like amount. By the time brushes 8: and 22 reach the positions shown in Figure l and. the cam is ready to operate, the switch brushes so and 92 will be bridging the gap between the resistors 50 and 52. Conducting bar will occupy the position shown by the brushes 9!! and 92 in Figure 1. Conducting bar It will be connecting resistors 52 and 84 in series. Conducting bar 12 will be connecting resistors 54 and SS in series, conducting bar ll will be in a position connecting resistors ll and B8 in series, and conducting bar 16 will be connecting the end of resistor 58 with contact point i". Conducting bar It, however, will not connect contact points 91 and 96, as these are contacted only by lorushesand 92. Similarly, with respect to drum 42, the contact points i and 122 will not be contacted by conducting bar MI.

The action just described will continue. The resistance between binding post 4 and terminal I is gradually increased by either resistor ID or ll and its coacting brushes and the increase of resistance represented by the sweep of the brushes being cumulated by the fixed resistors and the switching apparatus until the parts have reached the position shown in Figure 2.

As the knurled knob 2a is continued being rotated in a clockwise direction the same action occurs. The brushes 92 and 90 of rotor 40 and. the brushes I20 and ll! of rotor 42 being advanced in phase with each other and having a predetermined relationship to the rotation of the brush bearing arms l8 and 24, the overall resistance remains the same and the terminal 8 is alternately switched from one rotor brush to the 78 other in step by step fashion. There is a slight overlap in the period of time that the brushes and conducting bars remain in contact. To take a simple case, near the point at which the switch should work the conducting bar 70, for example, will be bridging resistors 50 and 52 for a short period after conducting bar as of rotor t2 has completed the circuit between resistors 60 and 62. To continue the action during the time while conducting bar 98 is bridging the circuit between resistors 60 and 62 associated with rotor iii the conducting bar It will have reached a position to connect resistors 52 and associated with rotor 40. In this manner if the switching takes place a little before it should the correct resistance value will be registered at terminal 3. Similarly, if there is a little lag at the switching point the correct resistance value will still be registered at terminal 8. This action is not at all critical be cause of this construction during the major portion of the movement. At a point when the terminal 8 is to be connected to terminal ii (zero value) and at a point when terminal a is connected to terminal 6 (maximum value) I am enabled to avoid'the critical adjustment at the extreme points by means of two auxiliary contact points at and 96 associated with rotor Gil and I22 and I251 associated with rotor d2.

Let us assume, further, that we do not know for certain whether the switch will be in the position shown in Figure l or in the position shown in Figure 2. If the switch is in the position shown in Figure 1 no diillculty is experienced since point 4 is connected to point 8 through conductor ilil brush I2 conductor IIZ conductor IIS brush 92 conductor I36 contact point I Be switch arm I62 conductor I845 to conductor Ifit. At the same time the resistance between the terminals =3 and 5 will be measured through a circuit completed by switch arm I 65 contact point let contact point I32 brush 9t resistors til, 52, 5t, 5t and 58, and their intermediate conducting segments, conductor I85 resistor I98 brush 2B and a resistance value along the resistor I8 equal to 2K+R to con ductor III to terminal t. Let us suppose, however, that at this point the switch instead of being in the position shown in Figure l is actu= ally in the position shown in Figure 2, the circuit can not be completed through brushes 9t and 92 since the gap between contact points I34 and Illa is open. The circuit path may now be traced as follows: from conductor I12 to contact segment I22 through conductor I44 to contact point I42 through the switch brush I65 through connecting conductor I64 through the switch arm I62 to contact point I38 through conductor I40 through brush II8 through conductor I14 and thence through the resistors 50, 52, 54, 56, and 58 through conductor I88 through resistor I913 and the resistance winding I8 through conductor III to the terminal 6. Due to the use of the auxiliary contact points I22 and I24 which are closed only by the brushes H8 and I and never by any of the conducting bars on rotor 42, I am enabled to achieve the zero position irrespective of the position of the cam operated switch. It will be noted further that contact points I26 and I28 are always open near the zero position so that the resistances of rotor 42 do not come into play.

Referring now to Figure 2, I have shown the same arrangement adjacent the maximum point. Let us rotate the brush 22 to bring it into contact with the conductor I'II at point 2|. If the switch does not operate to move from the position shown in Figure 2 until contact is made between brush 22 and the point 2|, terminal 8 is at maximum voltage ough condutor III brush 22 c0nductor lte'contact point are brush IIe conductor Mil contact point the brush I62 conductor I66 and conductor 6%. The circuit, furthermore, be-= tween terminal It and terminal :3 is completed through brush its conductor Id l brush in contact point I26 resistors I353, 55, be, 62 and all, conductor I82 resistor Itt brush It and a resistance along resistor It equal to ZK-I-R, and thence through conductor I It to terminal i. If, however, the rotary movement of the brush 22 the position occupied by brush to causes the earn it to throw the switch to the position shown in Figure 1, terminal a is placed at the potential of terminal 6 through the path comprising conductor III brush 22 conductor Ila contact seg ment at brush cc conductor I82 switch point Hit and switch arm I i-id. The resistance path across the potentiometer is completed through conduc tor its switch arm ItZ switch point I it conductor I brush or contact point st conductor I'Iii through resistors $8, at, $55, 52 and so, and thence through conductor I82 resistor lee brush It and the resistance winding It to conductor are and terminal In this case it will be noted that con tact points as and at are open, there being no conducting bar adjacent these contacts near the maximum position.

It will be clear from what has been said that a zero point and a maximum point can always be reached irrespective of the position of the switch at these points. At intermediate points of travel of my potentiometer the switching is likewise not critical because the correct value of the resistance between terminals will always be registered whether the path is through the resistors associated with rotor so or through the resistors associated with rotor Similarly, the terminal 8 will always be at its correct adjustment so that there will be no change in value whenever the switching does take place. It is understood, of course, that the switch arrangement shown is diagrammatic only and that a quick acting switch will be employed to minimize the momentary interruption of the circuit which takes place during the switching.

It will be noted that in operation the brushes c2 and at will rotate successively connecting adjacent resistors associated with drum Lit and the brushes I It and Ifib will rotate successively connecting intermediate resistors associated with drum 42. Taking contact points BI and as as an example, these will be bridged first by the brushes 92 and so. The circuit will next be completed across contact point Ito and the end of resistor all by the brushes I28 and lit. The cir-' cuit will next be completed across contact points 88 and SI by conducting bar 8t. The circuit will next be completed across contact point I81? and end of resistor to by conducting bar I 88. The action thus continues stepwise until the maximum is reached as described above.

It will be seen that I have accomplished the objects of my invention. I have provided a potentiometer which will have the desired overall resistance between narrow limits and in which a predetermined movement of the variable brush will produce a corresponding change in resistance within very narrow limits. I have provided a potentiometer in which voltage may be divided with a great linearity as ajunction of movement of the variable potentiometer brush. Due to the fact that the rheostats employed represent only a small fraction of the overall resistance a very minor error is introduced. These rheostats, furthermore, can be selected for the linearity and a very accurate overall system is produced.

It will be understood that certain features and sub-combinations are of utility and may be employed without reference to other features and sub-combinations. This is contemplated by and is within the scope oi my claims. It is further obvious that various changes may be made in details within the scope of my claims without departing from the spirit of my invention. It is, therefore, to be understood that my invention is not to be limited to the specific details shown and described.

Having thus described my invention, what I claim is:

1. A potentiometer including in combination a pair of terminals, a resistance across the terminals comprising a pluralityof separated fixed resistors and a rheostat having a variable brush, a multiple element switch for connecting the fixed resistors and the rheostat in series, a third terminal, means for connecting one of said switch elements to the third terminal and means for moving the switch in predetermined relation to the movement of the rheostat brush whereby the switch element connected to the third terminal will successively bridge successive fixed resistors.

2. A potentiometer including in combination a pair of terminals, a resistance across the ter minals comprising a pair of rheostats each having a pair of brushes adapted to intercept a predetermined resistance, means for rotating said rheostat brushes in unison, two groups of fixed resistors associated with respective rheostats each resistor oi which has a resistance equal to twice the predetermined resistance intercepted by the rheostat brushes, 9. pair of multiple element switches adapted to connect resistors of a respective group in series with each other and with one of the rheostat brushes, one of the elements of each of the multiple element switches comprising a pair of brushes, a double pole switch for alternately connecting the element brushes of one or the other of said multiple element switches, means for connecting the last resistor of each group of resistors to the other rheostat brush associated with the other group of resistors, means for moving the multiple element switches in unison and at a predetermined'function of the movement of the rheostat brushes, means for operating said double pole switch from a position bridging one pair of element brushes to a position bridging the other pair oi element brushes alternately after a substantially predetermined movement of the rheostat brushes in a given direction, a third terminal for the potentiometer, and means for connecting a switch arm of said double pole switch to said third terminal.

3. A potentiometer as in claim 2 in which each of said means for connecting the last resistor of each group of resistors to the other rheostat brush includes a fixed resistor having a resistance value equal to that intercepted by the rheostat brushes. v

4. A potentiometer as in claim 2 in which said means for operating said double pole switch comprises a cam and means for actuating said cam as a function of the movement of the rheostat brushes.

5. A potentiometer as in claim 2 including means for interconnecting the element brushes in parallel adjacent points of minimum and maximum positions of the multiple element switches whereby the one or the other of said first two-terminals will be connected to the third terminal irrespective of the position of the double pole switch at maximum and minimum setting.

6. In a voltage divider a pair of rheostats each having a resistance element, a pair of movable brushes each adapted to traverse respective resistance elements to vary the resistance between the brush and one end of the resistance element, means for moving said brushes in unison, a plurality of groups of fixed resistors each resistor of which has a predetermined resistance value, a pair of multiple element switches each associated with one group of fixed resistors adapted to connect the fixed resistors of one group in series with each other and with the rheostats, one element of each of said respective multiple element switches comprising a pair of brushes, means for moving said multiple element switches as a function of the movement of the rheostat brushes whereby to successively connect diflerent resistors of a group to each other by means of a pair of brushes, and switching means adapted alternately to bridge the brushes as a function of the movement of the rheostat brushes.

7. A voltage divider as in claim 6 in which the rheostat resistance elements are so disposed that when the first brush increases the resistance between the end of the resistor and the brush the second brush will reduce the resistance between the brush and the end of the rheostat resistance by the same amount.

8. .A voltage divider as in claim 6 in which each of said rheostats is provided with respective second brushes, means for mounting said second brushes to move in unison with said first brushes to intercept a predetermined resistance value, said resistance value being one-half of the resistance of each resistor of said groups of fixed resistors.

9. A voltage divider as in claim 6 in which each of said rheostats is provided with respective second brushes, means for mounting said second brushes to move in unison with said first brushes to intercept a predetermined resistance value, said resistance value being one-half of the resistance of each resistor of said groups of fixed resistors, and means for connecting the second brush of one rheostat to the last resistor of the group of fixed resistors associated with the other rheostat.

10. A voltage divider as in claim 6 in which each of said rheostats is provided with respective second brushes, means for mounting said second brushes to move in unison with said first brushes to intercept a predetermined resistance value, said resistance value being one-half of the resistance of each resistor oi. said groups of fixed resistors, means for connecting the second brush of one rheostat to the last resistor of the group of fixed resistors associated with the other rheostat, said means including a fixed resistor having a resistance value equal to that intercepted between .the rheostat brushes of a rheostat resistance.

11. A potentiometer including in combination a pair of term'inals, a pair 01' rheostats having circularly disposed resistors, a pair of brushes associated with each rheostat resistance element adapted to intercept a predetermined resistance value, means for connecting one terminal to one end of one of the rheostat resistors, means for connecting the other terminal to one end of the other of said rheostat resistors, a first group of separate fixed resistors each having a resistance value equal to twice the resistance intercepted by ll the rheostat brushes and associated with one oi said rheostats, a second group 01' fixed resistors associated with the other 01' said rheostats each resistor 01' which has a resistance equal to twice the resistance intercepted by the rheostat brushes, a rotar switch member associated with said first group of resistors, said rotary member having a plurality oi switching members adapted to connect the fixed resistors in series, one of said switching elements comprising a pair of brushes, means for connecting said brushes to two poles of a double pole double throw switch, a second rotar member associated with the second group of fixed resistors having switching elements adapted to connect the fixed resistors in series, one 'of said switching elements comprising a pair of brushes. means for connecting said brushes to another pair of poles of the double pole double throw switch. a contact point separated from the first resistor 01 the first group of fixed resistors, means for connecting the contact point to one or the brushes the first rheostat, a second contact point spaced from the end of the last resistor of the first group of resistors, means for connecting lhe second contact point to one or the brushes of the second rheostat, a contact point spaced from the last resistor of the second group of resistors, means for connecting this contact point to the second brush or the second rheostat, a contact point spaced from the first resistor of the second group of resistors, means for connecting this contact point to the second brush of the first rheostat, said contact points being adapted to be connected to adjacent resistors by means of said rotary switch elements, means for rotating said 12 rheostat brushes in unison, means for rotating said rotary switch members in predetermined phase relation with each other and with the move:

ances across the pair of terminals or the potentiometer, a third terminal, and means for connecting the third terminal to one or the other of said brush switch elements.

12. A potentiometer as in claim 11 in which said means for connecting the second contact point of the first group of resistors to one of the brushes of the second rheostat includes a fixed resistor having a resistance equal to that intercepted by the rheostat brushes.

13. A potentiometer as in claim 11 in which said means for connecting the contact point spaced from the first resistor of the second group of resistors to the second brush of the first rheo stat includes a fixed resistance having a resistance value equal to that intercepted by the rheostat brushes.

14. A potentiometer as in claim 11 in which said rheostat brushes are disposed substantially apart.

15. A potentiometer as in claim 11 including in combination means for connecting said rotary brush switch elements in parallel at positions of minimum and maximum adjustment.

CARL F. SC 1 No references citm.

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