US3383636A - High value electrical resistance transfer standard - Google Patents

Description

y 1968 M. MORGAN ETAL 3,383,636

HIGH VALUE ELECTRICAL RESISTANCE TRANSFER STANDARD Filed March 17, 1965 3 Sheets-Sheet 1 Jack C.R1'[e1 i Merle L.Morqc1n INVENTORS BY CQMAQQQW y 1968 M. MORGAN ETAL 3,383,636

HIGH VALUE ELECTRICAL RESISTANCE TRANSFER STANDARD Filed March 17, 1965 5 Sheets-Sheet 2 24 w Z Z Z6 da ck C. R1! ey Merle L.Morgan INVENTORS BY QQJL 6%,

y 1968 M. 1.. MORGAN ETAL 3,383,636

HIGH VALUE ELECTRICAL RESISTANCE TRANSFER STANDARD- Filed March 17, 1965 3 Sheets-Sheet 5 Jack C. Riley Merle LJ IOr'gcIn IN VE N TORS United States Patent 3,383,636 HIGH VALUE ELECTRICAL RESISTANCE TRANSFER STANDARD 'ierle L. Morgan and Jack C. Riley, Portland, Greg, as-

signors to Electra Scientific Industries, Inc., Portland,

Oreg., a corporation of Oregon Filed Mar. 17, 1965, Ser. No. 440,510 8 Claims. (Cl. 338-200) ABSTRACT OF THE DISCLOSURE A plurality of high value electrical resistance standards are connected electrically in series by electrical connector members adapted for connecting the junctions selectively to an external circuit. Connector members and adjacent resistors or associated insulators support each other out of contact with other parts of the structure, in order to eliminate insulation leakage currents to other parts of the circuit. The end connector members are supported by insulating members separately mounted on an electrically conductive electrical guard member for intercepting insulation leakage current.

BACKGROUND OF THE INVENTION This invention relates to electrical resistance standards used in the calibration of instruments for measuring high values of electrical resistance.

Resistors of very high value typically are less stable in resistance than those of lower value. Accordingly, it is advantageous to build high value resistance standards using two or more resistors connected in series to serve as a standard. Further, it is desirable that such an arrangement of a plurality of resistors be arranged to be calibrated from time to time by measurements at a lower resistance value, either of the individual resistors, or of groups of the resistors connected together in parallel, or in series-parallel. This calibration transfer from one re sistance level to another depends upon the assumption that the resistance of any series, parallel, or series-parallel connected group is accurately equal to the value calculated from the individual resistance values. The term resistance transfer standard is used to refer to a multiple-resistor standard designed for making such calibration transfers.

However, in high value resistance standards the insulating members which support the resistors and conductors must also be considered as high value resistors, since they are never perfect insulators. High value resistance standards therefore normally are built as three-terminal resistors" in order to separate the direct resistance between the two main terminals from the leakage resistances from these terminals to an electrically conducting member, generally referred to as a guard. Frequently this guard is provided by an electrically conducting supporting case, panel, or frame on which the insulators are mounted. High resistance measuring instruments are made to measure the direct resistance of a three-terminal resistor without being disturbed by the leakage resistance from the two main terminals to the third terminal which is connected to the guard. The guard functions to intercept insulation leakage currents, preventing such leakage currents from either main terminal from reaching the other main terminal, thereby eliminating these leakage resistances from the dire-ct resistance of a single threeterminal resistor or of any number of three-terminal resistors connected together in parallel.

However, when two or more three-terminal resistors are connected in series, or in series-parallel, any leakage resistances to the guard from resistor junctions other than 3,333,535 Patented May 14, 1968 those connected to measurement terminals, will make the effective direct resistance of the combination differ from the value calculated from the individual direct resistance values. Thus, in high value resistance transfer standards of the prior art, the effective resistance of a group of high value resistors connected in series, or series-parallel, is not precisely equal to the value calculated from the individual resistor values.

SUMMARY OF THE INVENTION In its basic concept, the electrical resistance transfer standard of the present invention involves an arrangement of a series-connected group of resistors in which the conductors at each junction between resistors, when not connected to an external circuit, are supported by and toner only resistors and insulating members which, in turn, are supported by and touch only conducting members connected to the opposite ends of the adjacent resistors.

Further, if the assembly includes binding posts or other measurement terminals, the only insulating members touched by the terminals at each junction between resistors, when not connected to an external circuit, are supported by and touch only conducting members connected to the opposite ends of the adjacent resistors.

Still further, if the assembly includes switching means for connecting junctions between resistors to measurement terminals, the only insulating members touched by the switch member connected to each junction between resistors, when the switch is open, are supported by and touch only conducting members connected to the opposite ends of the adjacent resistors.

Thus, it can be seen from the foregoing, that the resistors and insulating members touching each junction between resistors do not touch the guard member, nor do they touch any junction between resistors other than those at the opposite ends of the adjacent resistors. Further, any insulating member or members touching the conductors at both ends of any resistor does not touch any other insulating member or the guard member.

It is the principal object of the present invention to provide a resistance transfer standard in which, when a group of two or more resistors are connected in series, parallel, or series-parallel, there are no insulation leakage paths which can make the direct resistance of the group differ from the value calculated from the individual direct resistance values.

Another important object of this invention is the provision of a resistance transfer standard of the class described, which includes convenient switching facilities for connecting groups of resistors to binding posts or other terminals for connection to external circuits, without the inclusion of insulation leakage paths which can adversely affect the direct resistance of the group.

A further important object of the present invention is the provision of a resistance transfer standard of relatively simplified construction and yet characterized by extremely high accuracy of calibration transfer.

The foregoing and other objects and advantages of this invention will appear from the following detailed description, taken in connection with the accompanying drawings of the preferred embodiments.

BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 is a schematic representation illustrating the basic concept of the present invention;

FIG. 2a is an electrical schematic diagram of two three-terminal resistors connected in series, showing in dash lines the effect of leakage resistance to the guard member, and FIG. 2b is the equivalent circuit thereof;

FIG. 3 is a foreshortened view in side elevation showing one mechanical embodiment of the present invention;

FIG. 4 is an electrical schematic diagram of another embodiment of the present invention, wherein switching means is provided to connect each junction between resistors and the ends of the end resistor selectively to either of a pair of bus bars and terminals for connection to an external circuit;

FIG. 5 is a foreshortened view in side elevation of a mechanical arrangement incorporating the switching arrangement of FIG. 4;

FIG. 6 is a sectional view taken on the line 6,6 in FIG. 5; and

FIG. 7 is a sectional view taken on the line 7-7 in FIG. 5.

'DESCRIPT ION OF THE PREFERRED EMBODIMENTS Referring first to FIG. 1 of the drawings, there is illustrated schematically a series-connected group of a plurality of resistors, for example the five resistors '10 supported at the opposite ends of the string by electrical insulators 12 projecting from the common guard member 14 of electrically conductive material. The guard member may be a panel, frame, or case, as previously mentioned, or, for example, it may simply be an electrical conductor interconnecting electrically conductive members engaging the entire bottom surfaces of the insulators 12. In this basic form of the invention it is to be noted that the junction 16 between any two of the resistors does not have an insulation leakage path directly to the guard member 14. It is to be understood that the term insulation leakage refers to electrical leakage in insulating members, it being assumed that electrical leakage through the space surrounding the structural members is negligible. FIGS. 2a and 2b illustrate the adverse elfect which would result if such leakage were present. It can be seen from the equivalent circuit of FIG. 2b that if a significant leakage resistance R;, were present between the supporting member 14 and the junction 16 of resistors R and R (FIG. 2a), the direct resistance of the series combination will differ from the sum of the individual direct resistances by R R /R It is apparent, therefore, that in order for the direct resistance of the series combination to be exactly equal to the sum of the individual direct resistances, the leakage resistance R must be made infinite. This is achieved, in the basic structure illustrated in FIG. 1, by the elimination of insulation members between the guard member 14 and the junction 16 between adjacent resistors.

From the practical standpoint of instrumentation, it is desirable that greater structural stability be provided for support of the series-connected resistors making up the high value resistance standard. One embodiment thereof is illustrated in FIG. 3. A substantially U-shaped frame 20 of electrically conductive material supports between the upstanding spaced sides 22 thereof a pair of electrical insulators 12 which, in turn, support between them the assembly of alternately arranged spaced elements 24 of electrical insulating material and transverse mounting plates 26 of electrically conductive material, with the end plates 26 of the assembly abutting the insulators 12. The elongated assembly of insulators 12 and 24 and con doctors 26 may be secured together as an integral, substantially rigid unit betwen the end plates 22 by various means. In the embodiment illustrated in FIG. '3, they are secured together by adhesive bonds 28 of epoxy resin or other suitable material.

The upper end of each of the conductor plates 26 is offset to provide a convenient mounting for a conventional binding post 30, by which to make electrical connection of the conductor to an external circuit. At least one of the spaced sides of the frame also is similarly offset at its upper end for mounting a binding post 32 serving as the common guard terminal.

Adjacent pairs of conductor plates 26 serve as supports for the opposite ends of a resistor 10, as by the solder connections illustrated. The intermediate conductor plates thus also serve as the terminal junction 16 (FIG. 1) between two adjacent resistors. The end conductor plates serve as the end terminals for the opposite end resistors in the series string, and these end conductor plates are spaced electrically from the frame by means of the end insulators 12.

It is to be noted in the embodiment illustrated in FIG. 3 that each insulator 24 joins only two adjacent conductors, both mechanically and electrically, and that it does not touch any other conductor or insulator to provide a leakage path anywhere except directly in parallel with one of the resistors 10. Further, it is to be noted that the terminal binding posts 30 are not mounted on insulators touching the frame, except the terminal at each end of the string.

If the frame is provided with a cover panel 34, the latter must be spaced from the insulators 24 and plates 26 and provided with openings 36 each providing a clearance space around each binding post 30 except, optionally, those at the ends of the string. The provision of such an air gap around every binding post except those at the ends of the string is essential in the present invention, regardless of whether the panel is of conducting or insulating material. If the panel were conductive and the air gap were filled with insulating material, it would provide an electrical leakage path from the binding post to the frame through the panel. If the panel were made of an insulating material, it would provide electrical leakage paths from each binding post to the frame and to all the other binding posts.

In the embodiment illustrated in FIGS, 1 and 3, connection to individual resistors or to two or more resistors in series, parallel, or series-parallel, may be accomplished through the use of test leads. These test leads may lie on a table, for example, and thus produce a leakage path to the frame. However, these leakage paths are only from the ends of the resistance being measured, since no leads are connected to intermediate junctions not in use. Accordingly, these leakages will cause no error in the direct resistance value of the group.

The switching arrangement illustrated schematically in FIG. 4 provides convenient means by which to connect any series, parallel, or series-parallel group of resistors 10, or any individual resistor 10, between a pair of terminals A and B for connection to an external circuit. A pair of mechanically coupled switches at each junction between resistors 10, and at each end of the series string, serve to releasably connect the associated junction selectively through one of the bus bars 40 and 42 to one of the associated terminals A and B, respectively, or to leave the junction dis-connected from both bus bars as desired. In the embodiment illustrated, the contacts 44 and 46 in each pair of switches are mounted on the conductor plates 26 (FIGS. 3 and 5) which are separated by insulators 24 from the adjacent junctions 16. For purposes of this invention it is essential that these switch contacts not touch any other insulators when both of the contacts 44' and 46 are separated from their associated contacts 44 and 46, respectively. In this manner the only leakage paths from each junction 16, when it is disconnected from both bus bars, will be those directly in parallel with the adjacent resistors 10.

It will be apparent that each pair of contacts 44 and 46 may be replaced by the common pole of a single pole, double throw switch with an off position, provided that when the switch is in the oil? position, there is no leakage path from the common pole to guard.

The movable conductors 44 and 46' in each pair of switches are connected through the bus bars 40 and 42, respectively, to the measurement terminals A and B. Leakage paths to the frame from the conductors associated with each terminal A and B will not affect the direct resistance value. However, it is essential that there be no leakage path directly between the conductors associated with the terminals A and B.

The circuit of FIG. 4 also may include another measurement terminal C connected to one end of the string of series connected resistors 10. This terminal provides means by which to connect one or more of the resistors in series between terminal C and one of the terminals A and B, for connection to a external circuit. This arrangement provides simultaneously a (pair of three-terminal resistances with a common junction A and B. It is required, for purposes of this invention, that there be no leakage paths directly from the conductors connected to terminal C to those connected to terminals A and B.

FIGS. 5, 6- and 7 show a structural assembly accommodating the switching arrangement illustrated in FIG. 4 while controlling the insulation leakage paths in the manner described hereinbefore. In this embodiment the electrically conductive guard is provided by panel 50 and the connected end plates 52. Between these end plates is located the assembly of alternate insulator members 12 and 24 and transverse conductor plates 26, as in FIG. 3. In this embodnnent, however, the assembly is secured between the end plates 52 by means of the tie bolt 54 which extends through aligned opening in the insulators and conductor plates and is mounted adjacent its opposite ends in the end plates. By tightening the nut on the bolt, the assembly of insulators and conductor plates are secured together by compression.

It is important that the bolt 54 not touch any of the insulators 24 or plates 26, for otherwise the bolt would provide a leakage path to the frame.

A resistor is connected between each pair of adjacent conductor plates 26. Thus, the intermediate conductor plates serve as junctions 16 between adjacent resistors 10.

Mounted on each conductor plate 26 is a pair of fixed switch contacts 44 and 46. Associated with each pair of fixed contacts is a pair of movable contacts 44' and 46', respectively, one connected to each of the spaced bus bars and 42, respectively. These bus bars are supported between the end plates 5-2 by means of the insulator blocks 56 and 58, respectively, mounted in spaced relation on the end plates. The end plates thus serve as a guard, eliminating any direct leakage path between the bus bars.

The movable contacts normally are spaced from their associated fixed contacts, but are movable selectively into contact therewith for making connection to the corresponding bus bar. Means for effecting such movement is provided by the toggle switch actuator 60 mounted for pivotal movement on the panel, in electrical connection therewith. The toggle switch actuator is provided with a pair of separate electrical insulator caps 62 which engage the outer ends of the movable switch contacts 44 and 46', respectively, to insulate the latter from the actuator.

It is important that separate insulator caps 62 be used, as described, in order that the actuator 60, through its electrical connection to the panel, will serve as a guard to eliminate a direct leakage path between the two bus bars.

By pivoting the toggle actuator clockwise or counterclockwise, as indicated in FIG. 6, contact 44 or 46' is moved into engagement with the associated contact 44 or 46, respectively, thereby connecting the corresponding conductor plate 26 and associated resistor junction 16 to the desired bus bar.

The bus bars 40 and 42 are connected through the conductors 64 and 66 to the main binding posts A and B, respectively, and the end of the series-connected string of resistors is connected through the conductor 70' to the terminal C. The binding post G is connected electrically to the panel, as a guard connection.

It is to be noted that, for the purpose of this invention, the terminals A, B and C must be mounted on separated insulators in order that the panel will serve as a guard to eliminate leakage paths between them.

In all of the embodiments previously described, there has been achieved the complete elimination of insulation leakage paths which can affect the accuracy of calibration transfers. The only leakage paths in insulating members in the present invention are those of two types, namely those which are permanently connected in parallel with a single resistor, such as the insulating body of resistor 10 and the insulating spacer 24, and thus are always included as a part of its effective resistance value, and those which are present between measurement terminals and the guard terminal, and thus are eliminated from a direct resistance value.

It will be apparent to those skilled in the art that various changes may be made in the structural details described hereinbefore. For example, since any resistor 10 and its associated insulator 24 function as a single resistance between adjacent conductor plates 26, these may be replaced by any configuration of resistors, insulators and conductors interposed between said adjacent conductor plates 26. These and other changes may be made without departing from the spirit of this invention and the scope of the appended claims.

Having now described our invention and the manner in which it may be used, what we claim as new and desire to secure by Letters Patent is:

1. A high value electrical resistance transfer standard, comprising (a) a plurality of resistors,

(b) a plurality of electrically conductive intermediate connector members connecting said resistors together in an electrical series arrangement,

(0) a pair of electrically conductive end connector members connected one to each of the end resistors of the series arrangement,

(d) the intermediate and end connector members being adapted for connecting the junctions between the resistors and the ends of the end resistors selectively to an external circuit,

(e) an electrically conductive electrical guard member for intercepting insulation leakage currents,

(f) a support member of electrical insulation material mounted on the guard member and attached to one end connector member and supporting the assembly of resistors and connector members spaced from the guard member, and

(g) electrical insulator means associated with each resistor and arranged between adjacent connectors and separated from each other by said connector members, whereby each intermediate connector member when not connected to an external circuit touches only its adjacent resistors and insulator means, and each resistor touches only the connector members connected to its ends, whereby the effective resistance of each resistor includes the parallel resistance of its associated insulator means.

2. The high value electrical resistance transfer standard of claim 1 including a second support member on the guard member spaced from the first named support member and supporting the other of the pair of end connector members.

3. The high value electrical standard of claim 1 wherein (a) the connector members comprise spaced plates,

(b) the insulator means comprise rigid insulators interposed between said plates,

(c) the assembly of plates and insulators is secured together as an integral structural unit,

(d) one end plate is supported by the support member,

and

(e) the opposite ends of each resistor are connected electrically to an adjacent pair of said plates.

4. The high value electrical resistance transfer standard of claim 1 including resistance transfer (a) at least one electric switch associated with each connector member and having a pair of relatively movable contacts,

(b) insulator means connected to the guard member and supporting the first contact of said pair, said first contact being adapted for connection to an external circuit, and

(c) the second contact being supported by and connected electrically to the connector member, said second contact touching only the connector member when the switch is open.

5. The high value electrical resistance transfer standard of claim 4 including (a) actuator means for each switch comprising an electrically conductive operator member connected electrically to the guard member, and

(b) insulator means, one for each switch, interposed between the operator member and one of the re1atively movable contacts of the switch.

6. The high value electrical resistance transfer standard of claim 4 wherein said first contacts of the plurality of the switches are connected together electrically for connection to an external circuit.

7. The high value electrical resistance transfer standard of claim 4 including (a) a plurality of said electric switches arranged in a plurality of groups,

(b) a plurality of separate conductor members each connecting together electrically the first contacts of a different group of said switches, and

(c) insulator means on the guard member supporting each conductor member and associated contacts, the insulator means for each conductor member being spaced from each other.

8. The high value electrical resistance transfer standard of claim 4 including (a) a pair of electric switches associated with each connector member,

(b) a pair of electrical conductor members one connecting together the first contacts of one switch of every pair and the other connecting together the first contacts of the other switch of every pair, and

(c) insulator means on the guard member supporting each conductor member and associated contacts, the insulator means for each conductor member being spaced from each other.

References Cited UNITED STATES PATENTS 1,047,742 12/ 1912 Buchanan 338-319 X 1,956,859 4/1934 Everett 338-3 19 1,360,904- 11/1920 Dorgeloh 3383l9 3,283,285 11/1966 Zuk 338295 X RICHARD M. WOOD, Primary Examiner.

J. G. SMITH, Assistant Examiner.

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