CN201053998Y

CN201053998Y - Two-range potentiometer

Info

Publication number: CN201053998Y
Application number: CNU2007201099895U
Authority: CN
Inventors: 张春雷; 方李
Original assignee: Individual
Current assignee: Individual
Priority date: 2007-05-29
Filing date: 2007-05-29
Publication date: 2008-04-30
Anticipated expiration: 2017-05-29

Abstract

A two measurement range potentiometer is used for measuring direct voltage. A first stepping disk of the potentiometer comprises a measurement disk and two auxiliary disks without a welded resistance. The measurement disk of the second stepping disk comprises a circle-shaped resistance network, and the auxiliary disk of a second stepping disk comprises 10 resistances of 0.5 ohms. A third stepping disk comprises a measurement disk of ten 1 ohms and a substitution disk of ten 1ohms. Each contact point of a fourth stepping disk is connected with the corresponding contact point of the measurement disk of the third stepping disk. An electric brush of the fourth stepping disk is connected with a resistance of 100 k ohms in series. The substitution disk of the third stepping disk is connected with the resistance of 9995 ohms in series. The two series circuits and a resistance of 529.392 ohms are together connected into a parallel circuit in which a measurement range conversion is performed by using two resistance which are the resistance of 800 ohms and the resistance of 7200 ohms and a resistance measurement network to connect into the circuit by means of parallel circuits, and the minimum resolution is 1 microvolt. Measurement disks are connected by conducting wires without being switched by switches, so that the potentiometer can neglect the influence of the variation and the thermoelectric power when the potentiometer is used for measuring.

Description

Two-range potentiometer

Technical field

The utility model relates to the instrument that DC voltage is measured.

Background technology

Current for the potential difference meter that four measurement disks is arranged, in the connection between the four measurement disks, telophragma generally adopts switch to switch, and so just produces the variation of contact resistance, brings restriction to resolution.In order to overcome this problem, generally adopt big brush with the increase contact area, and adopt silver-carbon/carbon-copper composite material; Application number 2005100622602 discloses the new method that the potential difference meter that four measurement disks is arranged solves the switch contact resistance variation, its first, each is made up of second step disc measuring disk and replacement dish, measuring disk is identical with resistance on the replacement dish, resistance of the every increase of measuring disk, the replacement dish just reduces same resistance, its the 3rd, the 4th step disc respectively has measuring disk, replacement dish and bracket panel are formed, because the 3rd step disc was declined at 10 o'clock, the total resistance of circuit changed when the 4th step disc was put different indicating value, the 3rd step disc was put at 10 o'clock, the total resistance of circuit was constant when the 4th step disc was put different indicating value, the 3rd step disc removes for this reason measuring disk, outside the replacement dish, increased bracket panel distinguish step disc put 10 and the circuit of declining 10 two kinds of situations connect, the 4th step disc has also increased bracket panel, has 10 resistance to insert or cut out several resistance respectively above and makes the total resistance of circuit constant.Four measurement disks is connected two and measures between terminal, and the brush on the step disc switch is got rid of outside the measurement loop, does not exist switch to switch between the resistance on the four measurement disks, does not also just produce variation; Because 15 ° of angles are rotated in each stepping of potential difference meter step disc switch, every layer of 24 contact that can distribute, the measuring disk of first step disc and replacement dish are 21 contacts, two-layer inside and outside needing, every layer all has resistance, and internal layer resistance is overproof makes troubles to maintenance, the measuring disk of the 4th step disc, replacement dish and each half storey of bracket panel, the step disc switch also needs inside and outside two-layer, and internal layer also has resistance, and maintenance is also inconvenient.

The utility model content

The purpose of this utility model is a kind of two-range potentiometer of design, in the connection of four measurement disks, do not switch by switch, and first step disc cancellation replacement dish, and make four resistance on the step disc can both be contained in the switch skin, the switch internal layer is not welded with resistance.

The technical solution of the utility model is taked like this: from the positive pole of potential difference meter 3V working power through the resistance measurement network formed by resistance on four step discs and range transfer resistance setting resistance R to 463 Ω _NAnd the lockable adjustable resistance R of 0～1 Ω _P3, again to 60 Ω resistance R ₀, through 10 * 23 Ω adjustable resistance R _P1And 0～25 Ω adjustable resistance R _P2Get back to the negative pole of working power and form potential difference meter work loop; Standard cell E _NAnodal through being connected to the double-point double-throw switch K of galvanometer G between two normally closed contacts ₂To setting up resistance R _NAnd lockable adjustable resistance R _P3, process 75K Ω current-limiting resistance R is to standard cell E again _NNegative pole is formed the potential difference meter standard loop; Be used to connect measured " U _X" two terminals, anodal terminal through the four measurement disks resistor network after, again through being connected to the double-point double-throw switch K of galvanometer G between two normally closed contacts ₂Form the potential difference meter equalizing network to the negative pole terminal; First step disc has measuring disk I, it has 0,1,2 ... 22 totally 23 gears, except that between 0,1 contact, directly connecting, connect one of 10 Ω resistance between all the other each grade contacts, other has bracket panel I ' and bracket panel I ", the brush of bracket panel I ' and bracket panel I " brush be connected with lead, tie point is circuit node B, bracket panel I ' and bracket panel I " 0 contact isolated, all the other all contacts connect with lead; Second step disc is made up of measuring disk II and bracket panel II ', measuring disk II has 0,1,2 ... 10 totally 11 gears have the resistance of 11 11 Ω, the 1st resistance R above ₁One end welds the 2nd resistance R ₂One end, resistance R ₂The other end welds the 3rd resistance R ₃One end ... welding successively, the 10th resistance R ₁₀The other end and the 11st resistance R ₁₁An end tie point be circuit node C, the 11st resistance R ₁₁The other end and the 1st resistance R ₁The other end be connected in 0 point of measuring disk in the 3rd step disc, 0 of measuring disk is circuit node D in the 3rd step disc, resistance R ₁With resistance R ₂Tie point be connected resistance R through 20 Ω resistance with the 1st contact ₂With resistance R ₃Tie point be connected resistance R through 12 Ω resistance with the 2nd contact ₃With resistance R ₄Tie point be connected resistance R through 6 Ω resistance with the 3rd contact ₄With resistance R ₅Tie point be connected resistance R through 2 Ω resistance with the 4th contact ₅With resistance R ₆Tie point be connected resistance R with the 5th contact ₆With resistance R ₇Tie point be connected resistance R with the 6th contact ₇With resistance R ₈Tie point be connected resistance R through 2 Ω resistance with the 7th contact ₈With resistance R ₉Tie point be connected resistance R through 6 Ω resistance with the 8th contact ₉With resistance R ₁₀Tie point be connected resistance R through 12 Ω resistance with the 9th contact ₁₀With resistance R ₁₁Tie point be connected resistance R through 20 Ω resistance with the 10th contact ₁With resistance R ₁₁Connect node D o'clock being connected with 0 contact through 30 Ω resistance, is the resistance of 10 * 0.5 Ω on the bracket panel II ' of second step disc; The 3rd step disc is made up of measuring disk III that is both 10 * 1 Ω and replacement dish III ', and the brush of measuring disk III is connected with lead with the brush of replacement dish III '; The 4th step disc has only measuring disk IV, and the corresponding contact on its each contact and the measuring disk III connects, and resistance on the measuring disk III and measuring disk IV are public; Bracket panel II ' the 10th contact and 800 Ω resistance R ₁₈An end be parallel to node A, node A connects the positive pole of potential difference meter working power, 0 contact of bracket panel II ' connects bracket panel I " 0 contact, the brush of bracket panel II ' connects bracket panel I " other contact except that 0 contact, the brush of measuring disk II is through 190 Ω resistance R ₁₂The back connects the brush and bracket panel I of bracket panel I ' " the Node B that is connected of brush; other contacts connection measuring disk I 22nd contact of bracket panel I ' except that 0 contact; measuring disk I the 0th, 1 contact is connected with node C; 0 contact of bracket panel I ' is connected with node D after through 220 Ω resistance, the brush of the measuring disk IV 100k Ω resistance R of connecting ₁₅Back and 184.725 Ω resistance R ₁₆The tie point of one end is circuit node E, the 10th the series connection 9995 Ω resistance R of replacement dish III ' ₁₄Back connected node E, 529.392 Ω resistance R ₁₃One end connected node D, other end connected node E, 184.725 Ω resistance R ₁₆The other end and 7200 Ω resistance R ₁₇An end be connected in range selector K ₁* 1 range contact, 7200 Ω resistance R ₁₇The other end and 800 Ω resistance R ₁₈The other end be connected in range selector K ₁* 0.1 range contact, range selector K ₁Normally closed contact connect to set up resistance R _NNoble potential one end; Be used to connect measured " U _X" two measure terminal, anodally be connected with measuring disk I brush, negative pole passes through double-point double-throw switch K ₂The back is connected with the 10th of measuring disk III.

By above technical scheme, first step disc need not the replacement dish, does not all have resistance on the two-layer bracket panel, can be contained in the switch nexine, and measuring disk I is contained in the switch skin; Second step disc need not the replacement dish, 15 ° of angles are rotated in each stepping of switch, each 11 contact of the bracket panel II ' of second step disc and measuring disk II are distributed in same one deck of switch, resistance is contained in the switch skin, three, the 4th step disc does not all have bracket panel, resistance can be contained in the switch skin, unloads when resistance is overproof and loads onto easily, and this brings convenience for debugging and maintenance; This makes potential difference meter simple in structure, and volume-diminished has also reduced production cost; On the total circuit partly in potential difference meter internal compensation loop and work loop, there is not switch, so there is not variation influence, when the reset of potential difference meter four measurement disks, the zero potential of potential difference meter is the zero potential that D is ordered, in equalizing network, do not exist working current to flow through lead resistance, so this potential difference meter zero potential is very little, owing on the total circuit partly in potential difference meter internal compensation loop and work loop, do not have switch to switch, so this potential difference meter thermoelectrical potential and variable thermoelectrical potential are also very little.

Description of drawings

Accompanying drawing is the utility model schematic circuit.

Embodiment

Among the figure, measuring disk II is 11 the 11 end to end resistance rings of Ω between node C and node D, when measuring disk II puts " 5 " or puts " 6 ", the brush of measuring disk II is to being that 5 11 Ω resistance are in parallel with 6 11 Ω resistance between the node D, back in parallel resistance is 30 Ω to the maximum, other contact of measuring disk II all will be connected to 30 Ω to the resistance between the node D, and corresponding point directly are connected on the 5th, 6 contacts and the resistance ring; When measuring disk II put " 4 " or puts " 7 ", the brush of measuring disk II was to being that 4 11 Ω resistance are in parallel with 7 11 Ω resistance between the node D, and back in parallel resistance is 28 Ω, so 4,7 contacts are connected with corresponding point on the resistance ring through 2 Ω resistance; When measuring disk II put " 3 " or puts " 8 ", the brush of measuring disk II was to being that 3 11 Ω resistance are in parallel with 8 11 Ω resistance between the node D, and back in parallel resistance is 24 Ω, so 3,8 contacts are connected with corresponding point on the resistance ring through 6 Ω resistance; When measuring disk II put " 2 " or puts " 9 ", the brush of measuring disk II was to being that 2 11 Ω resistance are in parallel with 9 11 Ω resistance between the node D, and back in parallel resistance is 18 Ω, so 2,9 contacts are connected with corresponding point on the resistance ring through 12 Ω resistance; When measuring disk II set or when putting " 10 ", the brush of measuring disk II is to being that 1 11 Ω resistance is in parallel with 10 11 Ω resistance between the node D, and back in parallel resistance is 10 Ω, so 1,10 contacts are connected with corresponding point on the resistance ring through 20 Ω resistance; When measuring disk II reset, the brush of measuring disk II is between the node D being the connections of 30 Ω resistance.

The first step disc reset, second step disc are put " n " (n=0,1,2,3 ... 10) time, Node B is that two resistance that are both 220 Ω are in parallel with resistance value between the node D, is 110 Ω therefore.

First step disc is put " n " (n=0,1,2,3 ... 22), during the second step disc reset, Node B and resistance value between the node D also are that the resistance of two 220 Ω is in parallel, so resistance also is 110 Ω.

When first, second step disc during not reset, except that second step disc was put " 10 ", the calculating of the resistance value between Node B and the node D need be carried out triangle-star conversion.

During the second step disc set, the calculating of resistance value between Node B and the node D: establish resistance (R ₂+ R ₃+ ... + R ₉+ R ₁₀) and resistance R ₁₁The both sides resistance is equivalent to resistance r ₁, resistance R ₁₁With resistance R ₁The both sides resistance is equivalent to resistance r ₁' resistance (R ₂+ R ₃+ ... + R ₉+ R ₁₀) and resistance R ₁The both sides resistance is equivalent to resistance r ₁", be equivalent to resistance r ₁, r ₁', r ₁" intersection point is Q ₁:

R then ₁=(R ₂+ R ₃+ ... + R ₉+ R ₁₀) * R ₁₁/ (R ₁+ R ₂+ ... + R ₁₀+ R ₁₁)=9 * 11 * 11/11 * 11 Ω=9 Ω

r ₁’＝R ₁×R ₁₁/(R ₁+R ₂+…+R ₁₀+R ₁₁)＝11×11/11×11Ω＝1Ω

r ₁”＝(R ₂+R ₃+…+R ₉+R ₁₀)×R ₁/(R ₁+R ₂+…+R ₁₀+R ₁₁)＝9×11×11/11×11Ω＝9Ω

Resistance value equals (210 Ω+r between Node B and the node D ₁) * (190 Ω+20 Ω+r ₁")/2 * 219 Ω+r ₁'=219 Ω/2+1 Ω=109.5 Ω+1 Ω=110.5 Ω

When second step disc is put " 2 ", the calculating of resistance value between Node B and the node D: establish resistance (R ₃+ R ₄+ ... + R ₉+ R ₁₀) and resistance R ₁₁The both sides resistance is equivalent to resistance r ₂, resistance R ₁₁With resistance (R ₁+ R ₂) the both sides resistance is equivalent to resistance r ₂' resistance (R ₃+ R ₄+ ... + R ₉+ R ₁₀) and resistance (R ₁+ R ₂) the both sides resistance is equivalent to resistance r ₂", be equivalent to resistance r ₂, r ₂', r ₂" intersection point is Q ₂:

R then ₂=8 Ω r ₂'=2 Ω r ₂"=16 Ω

Resistance value equals (210 Ω+r between Node B and the node D ₂) * (190 Ω+12 Ω+r ₂")/2 * 218 Ω+r ₂'=218 Ω/2+2 Ω=109 Ω+2 Ω=111 Ω.

In like manner, when second step disc was put " 3 ", resistance value was 111.5 Ω between Node B and the node D,

When second step disc was put " 4 ", resistance value was 112 Ω between Node B and the node D,

When second step disc was put " 5 ", resistance value was 112.5 Ω between Node B and the node D,

……

When second step disc was put " 10 ", resistance value was 115 Ω between Node B and the node D.Because the every stepping of measuring disk II increases by 0.5 Ω, so the every stepping of bracket panel II ' reduces by 0.5 Ω, makes the total resistance of circuit constant.

When the first or second step disc reset, the resistance value between Node B and the node D is 110 Ω, and 10 * 0.5 Ω resistance of bracket panel II ' all enter circuit, and keeping the resistance between node A and the node D is 115 Ω.

When three, the 4th step disc was put different indicating value, the resistance between node D and node E changed between 500.2728 Ω～500.2780 Ω, and node E connects 184.725 Ω resistance R ₁₆After, pass through node D to K switch from node A ₁* resistance value between 1 range contact is 800 Ω, and the change in resistance of 0.005 Ω is 7/1000000ths to the relative variation of 800 Ω, and influence can be ignored.

Electric current is 2.2mA during the standardization of potential difference meter working current, when * 1 range, passes through node D to K switch from node A ₁* 800 Ω resistance values between 1 range contact and the resistance R of 8000 Ω ₁₇, resistance R ₁₈The sum parallel connection, the electric current that therefore flows through bracket panel II ' is 2mA, flows through resistance R ₁₇, resistance R ₁₈Electric current be 0.2mA.

I is to equivalent resistance r for Node B process measuring disk _n, r _n', r _n" intersection point Q _n(n=1,2,3 ... 10) with Node B through 190 Ω resistance R ₁₂To equivalent resistance intersection point Q _nResistance equates, so flow through measuring disk I and 190 Ω resistance R ₁₂Electric current respectively be 1mA.

When not considering measuring disk I, for ten one the 11 Ω end to end resistance rings of measuring disk II between node C and node D, resistance R during measuring disk II set ₁To be all 11 Ω resistance in parallel with 10 resistances, flows through resistance R ₁₁Electric current be 1/11mA, the voltage U between node C and the node D _CD=1/11 * 11mV=1mV; Resistance (R when measuring disk II puts " 2 " ₁+ R ₂) to be all 11 Ω resistance in parallel with 9 resistances, flows through resistance R ₁₁Electric current be 2/11mA, the voltage U between node C and the node D _CD=2/11 * 11mV=2mV; (n=1,2,3 when in like manner, measuring disk II puts " n " ... 10) voltage U between resistance nodes C and the node D _CD=n mV; During measuring disk II reset, electric current is without resistance R ₁₁, U _CD=0mV.

When not reset of measuring disk I, the electric current that flows through measuring disk I is superimposed upon on the measuring disk II at the 10mV voltage between node C, the D, replaces the resistance between 0,1 contact.When measuring disk I reset, electric current is without resistance R ₁₁, U _CD=0mV.

Potential difference meter 2mA working current is divided into three the tunnel: the one tunnel through measuring disk IV through first, second step disc behind node D, another road is through replacement dish III ', and one the tunnel through 529.392 Ω resistance R again ₁₃, three road electric currents meet at node E.When three, the 4th step disc is put different indicating value between node D and the node E resistance change, three, resistance minimum between node D and node E during the 4th step disc reset indicating value, resistance maximum between node D and node E when three, the 4th step disc is put " 10 " indicating value, get intermediate value, resistance is not having resistance R between node D and the node E when the 3rd, the 4th step disc is put " 5 " indicating value for this reason ₁₃When in parallel is (10 ⁵÷ 11+5) Ω is 0.11mA in order to make the total current that flows through the 3rd, the 4th step disc, with 529.392 Ω resistance R ₁₃The electric current of shunting 1.89mA; Three, during the 4th step disc reset indicating value, the total current that flows through the 3rd, the 4th step disc is 0.11001mA, and when the 3rd, the 4th step disc was put " 10 " indicating value, the total current that flows through the 3rd, the 4th step disc was 0.10999mA, error is ten thousand/, influence can be ignored; Flow through resistance R ₁₄With resistance R ₁₅The current ratio exact value be 10, when the 3rd, the 4th step disc reset indicating value, flow through resistance R ₁₄With resistance R ₁₅Current ratio be 9.995, when the 3rd, the 4th step disc is put " 10 " indicating value, flow through resistance R ₁₄With resistance R ₁₅Current ratio be 10.005, error is 5/10000ths owing to be last two dishes, influence also can be ignored.

During the working current standardization, first step disc is put n ₁, second step disc puts n ₂, the 3rd step disc puts n ₃, the 4th step disc puts n ₄, " U at this moment _x" two measure that voltage is between terminal:

U _x＝1×10n ₁+1×n ₂/11×11+0.1×n ₃+0.01×n ₄(mV)

＝10n ₁+n ₂+0.1n ₃+0.01n ₄(mV)

When * 0.1 range, pass through node D to K switch from node A ₁* 800 Ω resistance and 7200 Ω resistance R between 1 range contact ₁₇Be connected into 8000 Ω heels, 800 Ω resistance R ₁₈Parallel connection, the electric current that therefore flows through bracket panel II ' is 0.2mA, flows through resistance R ₁₈Electric current be 2mA, at this moment first step disc is put n ₁, second step disc puts n ₂, the 3rd step disc puts n ₃, the 4th step disc puts n ₄, " U _x" two measure that voltage is between terminal:

U _x＝0.1×10n ₁+0.1×n ₂/11×11+0.01×n ₃+0.001×n ₄(mV)

＝1n ₁+0.1n ₂+0.01n ₃+0.001n ₄(mV)

Because the electromotive force of standard cell disperses, between 1.0188V～1.0196V, standardized working current is 2.2mA, therefore sets up resistance R _NGet 463 Ω, add the lockable adjustable resistance R of 0～1 Ω _P3, variation range that can the coverage criteria cell emf.

Node A is to range selector K ₁Normally closed contact between the resistance of two ranges all be 8000/11 Ω, the setting resistance R of 463 Ω _NWith 0～1 Ω adjustable resistance R _P3Be 464 Ω, add 60 Ω resistance R ₀, amount to 1251 Ω, bear about 2.75V voltage; Potential difference meter adopts two groups of dry cell power supplies, and electromotive force was about 1.65V when dry cell was new, with old to 1.35V when following, the electric current shakiness, in order to make dry cell under new, former affection condition, can both make the working current of potential difference meter be adjusted to standardization, for this reason, get adjustable resistance R _P1Be 10 * 23 Ω, adjustable resistance R _P2Be 0～25 Ω, dry cell voltage usable range is between 2.75V～3.31V.

The normalized current of potential difference meter is such acquisition: 200mV standard signal voltage is pressed polarity and potential difference meter " U _x" two measure terminal and connect, the total indicating value of each step disc of potential difference meter is identical with the standard signal magnitude of voltage, double-point double-throw switch K ₂Throw to the left side, regulate adjustable resistance R _P1And adjustable resistance R _P2, make galvanometer G nulling; Again with double-point double-throw switch K ₂Throw to the right, regulate adjustable resistance R _P3, make galvanometer G nulling, repeat once again after, adjustable resistance R _P3Locking, at this moment the working current of potential difference meter is with regard to standardization.

Claims

1. two-range potentiometer, from the positive pole of potential difference meter 3V working power through the resistance measurement network formed by resistance on four step discs and range transfer resistance setting resistance R to 463 Ω _NAnd the lockable adjustable resistance R of 0～1 Ω _P3, again to 60 Ω resistance R ₀, through 10 * 23 Ω adjustable resistance R _P1And 0～25 Ω adjustable resistance R _P2Get back to the negative pole of working power and form potential difference meter work loop; Standard cell E _NAnodal through being connected to the double-point double-throw switch K of galvanometer G between two normally closed contacts ₂To setting up resistance R _NAnd lockable adjustable resistance R _P3, process 75K Ω current-limiting resistance R is to standard cell E again _NNegative pole is formed the potential difference meter standard loop; Be used to connect measured " U _X" two terminals, anodal terminal through the four measurement disks resistor network after, again through being connected to the double-point double-throw switch K of galvanometer G between two normally closed contacts ₂Form the potential difference meter equalizing network to the negative pole terminal; It is characterized in that first step disc has measuring disk I, it has 0,1,2 ... 22 totally 23 gears, except that between 0,1 contact, directly connecting, connect one of 10 Ω resistance between all the other each grade contacts, other has bracket panel I ' and bracket panel I ", the brush of bracket panel I ' and bracket panel I " brush be connected with lead, tie point is circuit node B, bracket panel I ' and bracket panel I " 0 contact isolated, all the other all contacts connect with lead; Second step disc is made up of measuring disk II and bracket panel II ', measuring disk II has 0,1,2 ... 10 totally 11 gears have the resistance of 11 11 Ω, the 1st resistance R above ₁One end welds the 2nd resistance R ₂One end, resistance R ₂The other end welds the 3rd resistance R ₃One end ... welding successively, the 10th resistance R ₁₀The other end and the 11st resistance R ₁₁An end tie point be circuit node C, the 11st resistance R ₁₁The other end and the 1st resistance R ₁The other end be connected in 0 point of measuring disk in the 3rd step disc, 0 of measuring disk is circuit node D in the 3rd step disc, resistance R ₁With resistance R ₂Tie point be connected resistance R through 20 Ω resistance with the 1st contact ₂With resistance R ₃Tie point be connected resistance R through 12 Ω resistance with the 2nd contact ₃With resistance R ₄Tie point be connected resistance R through 6 Ω resistance with the 3rd contact ₄With resistance R ₅Tie point be connected resistance R through 2 Ω resistance with the 4th contact ₅With resistance R ₆Tie point be connected resistance R with the 5th contact ₆With resistance R ₇Tie point be connected resistance R with the 6th contact ₇With resistance R ₈Tie point be connected resistance R through 2 Ω resistance with the 7th contact ₈With resistance R ₉Tie point be connected resistance R through 6 Ω resistance with the 8th contact ₉With resistance R ₁₀Tie point be connected resistance R through 12 Ω resistance with the 9th contact ₁₀With resistance R ₁₁Tie point be connected resistance R through 20 Ω resistance with the 10th contact ₁With resistance R ₁₁Connect node D o'clock being connected with 0 contact through 30 Ω resistance, is the resistance of 10 * 0.5 Ω on the bracket panel II ' of second step disc; The 3rd step disc is made up of measuring disk III that is both 10 * 1 Ω and replacement dish III ', and the brush of measuring disk III is connected with lead with the brush of replacement dish III '; The 4th step disc has only measuring disk IV, and the corresponding contact on its each contact and the measuring disk III connects; Bracket panel II ' the 10th contact and 800 Ω resistance R ₁₈An end be parallel to node A, node A connects the positive pole of potential difference meter working power, 0 contact of bracket panel II ' connects bracket panel I " 0 contact, the brush of bracket panel II ' connects bracket panel I " other contact except that 0 contact, the brush of measuring disk II is through 190 Ω resistance R ₁₂The back connects the brush and bracket panel I of bracket panel I ' " the Node B that is connected of brush; other contacts connection measuring disk I 22nd contact of bracket panel I ' except that 0 contact; measuring disk I the 0th, 1 contact is connected with node C; 0 contact of bracket panel I ' is connected with node D after through 220 Ω resistance, the brush of the measuring disk IV 100k Ω resistance R of connecting ₁₅Back and 184.725 Ω resistance R ₁₆The tie point of one end is circuit node E, the 10th the series connection 9995 Ω resistance R of replacement dish III ' ₁₄Back connected node E, 529.392 Ω resistance R ₁₃One end connected node D, other end connected node E, 184.725 Ω resistance R ₁₆The other end and 7200 Ω resistance R ₁₇An end be connected in range selector K ₁* 1 range contact, 7200 Ω resistance R ₁₇The other end and 800 Ω resistance R ₁₈The other end be connected in range selector K ₁* 0.1 range contact, range selector K ₁Normally closed contact connect to set up resistance R _NNoble potential one end; Be used to connect measured " U _X" two measure terminal, anodally be connected with measuring disk I brush, negative pole passes through double-point double-throw switch K ₂The back is connected with the 10th of measuring disk III.