10. Electrical Measuring Devices and Measurement

ELECTRICAL MEASURING ELECTRICAL MEASURING DEVICES AND DEVICES AND

MEASUREMENT MEASUREMENT

Compiled and presented by Compiled and presented by Doren NedrickDoren Nedrick

Measuring InstrumentsMeasuring Instruments

The majority of electrical measuring The majority of electrical measuring instruments use the magnetic effect of the instruments use the magnetic effect of the electric current to measure electrical electric current to measure electrical quantities quantities (e.g., (e.g., current, voltage). The current, voltage). The simplest application of this method is simplest application of this method is illustrated in Fig. 10.1.illustrated in Fig. 10.1.

Simple Moving-iron Simple Moving-iron InstrumentInstrument

Principle: Principle: When an electric current is passed When an electric current is passed through a coil, an electro magnet is through a coil, an electro magnet is produced (recall principle of produced (recall principle of electromagnetism).electromagnetism).

Operation: (a) Operation: (a) A piece of soft iron suspended A piece of soft iron suspended on a spindle outside the coil will be attracted on a spindle outside the coil will be attracted towards the electromagnet (the coil).towards the electromagnet (the coil).

(b) (b) A pointer, fixed in the spindle, will move A pointer, fixed in the spindle, will move along the scale.along the scale.

The coil is termed the operating device.The coil is termed the operating device.

The Moving iron instrumentThe Moving iron instrument

This simple instrument has two This simple instrument has two disadvantages:disadvantages:

(a) The soft iron segment will be drawn into (a) The soft iron segment will be drawn into the coil unless its movement can be the coil unless its movement can be controlled.controlled.

(b) (b) When the instrument is operating the When the instrument is operating the soft iron segment will swing backwards and soft iron segment will swing backwards and forwards (oscillate) like a pendulum. A forwards (oscillate) like a pendulum. A device is required; to dampen this device is required; to dampen this movement, or oscillation.movement, or oscillation.

Basic Parts of an Basic Parts of an InstrumentInstrument

Every instrument requires three basic Every instrument requires three basic devices:devices:

1. 1. Operating device: Operating device: Usually the Usually the electromagnet formed by the current electromagnet formed by the current flowing through a coil.flowing through a coil.

2. 2. Controlling device: Controlling device: There are two There are two types of controlling devices: (types of controlling devices: (aa) gravity ) gravity control and control and (b) (b) spring control.spring control.

Gravity Control (Fig a):Gravity Control (Fig a): Two weights are attached to the Two weights are attached to the spindle: (a) a counter-balance weight to balance the spindle: (a) a counter-balance weight to balance the weight of the pointer, and weight of the pointer, and (b (b : weight used to control : weight used to control the movement of the pointer and restore it to its the movement of the pointer and restore it to its position after movement.position after movement.

Spring Control (Fig b):Spring Control (Fig b): Spring control is obtained by Spring control is obtained by attaching two: counter-wound phosphor-bronze (non-attaching two: counter-wound phosphor-bronze (non-magnetic) springs on the spindle. Variations in the magnetic) springs on the spindle. Variations in the pointer for zero setting can be obtained by slackening pointer for zero setting can be obtained by slackening or tightening one of the springs.or tightening one of the springs.

3. 3. Damping device. Damping device. Two types of Two types of damping are used: damping are used: (a) (a) air dashpot air dashpot damping and damping and (b) (b) eddy current damping. eddy current damping.

Air Dashpot Damping (Fig. 10.3a).Air Dashpot Damping (Fig. 10.3a). In this In this method a piston, or vane, is attached to method a piston, or vane, is attached to the spindle. The movement of the the spindle. The movement of the aluminium piston in an air dashpot aluminium piston in an air dashpot minimizes the movement of the pointer minimizes the movement of the pointer when quantities are being measured.when quantities are being measured.

Eddy Current Damping (Fig. 10.3b). When a metal disc is Eddy Current Damping (Fig. 10.3b). When a metal disc is rotated between the poles of a permanent magnet a rotated between the poles of a permanent magnet a current is induced into the disc. The magnetic field due to current is induced into the disc. The magnetic field due to this current is in opposition to the force producing it this current is in opposition to the force producing it (Lenz's Law). This effect is used in electrical measuring (Lenz's Law). This effect is used in electrical measuring instruments to produce a magnetic brake to dampen the instruments to produce a magnetic brake to dampen the oscillations of the pointer. An aluminium disc is fitted to oscillations of the pointer. An aluminium disc is fitted to the pointer, and a small permanent magnet is fixed on the the pointer, and a small permanent magnet is fixed on the coil former.coil former.

Moving-iron Repulsion-type Moving-iron Repulsion-type InstrumentInstrument

Fig. 10.4 shows a complete moving-iron, repulsion-type instrument.

Moving-iron Repulsion-type Moving-iron Repulsion-type Instrument cont’dInstrument cont’d

Operating Device:Operating Device: This type of instrument This type of instrument uses the principle of the repulsion between uses the principle of the repulsion between like poles like poles (i.e., (i.e., like poles repel). If two like poles repel). If two pieces of soft iron are placed in the field of an pieces of soft iron are placed in the field of an electromagnet, repulsion will take place electromagnet, repulsion will take place between them, as both pieces will be of the between them, as both pieces will be of the same polarity. same polarity.

Construction: Construction: In this instrument a piece of In this instrument a piece of soft iron is fixed inside the coil former and soft iron is fixed inside the coil former and another piece is free to move on a steel another piece is free to move on a steel spindle. The repulsion between these two spindle. The repulsion between these two pieces gives the deflection of the pointer. pieces gives the deflection of the pointer.


Controlling Device:Controlling Device: Spring control is used in Spring control is used in the illustration (Fig. 10.4). Gravity control the illustration (Fig. 10.4). Gravity control may also be used. may also be used.

Damping Device:Damping Device: Air dashpot damping is Air dashpot damping is generally used. generally used.

Scale:Scale: The scale is cramped at the beginning The scale is cramped at the beginning because the torque, or twisting power, of because the torque, or twisting power, of the operating device, depends on the the operating device, depends on the square of the operating current (Isquare of the operating current (I22). The ). The scale is useless below 10 per cent of the scale is useless below 10 per cent of the instrument's range. instrument's range.


This is partly overcome by using a nickel-This is partly overcome by using a nickel-iron tongue-shaped section. This scale is iron tongue-shaped section. This scale is called non-linear (unevenly spaced). called non-linear (unevenly spaced).

Operating Coil:Operating Coil: The operating coil has a The operating coil has a few turns of thick, cotton-insul ated wire few turns of thick, cotton-insul ated wire when the instrument is used as an when the instrument is used as an ammeter, and many turns of thin, ammeter, and many turns of thin, varnish-insulated wire when the varnish-insulated wire when the instrument is used for voltage readings.instrument is used for voltage readings.

Moving-iron Repulsion-type Moving-iron Repulsion-type InstrumentInstrument

AdvantagesAdvantages 1. Cheap.1. Cheap. 2. Robust.2. Robust. 3. Can be used on a.c. or d.c. (it is universal). 3. Can be used on a.c. or d.c. (it is universal). DisadvantagesDisadvantages 1. Cramped scale.1. Cramped scale. 2. Accuracy affected by 2. Accuracy affected by (a) (a) temperature temperature

variations and variations and (b) (b) stray magnetic fields.stray magnetic fields. Applications: Applications: The moving-iron repulsion-type The moving-iron repulsion-type

is the most commonly used. Applications is the most commonly used. Applications include industrial panels, particularly where include industrial panels, particularly where vibration is con siderable and motor starters.vibration is con siderable and motor starters.

Moving-coil InstrumentMoving-coil Instrument

Moving Coil InstrumentMoving Coil Instrument

Operating DeviceOperating Device: When a current-: When a current-carrying conductor is placed in a carrying conductor is placed in a magnetic field interaction takes place magnetic field interaction takes place between the field of the magnet and between the field of the magnet and the field due to the current flowing the field due to the current flowing through the conductor. If the through the conductor. If the conductor is suspended in the conductor is suspended in the magnetic field, it will move.magnetic field, it will move.


The moving-coil instrument operates The moving-coil instrument operates through the interaction of two magnetic through the interaction of two magnetic fields: fields:

(a) The permanent magnet field.(a) The permanent magnet field. (b) (b) The field due to the current flowing The field due to the current flowing

through a moving coil (a few turns of thin, through a moving coil (a few turns of thin, varnish-insulated wire).varnish-insulated wire).

Controlling Device:Controlling Device: Control is obtained by Control is obtained by using two counter-wound phosphor bronze using two counter-wound phosphor bronze springs. These springs are also used to carry springs. These springs are also used to carry current to the moving coil.current to the moving coil.


Damping Device:Damping Device: Eddy current damping is Eddy current damping is used. The movement of the aluminium coil used. The movement of the aluminium coil former through the permanent magnet field former through the permanent magnet field produces eddy currents.produces eddy currents.

Scale:Scale: The scale is linear (evenly spaced) The scale is linear (evenly spaced) because the deflection of the moving coil is because the deflection of the moving coil is directly proportional to the current flowing directly proportional to the current flowing through it and the magnetic field, due to the through it and the magnetic field, due to the permanent magnet, is evenly distributed.permanent magnet, is evenly distributed.

NOTE: The moving-coil instrument can only NOTE: The moving-coil instrument can only be used on d.c. as it is 'polarized': it be used on d.c. as it is 'polarized': it contains two separate fields.contains two separate fields.


AdvantagesAdvantages 1. Very accurate.1. Very accurate. 2. Even scale.2. Even scale. 3. Unaffected by stray magnetic fields.3. Unaffected by stray magnetic fields.

DisadvantagesDisadvantages 1. Expensive.1. Expensive. 2. Unsuitable for a.c.2. Unsuitable for a.c. 3. Easily damaged.3. Easily damaged.

Practical Moving coil Practical Moving coil InstrumentInstrument

A more practical moving coil instrument is A more practical moving coil instrument is shown below: shown below:

Parts of a Moving Coil Parts of a Moving Coil InstrumentInstrument

The parts which make up a moving coil The parts which make up a moving coil voltmeter are:voltmeter are:

(i)(i) PointerPointer (ii)(ii) Graduated ScaleGraduated Scale (iii)(iii) Permanent MagnetPermanent Magnet (iv)(iv) Moving CoilMoving Coil (v)(v) Spiral SpringSpiral Spring (vi)(vi) Jewelled BearingsJewelled Bearings (vii) Casing(vii) Casing

Functions of each part of the Functions of each part of the moving coil voltmetermoving coil voltmeter

(i)(i) Pointer:- indicates the magnitude Pointer:- indicates the magnitude of the voltage being measuredof the voltage being measured

(ii)(ii) Graduated Scale: displays the Graduated Scale: displays the range of the meter movement.range of the meter movement.

(iii) Permanent Magnet: provides the (iii) Permanent Magnet: provides the main magnetic field which interacts main magnetic field which interacts with that of the moving coil.with that of the moving coil.

Functions of each part of the Functions of each part of the moving coil voltmeter (Cont’d)moving coil voltmeter (Cont’d)

(iv) Moving Coil: - channels the current (iv) Moving Coil: - channels the current in and out of the meter movement, in and out of the meter movement, thus causing a magnetic field thus causing a magnetic field interaction which results in the interaction which results in the rotation of the coil.rotation of the coil.(v) Spring Coil - controls the rotating (v) Spring Coil - controls the rotating action of the moving coil meter.action of the moving coil meter.

(vi) Jeweled Bearing - reduces the (vi) Jeweled Bearing - reduces the friction encountered by the rotating friction encountered by the rotating parts of the meter movementparts of the meter movement(vii) Case/casing - Protection(vii) Case/casing - Protection

Insulation Resistance Insulation Resistance TesterTester

The insulation resistance tester The insulation resistance tester (e.g., (e.g., the the megger) consists of a hand-driven d.c. megger) consists of a hand-driven d.c. generator which operates a moving-coil generator which operates a moving-coil instrument. This instrument (Fig. 10.7) has a instrument. This instrument (Fig. 10.7) has a magnetic circuit similar to that of the moving-magnetic circuit similar to that of the moving-coil instrument, but two coils are fitted on the coil instrument, but two coils are fitted on the steel spindle:steel spindle:

(a) the voltage or pressure coil, which is (a) the voltage or pressure coil, which is connected across the generator; andconnected across the generator; and

(b) (b) the current or control coil, which is the current or control coil, which is connected in series with the external connected in series with the external resistance. resistance.

Insulation Resistance Insulation Resistance TesterTester

Operation Operation

1. When the external circuit is open 1. When the external circuit is open (i.e., (i.e., no no resistor connected) only the voltage coil is resistor connected) only the voltage coil is energized and the field of this coil lines up energized and the field of this coil lines up with the permanent magnet field, thus giving with the permanent magnet field, thus giving infinity reading.infinity reading.

2. If a resistor is connected in the external 2. If a resistor is connected in the external circuit, a current will flow through the current circuit, a current will flow through the current coil.coil.

3. Interaction takes place between the fields 3. Interaction takes place between the fields of both coils and the finalof both coils and the finalreading will be the resultant of these two reading will be the resultant of these two opposing forces.opposing forces.

OhmmeterOhmmeter

A similar movement is used in the ohmmeter, A similar movement is used in the ohmmeter, but a battery replaces the hand-driven but a battery replaces the hand-driven generator.generator.

Application:Application: The insulation resistance tester The insulation resistance tester measures resistance values from 10,000 Ω to measures resistance values from 10,000 Ω to infinity and is used for the following tests:infinity and is used for the following tests:

(a) (a) Between conductors (polarity test).Between conductors (polarity test). (b) (b) Between conductors and the general mass of Between conductors and the general mass of

earth (the earth insulation resistance tests).earth (the earth insulation resistance tests). The ohmmeter is used to measure smaller The ohmmeter is used to measure smaller

values of resistance; it is also used for circuit values of resistance; it is also used for circuit tracing.tracing.

Ohm MeterOhm Meter

Using the ohm meterUsing the ohm meterZERO ADJUSTMENT POTENTIOMETER KNOB

Using the ohm meterUsing the ohm meterOhmmeters are connected in parallel Ohmmeters are connected in parallel

with the terminals of the device or with the terminals of the device or circuit to be tested. The device or circuit circuit to be tested. The device or circuit must be disconnected from power and must be disconnected from power and from any portion of the circuit that is not from any portion of the circuit that is not to be tested.to be tested.

False readings will result if other False readings will result if other components are connected to the test components are connected to the test circuit. Do not let the fingers of both circuit. Do not let the fingers of both hands touch the tips of the test leads hands touch the tips of the test leads while measuring resistance. while measuring resistance.

Using the ohm meterUsing the ohm meter

If you do, the ohm-meter will measure If you do, the ohm-meter will measure the combined resistance of your body the combined resistance of your body and the circuit being tested. Be very and the circuit being tested. Be very careful of this when measuring high careful of this when measuring high values of resistance, as your body values of resistance, as your body resistance is high and will affect the total resistance is high and will affect the total resistance of another high resistance. A resistance of another high resistance. A false reading can cause you to make a false reading can cause you to make a poor decision when troubleshooting a poor decision when troubleshooting a circuit.circuit.

Past Paper QuestionPast Paper Question

Figure VI indicates three resistance readings Figure VI indicates three resistance readings taken by an ohmmeter. In your answer booklet.taken by an ohmmeter. In your answer booklet.

write the letters (X) , (Y) and (Z) . Next to EACH write the letters (X) , (Y) and (Z) . Next to EACH letter, write the value of reading indicated.letter, write the value of reading indicated.

VoltmeterVoltmeter

A voltmeter is a testing device used to A voltmeter is a testing device used to measure voltage. The meter test leads measure voltage. The meter test leads or wire connections are attached to or wire connections are attached to the tested circuit in parallel with the the tested circuit in parallel with the device that is expected to have a device that is expected to have a voltage drop. voltage drop.

Using the VoltmeterUsing the Voltmeter


Voltmeters are always connected in Voltmeters are always connected in parallel to the device or circuit across parallel to the device or circuit across which the voltage is to be measured. A dc which the voltage is to be measured. A dc voltmeter is a polarized instrument. This voltmeter is a polarized instrument. This means that care must be taken to means that care must be taken to connect the test leads to the correct connect the test leads to the correct polarity, or + to + and - to - . If this is not polarity, or + to + and - to - . If this is not done, a digital meter will indicate a done, a digital meter will indicate a negative voltage, which might confuse negative voltage, which might confuse the data. the data.


Also, the pointer of a moving coil Also, the pointer of a moving coil meter will move in the wrong direction meter will move in the wrong direction and damage the mechanism. and damage the mechanism.

As you have learned, alternating As you have learned, alternating current reverses direction every half current reverses direction every half cycle and, therefore, ac does not have cycle and, therefore, ac does not have polarity. So, an ac volt meter can be polarity. So, an ac volt meter can be connected across two points under connected across two points under test regardless of polarity.test regardless of polarity.

Using AmmetersUsing Ammeters Ammeters are connected in series with the Ammeters are connected in series with the

conductors and the load being tested. They conductors and the load being tested. They can be permanently damaged if connected can be permanently damaged if connected into circuits with too much current. A dc into circuits with too much current. A dc ammeter should be connected into a circuit ammeter should be connected into a circuit with the correct polarity. A digital ammeter with the correct polarity. A digital ammeter which has been connected incorrectly will which has been connected incorrectly will indicate a reverse polarity on the display. If indicate a reverse polarity on the display. If a moving coil meter is connected in reverse, a moving coil meter is connected in reverse, the pointer will move in the wrong direction the pointer will move in the wrong direction and may be damaged. An ac ammeter can and may be damaged. An ac ammeter can be connected into a circuit without regard be connected into a circuit without regard for polarity.for polarity.

Using the AmmeterUsing the Ammeter

Past Paper questionPast Paper question

(a) In your answer (a) In your answer booklet, write the booklet, write the numbers (numbers (1), (2), (3), 1), (2), (3), (4), (5), (6) (4), (5), (6) and and (7) – (7) – Next to EACH Next to EACH number, write the number, write the name of the name of the corresponding part. corresponding part.

(b)(b) State TWO State TWO characteristics of characteristics of moving-coil moving-coil instruments. instruments.

Figure V shows a diagram of a moving-coil instrument.

Varying Range of VoltmetersVarying Range of Voltmeters

The range of a voltmeter may be extended The range of a voltmeter may be extended by connecting a resistor (known as a by connecting a resistor (known as a multiplier) in series with the instrumentmultiplier) in series with the instrument..

Example:Example: A moving-coil instrument gives a full-scale A moving-coil instrument gives a full-scale

deflection (F.S.D.) with a current of 30mA deflection (F.S.D.) with a current of 30mA (0.030 A) and a voltage of 90 mV (0.090 V). (0.030 A) and a voltage of 90 mV (0.090 V). Calculate the value of a resistor to be Calculate the value of a resistor to be connected in series with the instrument so connected in series with the instrument so that it can be used to read 100V.that it can be used to read 100V.

SolutionSolution I (at F.S.D.) = 0.030AI (at F.S.D.) = 0.030AV (at F.S.D.) = 0.090VV (at F.S.D.) = 0.090VVVTT (Voltage across terminals) = 100V (Voltage across terminals) = 100VRm = resistance of multiplierRm = resistance of multiplierVoltage (across resistor) = 100V – Voltage (across resistor) = 100V –

0.090V 0.090V = 99.91V= 99.91VAs R = V/IAs R = V/IRm = 99.91V/0.030ARm = 99.91V/0.030A = 3330.33Ω = 3330.33Ω

Ammeter Ammeter

The range of an ammeter can be extended The range of an ammeter can be extended by connecting a resistor (known as a shunt) by connecting a resistor (known as a shunt) across the instrument so that a small across the instrument so that a small proportion of the current in the circuit proportion of the current in the circuit passes through the instrument. passes through the instrument.

ExampleExample

A moving-coil instrument gives a full-A moving-coil instrument gives a full-scale deflection with a p.d. of 70 mV and scale deflection with a p.d. of 70 mV and a current of 20 mA. Calculate the value of a current of 20 mA. Calculate the value of shunt required to give a range of 0-10 A. shunt required to give a range of 0-10 A.

SolutionSolution

Rsh Rsh = resistance of shunt= resistance of shunt Ish (current through shunt) = 10A – Ish (current through shunt) = 10A –

0.02A 0.02A = 9.98A= 9.98AVsh (voltage across shunt) = 0.070VVsh (voltage across shunt) = 0.070V Rsh = Rsh = 0.070V0.070V 9.98A9.98A Rsh = 0.007014Ω Rsh = 0.007014Ω

ExerciseExerciseA moving-coil instrument having a resistance

of 10Ω, gives a f.s.d. when the current is 8 mA. Calculate the value of the multiplier to be connected in series with the instrument so that it can be used as a voltmeter for measuring p.d.s. up to 100 V.

A moving-coil instrument gives a f.s.d. when the current is 40 mA and its resistance is 25 . Calculate the value of the shunt to be connected in parallel with the meter to enable it to be used as an ammeter for measuring currents up to 50 A.

ExerciseExercise A moving-coil instrument gives f.s.d. for a current of

10 mA. Neglecting the resistance of the instrument, calculate the approximate value of series resistance needed to enable the instrument to measure up to (a) 20 V (b) 100 V (c) 250 V. [(a) 2 k (b) 10 k (c) 25 k]

A meter of resistance 50 has a f.s.d. of 4 mA. Determine the value of shunt resistance required in order that f.s.d. should be (a) 15 mA (b) 20 A (c) 100 A. [(a) 18.18 (b) 10.00 m (c) 2.00 m]

A moving-coil instrument having a resistance of 20 , gives a f.s.d. when the current is 5 mA. Calculate the value of the multiplier to be connected in series with the instrument so that it can be used as a voltmeter for measuring p.d.s up to 200 V. [39.98 k]

ExerciseExercise

A moving-coil instrument has a f.s.d. of 20 mA and a resistance of 25Ω. Calculate the values of resistance required to enable the instrument to be used (a) as a 0–10 A ammeter, and (b) as a 0–100 V voltmeter. State the mode of resistance connection in each case.

[(a) 50.10 m in parallel (b) 4.975 k in series]

WattmeterWattmeter

A wattmeter is an instrument for measuring electrical power in a circuit.

Figure 10.8 shows typical connections of a wattmeter used for measuring power supplied to a load. The instrument has two coils:

(i) a current coil, which is connected in series with the load, like an ammeter, and

(ii) a voltage coil, which is connected in parallel with the load, like a voltmeter.

WattmeterWattmeter

The multimeterThe multimeter

Instruments are manufactured that combine a moving-coil meter with a number of shunts and series multipliers, to provide a range of readings on a single scale graduated to read current and voltage.

If a battery is incorporated then resistance can also be measured.

Such instruments are called multimeters or universal instruments or multi-range instruments.

Past Paper QuestionPast Paper Question

1. (A) You are given a Volt/Amp meter. State how you will1. (A) You are given a Volt/Amp meter. State how you will increaseincrease (i) the voltage range of the instrument (2 marks)(i) the voltage range of the instrument (2 marks) (ii) the current range of the instrument. (2 marks)(ii) the current range of the instrument. (2 marks) (B) (i) What type of meter shows readings on a linear(B) (i) What type of meter shows readings on a linear scale (2 marks)scale (2 marks) (ii) What type of meter shows readings on a non (ii) What type of meter shows readings on a non linear scale (2 marks)linear scale (2 marks)

Definition of SensitivityDefinition of Sensitivity A source of 150V is applied to a series circuit A source of 150V is applied to a series circuit

consisting of two 10kΩ resistors. The voltage consisting of two 10kΩ resistors. The voltage drop across each resistor is 75V. In the 150V drop across each resistor is 75V. In the 150V range, the voltmeter to be used has a total range, the voltmeter to be used has a total internal resistance of 10kΩ. When the internal resistance of 10kΩ. When the voltmeter connected across the circuit. The voltmeter connected across the circuit. The parallel combination of Rparallel combination of R22 and the meter now and the meter now present a total resistance of 5kΩ. Because of present a total resistance of 5kΩ. Because of the addition of the voltmeter, the voltage drops the addition of the voltmeter, the voltage drops change to 100V across Rchange to 100V across R11 and 50 volts across and 50 volts across RR22. Notice that this is not the normal voltage . Notice that this is not the normal voltage drop across Rdrop across R22. Actual circuit conditions have . Actual circuit conditions have been altered because of the voltmeter. been altered because of the voltmeter.

Voltmeter Sensitivity Voltmeter Sensitivity The sensitivity of a voltmeter is given in The sensitivity of a voltmeter is given in

ohms per volt. ohms per volt. This is the same as saying the sensitivity This is the same as saying the sensitivity

is equal to the reciprocal of the full-scale is equal to the reciprocal of the full-scale deflection current. deflection current.

Sensitivity Cont’dSensitivity Cont’d

Therefore, the sensitivity of a 100-Therefore, the sensitivity of a 100-microampere movement is the microampere movement is the reciprocal of 0.0001 ampere, or reciprocal of 0.0001 ampere, or 10,000 ohms per volt. 10,000 ohms per volt.

Factors that affect the Factors that affect the sensitivity of measuring sensitivity of measuring

equipmentequipmentTemperature variationsTemperature variationsStray magnetic fieldsStray magnetic fields Internal resistance of the instrumentInternal resistance of the instrumentDamping device not working properlyDamping device not working properly

Wheatstone BridgeWheatstone Bridge

Figure 10.21 shows a Wheatstone bridge circuit which compares an unknown resistance Rx with others of known values,

Wheatstone bridgeR1 and R2, which have fixed values,

and R3, which is variable. R3 is varied until zero deflection is obtained on the galvanometer G. No current then flows through the meter, VA = VB, and the bridge is said to be ‘balanced’.

ExampleExampleIn a Wheatstone bridge ABCD, a

galvanometer is connected between A and C, and a battery between B and D. A resistor of unknown value is connected between A and B. When the bridge is balanced, the resistance between B and C is 100, that between C and D is 10 and that between D and A is 400 .

Calculate the value of the unknown resistance.

SolutionSolution

At balance, equating the products of opposite ratio arms, gives:

Rx x 10 = 100 x 400and Rx = 40 000 10 Hence the unknown

resistance, Rx = 4000Ω

FormulaFormula

Analogue vs. DigitalAnalogue vs. Digital Paralax error could occur when reading analogue Paralax error could occur when reading analogue

metermeter Analogue meters uses a needle and calibrated scale to Analogue meters uses a needle and calibrated scale to

indicate values.indicate values. Reading analog meters usually requires simple mental Reading analog meters usually requires simple mental

calculations.calculations. Digital meters display those values on a digital display.Digital meters display those values on a digital display. Digital meters are simpler to read and many will adjust Digital meters are simpler to read and many will adjust

to the proper range required for the circuit or device to the proper range required for the circuit or device they are connected to. These meters are known as they are connected to. These meters are known as auto-ranging.auto-ranging.

Most digital voltmeters have 50 times more Most digital voltmeters have 50 times more impedance than analog voltmeters, digital meters are impedance than analog voltmeters, digital meters are more accurate when measuring voltage in high more accurate when measuring voltage in high resistance circuits.resistance circuits.

Analog vs. DigitalAnalog vs. Digital

Digital ammeters are often capable of Digital ammeters are often capable of measuring smaller currents, all the way down measuring smaller currents, all the way down to micro amperes.to micro amperes.

They are easier to use because they give a They are easier to use because they give a specific value eliminating the need to interpret specific value eliminating the need to interpret the analog meter’s needle on its scale.the analog meter’s needle on its scale.

Analog ohm meter should be calibrated Analog ohm meter should be calibrated regularly by connecting the two leads together regularly by connecting the two leads together and zeroing the meter with the adjust knob. and zeroing the meter with the adjust knob. This compensates for the changes in the state This compensates for the changes in the state of charge of the internal battery.of charge of the internal battery.

10. Electrical Measuring Devices and Measurement

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