Category: Electrical Measurements Objective Questions

Electrical Measurements Assessment Questions and Answers on “Measurement Errors in a Wheatstone Bridge”.

1. The Wheatstone bridge is used for the measurement of _________
a) 1ῼ to a few megaohms
b) 1kῼ to a few megaohms
c) 1Mῼ to a few gigaohms
d) 1mῼ to a few ohms
Answer: a
Clarification: A Wheatstone bridge is used for the measurement of medium resistance in the range of 1ῼ to a few megaohms. Kelvin bridge is used for the measurement of low resistance.

2. The source of error in a Wheatstone bridge is due to _________
a) range of galvanometer used
b) type of the source of emf used
c) limiting errors of the three known resistances
d) balance condition
Answer: c
Clarification: The main source of error in a Wheatstone bridge is because of the limiting errors of the three known resistances. As a result very precise resistances are required with 1 % tolerance.

3. What is the main source of error in a null detector?
a) lower current detection ability
b) insufficient sensitivity of the null detector
c) wiring resistance
d) power dissipation
Answer: b
Clarification: Resistance of contact wires and power dissipation are some of the causes for error in a null detector. But insufficient sensitivity is the major source of error in a null detector such as a galvanometer.

4. When current passes through the resistances in the Wheatstone bridge what happens?
a) voltage drop occurs
b) null detector burns
c) back emf is induced
d) temperature increases due to power dissipation
Answer: d
Clarification: Current flow through the resistances causes power dissipation i.e. I²R leading to an increase in the temperature. As the bridge consists of pure resistances, there is no back e.m.f induced.

5. What is the effect of the increase in temperature on the resistance elements?
a) directly proportional
b) independent
c) inversely proportional
d) changes by a factor of 10
Answer: a
Clarification: Temperature affects resistance of the circuit. Resistance has a positive temperature coefficient and increases linearly with rise in temperature.

6. Heating effect in the Wheatstone bridge can be avoided by _________
a) using an expensive emf source
b) limiting current to a safe value
c) using heating sinks
d) immersing the setup in ice
Answer: b
Clarification: Heating effect can be eliminated by limiting the current through the circuit to a safe value and by calculating the power dissipation in the arms. Heat sinks are typically used in power electronic applications.

7. The main error when measuring low resistances is _________
a) power dissipation of the arms
b) voltage drop across the emf source
c) lead and contact resistance
d) current flowing through the circuit
Answer: c
Clarification: Voltage drop across the circuit along with power dissipation lead to errors in the measurement of low resistances. The resistance of leads and contacts of the external circuit adds to the values in the circuit. This is the main source of error while measuring low resistances.

8. How is thermal emf generated?
a) when current flows
b) when voltage is applied
c) when the null detector is connected
d) when dissimilar metals come in contact
Answer: d
Clarification: When dissimilar metals come into contact with each other in a galvanometer circuit, thermal emf is generated. Null detector is used for detecting the balance condition.

9. Thermal emf can be prevented by _________
a) using more sensitive galvanometers
b) using similar metals
c) by separating the dissimilar metals
d) reducing the supply emf
Answer: a
Clarification: Thermal emf can be eliminated by making use of sensitive galvanometers consisting of copper coils and copper suspension systems. Dissimilar metals can’t be physically separated.

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Electrical Measurements Interview Questions and Answers on “Anderson Bridge, Advantages & Disadvantages of Anderson Bridge”.

1. Anderson bridge is used for _________
a) the measurement of self-inductance
b) the measurement of resistance
c) the measurement of capacitance
d) the measurement of impedance
Answer: a
Clarification: The Anderson bridge is one of the important bridges used for the measurement of self-inductances in terms of standard capacitance value. Resistance is usually measured by making use of a Wheatstone’s bridge or Kelvin’s double bridge.

2. Anderson bridge is a modified form of ________
a) Wheatstone’s bridge
b) Maxwell bridge
c) Kelvin double bridge
d) Schering bridge
Answer: b
Clarification: The Anderson’s bridge is a modified form of Maxwell’s bridge which is used for the measurement of self-inductances in terms of standard capacitance value. Wheatstone bridge and Kelvin bridge are used for the measurement of medium and low resistances respectively.

3. Anderson’s bridge is basically used for ________
a) measurement of capacitance
b) measurement of resistance
c) measurement of inductance
d) measurement of voltage
Answer: c
Clarification: Anderson’s bridge is used basically for the precise measurement of self-inductances in terms of a standard capacitance value over a wide range of values.

4. Balance equation for computing the inductance is ________
a) L_x = R₃ R₅
b) L_x = CR₅
c) L_x = CR₃
d) L_x = CR₃ R₅
Answer: d
Clarification: The balance equation for computing the self-inductance in an Anderson’s bridge is given by the equation, L_x = CR₃ R₅.
where, C is the standard capacitance
R₃ and R₅ are the known non-inductive resistances.

5. Which of the following is a balance equation for computing the resistance?
a) R₁ = ^{R₂ R₃}⁄_R4
b) R₁ = ^R₂ ⁄ _R4
c) R₁ = ^R₃ ⁄ _R4
d) R₁ = R₂ R₃
Answer: a
Clarification: The balance equation for computing the resistance in an Anderson’s bridge is given by the equation R₁ = ^{R₂ R₃}⁄_R4.
where, R₂, R₃ and R₄ are the known non-inductive resistances.

6. When the capacitor used is imperfect, the inductance value changes.
a) True
b) False
Answer: b
Clarification: For an imperfect capacitor used in the Anderson bridge, the value of inductance remains unaffected. The value of R₁ changes.

7. Anderson’s bridge is used for the measurement of capacitance.
a) True
b) False
Answer: a
Clarification: When a calibrated self-inductance is available, the value of unknown capacitance can be computed by making use of Anderson’s bridge.

8. Anderson’s bridge is used for the measurement of ________
a) capacitance
b) resistance
c) inductance
d) impedance
Answer: a
Clarification: Anderson’s bridge is used for the measurement of capacitance. Unknown capacitance value can be measured accurately in terms of the self-inductance of one of the ratio arms of the bridge.

9. Anderson’s bridge makes use of a variable capacitance.
a) True
b) False
Answer: b
Clarification: An Anderson’s bridge makes use of a fixed capacitance value. Most of the other AC bridges used for the measurement of either capacitance, inductance or impedance make use of a variable capacitance.

10. Anderson’s bridge is very basic.
a) True
b) False
Answer: b
Clarification: An Anderson’s bridge is basically used for the measurement of unknown capacitance value in terms of the self-inductance of one of the standard ratio arms. It is a complex bridge comprising of equivalent star-delta networks for computation of resistance and inductance.

11. Bridge balance equations are ________
a) easy to derive
b) independent of the components
c) complex in nature
d) real in nature
Answer: c
Clarification: An Anderson’s bridge is basically used for the measurement of unknown capacitance value in terms of the self-inductance of one of the ratio arms. It consists of real as well as complex terms. It also comprises of star-delta equivalent networks for impedance computations.

12. Components in an Anderson’s bridge are ________
a) less
b) zero
c) intermediate
d) more
Answer: d
Clarification: An Anderson’s bridge is a complex type of bridge used basically for the measurement of unknown capacitance. It consists of several resistances, inductances and capacitances in the ratio arms.

13. Anderson’s bridge ________
a) can’t be shielded
b) can be fully shielded
c) can be partially shielded
d) can be shielded based on the components used
Answer: a
Clarification: An Anderson’s bridge is a complex circuit and a modified form of the Maxwell bridge. An Anderson’s bridge cannot be shielded due to the presence of an additional junction point.

14. What is the significance of Anderson bridge with respect to Q factor?
a) difficult to balance the bridge
b) easy to balance the bridge
c) intermediate balance can be achieved
d) no balance can be done
Answer: b
Clarification: An Anderson’s bridge is a modified form of the Maxwell bridge and is used for the measurement of unknown capacitance in terms of self-inductance of standard value. It is easy to achieve balance condition in an Anderson bridge.

15. An Anderson’s bridge can be used to ________
a) measure mutual inductance
b) measure impedance
c) measure self inductance
d) measure stray capacitance
Answer: c
Clarification: An Anderson bridge can be used for the measurement of the self-inductance of a coil. A coil with unknown capacitance can be used to determine its self-inductance by making use of an Anderson bridge.

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Electrical Measurements & Measuring Instruments Multiple Choice Questions on “Characteristics of Potential Transformers”.

1. Increasing secondary burden _____________
a) increases I_s
b) decreases I_s
c) keeps I_s constant
d) decreases I_p
Answer: a
Clarification: When the secondary burden of a potential transformer increases, it leads to an increase in the secondary current. As a result the primary current also increases.

2. For a given V_p the V_s __________
a) increases
b) decreases
c) remains constant
d) depends on the supply
Answer: b
Clarification: For a particular value of primary voltage V_p, the value of the secondary voltage V_s decreases. This leads to a decrease in the actual ratio.

3. What is effect of the secondary burden on the ratio error?
a) ratio error is constant with secondary burden
b) ratio error decreases with secondary burden
c) ratio error increases with secondary burden
d) depends on the supply voltage
Answer: c
Clarification: As the secondary burden is increased, the ratio error also increases and becomes more negative. Ratio error varies linearly with respect to the change in the secondary burden.

4. V_p is leading in phase, while V_s is lagging in phase.
a) True
b) False
Answer: a
Clarification: As the secondary burden is increased, winding voltage drop increases. Voltage V_p is leading in phase while V_s is lagging in phase. As a result the phase angle increases with the secondary burden.

5. What is the effect of p.f. on secondary burden?
a) I_p shifts towards I_o
b) I_p deviates from I_o
c) I_p is independent of I_o
d) I_p and I_o cancel eachother
Answer: a
Clarification: When the power factor of the secondary burden is reduced, I_p shifts towards I_o. Voltages V_p and V_s come closer to E_p and E_s.

6. How is the transformation ratio dependent on the secondary burden?
a) decreases
b) increases
c) remains constant
d) depends on the supply
Answer: b
Clarification: As the primary voltage V_p is constant, the induced primary e.m.f E_p is reduced. Similarly the secondary voltage V_s is reduced with respect to E_s. As a result the transformation ratio increases.

7. Decrease in p.f. makes V_s to lead V_p.
a) True
b) False
Answer: a
Clarification: As the power factor is decreased, Vs leads in phase while V_p decreases in phase. Thus the phase angle reduces with the decrease in power factor (lagging).

8. At constant voltage how is frequency affected by flux?
a) decreases with an increase in flux
b) increases with a decrease in flux
c) remains constant
d) depends on the current
Answer: b
Clarification: When frequency is increased at a constant voltage, the flux decreases. Voltage ratio decreases as the magnetizing and iron loss components of current are reduced.

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Electrical Measurements Question Bank on “Consideration in Selecting Voltmeter”.

1. Input impedance of a voltmeter is ___________
a) high
b) low
c) medium
d) zero
Answer: a
Clarification: In order to prevent the loading effect in a voltmeter, the input impedance is maintained as high as possible. It must be maintained higher than the impedance of the circuit being used.

2. Input impedance depends on _________
a) resistance
b) capacitance
c) inductance
d) voltage
Answer: b
Clarification: Shunt capacitance appearing across the input terminals affects the input impedance. Capacitive reactance of the capacitance is inversely proportional to the frequency.
X_c ∝ ¹⁄_f
where, X_c is the capacitive reactance
f is the frequency.

3. At high frequencies the capacitive reactance _________
a) is constant
b) increases
c) decreases
d) becomes zero
Answer: c
Clarification: We know that the capacitive reactance is inversely proportional to the frequency.
X_c ∝ ¹⁄_f
At high frequencies, the capacitive reactance becomes very small.

4. Voltage ranges in a meter are _________
a) 5-10-15
b) 20-50-100
c) 0.5-1-1.5
d) 1-3-10
Answer: d
Clarification: Meter scale will have a calibration of 1-3-10 for the voltage range with a separation of 10 dB. It can also be in the sequence of 1.5-5-15. We can also make use of a single scale.

5. Linear meter with 1% accuracy must have _________
a) 100 divisions
b) 50 divisions
c) 200 divisions
d) 500 divisions
Answer: a
Clarification: In order to recognize a reading with 1% accuracy it must have 100 divisions on the 1 V range.

6. Decibel covers a small range of values.
a) True
b) False
Answer: b
Clarification: As the unit decibel makes of the logarithmic scale, it covers a wide range of values of any given quantity.

7. Noise is a function of _________
a) voltage
b) current
c) bandwidth
d) frequency
Answer: c
Clarification: Noise in general depends on the value of the bandwidth. Voltmeter having a large bandwidth takes up more noise. Usually, the sensitivity of a voltmeter is 1mV for a bandwidth of 10Hz – 10MHz.

8. Current can be measured effectively by making use of _________
a) d.c. voltmeter
b) a.c. ammeter
c) d.c. ammeter
d) a.c. voltmeter
Answer: d
Clarification: By making use of an a.c. voltmeter in series with resistance we can obtain the current value. We can also make use of the current probe method to obtain the value of the current without effectively disturbing the circuit.

9. Average responding voltmeter is cheap and accurate.
a) True
b) False
Answer: a
Clarification: Average responding voltmeter is used for a.c. measurements for a sine wave with 10% distortion or less. Hence the average responding voltmeter is cheap and accurate.

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Electrical Measurements & Measuring Instruments Multiple Choice Questions on “Applications of Wheatstone Bridge”.

1. Wheatstone bridge is a __________
a) a.c. bridge
b) d.c. bridge
c) high voltage bridge
d) power dissipation bridge
Answer: b
Clarification: The Wheatstone bridge is a d.c. bridge that is used for the measurement of medium resistance. Schering bridge is used for measurement of high voltages. AC bridges comprise of Anderson bridge, Maxwell Bridge, etc.

2. Wheatstone bridge is used to measure resistance in the range of ________
a) 1Ω to a few megaohms
b) 10kΩ to a few megaohms
c) 100MΩ to a few gegaohms
d) 100Ω to a few teraohms
Answer: a
Clarification: Wheatstone bridge is the simplest form of bridge circuit. It is basically used for the measurement of medium resistances in the range of 1Ω to a few megaohms.

3. Wheatstone bridge is used to measure the d.c. resistance of various types of wires for _________
a) determining their effective resistance
b) computing the power dissipation
c) quality control of wire
d) maintaining a source of constant e.m.f
Answer: c
Clarification: Wheatstone bridge is used to measure the d.c. resistance of various types of wires for controlling the quality of the wires. Voltage source maintains a constant e.m.f in the bridge circuit.

4. Telephone companies make use of the Wheatstone bridge for _________
a) measuring the telephone resistance
b) computing the line strength
c) maintaining dialtone
d) locating the cable faults
Answer: d
Clarification: Cable faults in telephones can be located by telephone companies by making use of a Wheatstone bridge. Telephonic resistances are determined using suitable techniques. Dialtone is maintained through optical fibre technology.

5. The types of faults in a telephone line are ________
a) line to line or line to ground
b) triple line to line or line to ground
c) open circuit and short circuit
d) symmetrical and unsymmetrical
Answer: a
Clarification: In a telephone line, line to line and line to ground faults occur. Symmetrical, unsymmetrical, open circuit, short circuit, triple line to line and line to ground faults occur in power systems.

6. How can a Wheatstone bridge be used for the measurement of physical parameters?
a) in conjunction with a rectifier
b) along with an op amp
c) by connecting it to a thermistor
d) by making use of a transducer
Answer: b
Clarification: A Wheatstone bridge can be used for the measurement of physical parameters such as temperature, strain, light, etc by making use of an operational amplifier. Rectifier circuits are used for the conversion of ac to dc.

7. By using the variations on a Wheatstone bridge we can _________
a) measure quantities such as voltage, current and power
b) measure high resistance values
c) measure quantities such as complex power
d) measure quantities such as capacitance, inductance and impedance
Answer: d
Clarification: In its simplest form a Whetstone bridge consists of resistive arms. A Wheatstone bridge is used for the measurement of quantities such as capacitance, inductance and impedance by making use of the variations.

8. One of the simplest applications of a Wheatstone bridge is ________
a) voltage measurement
b) current measurement
c) light measurement
d) power measurement
Answer: c
Clarification: Wheatstone bridge consists of simple resistances in the ratio arms. One of the simplest applications of a Wheatstone bridge is the measurement of light by making use of a photo resistive device.

9. Thermal compensation can be provided in a Wheatstone bridge by ________
a) using more than one resistive sensor
b) making use of a heat sink
c) using cooling fans
d) immersing the circuit into a liquid
Answer: a
Clarification: By making use of more than one resistive sensor within the four arms of a Wheatstone bridge we get a full bridge, half bridge or a quarter bridge setup with an automatic balancing effect.

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