Electrical Measurements & Measuring Instruments Multiple Choice Questions on “Kelvin Bridge”.

1. Kelvin’s bridge consists of _________

a) double bridge

b) single bridge

c) half bridge

d) three fourth bridge

Answer: a

Clarification: Kelvin’s bridge consists of two additional arms compared to a Wheatstone bridge. Hence it is also known as a double bridge.

2. The range of resistance measured in a Kelvin bridge is _________

a) 10Ω to 10 mΩ

b) 1Ω to 10 μΩ

c) 0.01Ω to 10 MΩ

d) 0.1Ω to 10 nΩ

Answer: b

Clarification: Kelvin bridge is used for the measurement of low resistances. Low resistances vary in the range of 1Ω to 10 μΩ.

3. Accuracy of Kelvin bridge is of the order of _________

a) ±0.5 to ±2 %

b) ±0.05 to ±0.02 %

c) ±0.05 to ±0.2 %

d) ±0.005 to ±0.02 %

Answer: c

Clarification: As Kelvin bridge is used for the measurement of low resistance values, the accuracy of measurement of low resistances in a Kelvin bridge is of the order of ±0.05 to ±0.2 %.

4. What is the balance equation of Kelvin bridge?

a) R_{x} = (frac{R_2 R_3}{R_1})

b) R_{x} = (frac{R_1 R_2}{R_3})

c) R_{x} = (frac{R_1}{R_2})

d) R_{x} = (frac{R_1 R_3}{R_2})

Answer: d

Clarification: The balance equation in a Kelvin bridge is given by the relation R_{x} = (frac{R_1 R_3}{R_2})

where,

R_{1}, R_{2}, R_{3}, and R_{x} form the ratio arms

Rx is the value of the unknown resistance.

5. What is the effect of load and contact resistance in Kelvin bridge?

a) independent

b) fully dependent

c) partially dependent

d) depends on the resistance value

Answer: a

Clarification: Effect of contact and lead resistances are completely eliminated in a Kelvin bridge as they don’t appear in the balance equation. Hence the Kelvin bridge is independent of the load and contact resistances.

6. The relation between ratio of resistance arms and ratio of resistance arms of second bridge is _________

a) unequal

b) equal

c) twice

d) one forth

Answer: b

Clarification: The relation between the ratio of the resistances of resistance arm and second resistance arm is equal for balance condition.

7. Why Kelvin bridge is used for measurement of low resistance?

a) due to e.m.f source used

b) due to a large current flow

c) due to contact and lead resistance

d) due to power dissipation across the circuit

Answer: c

Clarification: While measuring very low resistances the contact and lead resistances cause significant errors in the value of the measured resistance. As a result Kelvin bridge is used for measurement of low resistances.

8. What is the condition to achieve a high sensitivity in a Kelvin bridge?

a) low voltage

b) high power

c) medium resistance

d) high current

Answer: d

Clarification: The condition to achieve a high sensitivity in a Kelvin bridge is that the measuring current should be high enough so as to sensitize the null detector.

9. Kelvin bridge can be calibrated to read _________

a) inductance and Quality factor value

b) capacitance only

c) power and voltage

d) current and frequency

Answer: a

Clarification: In its basic form a Kelvin bridge is used for the measurement of low resistance. A Kelvin bridge can be used for the measurement of inductance and Quality factor value through calibration.

10. Why can’t a Kelvin bridge be used for the measurement of low Quality factor value?

a) due to thermoelectric effect

b) due to balance problem

c) due to the dull detector used

d) due to temperature

Answer: b

Clarification: A Kelvin bridge can be used for the measurement of high Quality factor values. Due to bridge imbalance problem, Kelvin bridge can’t be used for measuring low Quality factor values.