# 1. Kelvin Bridge circuit is used for the measurement of

1. Low resistance
2. High resistance
3. Medium Resistance
4. Capacitance

Explanation:-

Kelvin bridge is a modification of the Wheatstone bridge. It consists of a double set of ratio arms. There are many difficulties that arise in the Wheatstone bridge when measuring resistance that is so small that the resistance of the leads and contacts becomes appreciable. This bridge provides accurate results for measuring low-value resistances as the effect of the connecting leads is eliminated. They are used for the measurement of low values of resistance from some micro-ohms up to 10 ohms.

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

1. 10Ω to 10 mΩ
2. 1Ω to 10 μΩ
3. 0.01Ω to 10 MΩ
4. 0.1Ω to 10 MΩ

Explanation:-

The Kelvin bridge is used for measuring low resistance values. In a typical Kelvin’s double bridge the range of a resistance covered 1Ω to 10µΩ with an accuracy of+0.05% to +0.2%.

# 3. Kelvin’s bridge consists of ________

1. Double bridge
2. Single bridge
3. Half-bridge
4. Three fourth bridge

Explanation:-

When the resistance to be measured is of the order of magnitude of bridge contact and lead resistance, a modified form of Wheatstone’s bridge, the Kelvin bridge is employed. Kelvin’s Bridge is also known as a Double bridge because it incorporates a second set of ratio arms as compared to the Wheatstone bridge.

The second set of ratio arms is the resistances ‘a’ and “b with the help of these resistances, the galvanometer is connected to point ‘c’. The galvanometer gives a null indication when the potential of the terminal ‘c’ is the same as the potential of the terminal E.

This additional ratio arms eliminate the effect of lead and contact resistances. The important condition for this bridge balance condition is that the ratio of the resistances of ratio arms must be the same as the ratio of the resistances of the second ratio arms.

# 4. Which of the following is the balanced equation of kelvin Bridge?

1. Rx = R2.R3 ⁄ R1
2. Rx = R2.R1 ⁄ R3
3. Rx = R1.R3 ⁄ R2
4. Rx = R1.R3 ⁄ R2R4

Answer.3. Rx = R1.R3 ⁄ R2

Explanation:-

Kelvin’s bridge is a modification of Wheatstone’s bridge and is used to measure values of resistance below 1Ω. In low resistance measurement, the resistance of the leads connecting the unknown resistance to the terminal of the bridge circuit may affect the measurement. In a typical Kelvin’s bridge, the range of resistance covered is 1Ω to 10 μΩ.

The Wheatstone’s bridge is not suitable for comparing two very low resistance such as metal rods because the junction resistances and resistances of connecting wires are larger compared to the low resistance to be measured. Consider the circuit in Fig., where Ry represents the resistance of the connecting leads from R3 to RX (unknown resistance). The galvanometer can b connected either to point c or to point a. When it is connected to point “a” the resistance Ry of the connecting lead is added to the unknown resistance Rx, resulting in too high indication for Rx

When the connection is made to point “c” Ry is added to the bridge arm R3 and the resulting measurement of RX is lower than the actual value because now the actual value of R3 is higher than its nominal value by the resistance Ry. If the galvanometer is connected to point “b” in-between points “c” and “a”, in such a way that the ratio of the resistance from “c” to “b” and the from “a” to “b” equals the ratio of resistances R1 and R2, then

Rcb/Rab = R1/R2

The actual equation of Kelvin’s bridge is

Rx = R1.R3 ⁄ R2

# 5. Kelvin bridge is used for the measurement of low resistance due to

1. E.m.f source used
2. Large current flow
4. Power dissipation across the circuit

Explanation:-

One of the major drawbacks of the Wheatstone bridge is that it can measure the resistance from a few ohms to several megaohms but low resistance measurements give the significant error. There are many difficulties that arise in the Wheatstone bridge when measuring resistance that is so small that the resistance of the leads and contacts becomes appreciable. So, we need some modification in the Wheatstone bridge itself, and the modified bridge so obtained is the Kelvin bridge, which is not only suitable for measuring the low value of resistance but has a wide range of applications in the industrial world. The balanced equation of Kelvin’s bridge is

Rx = R1.R3 ⁄ R2

Hence, the value of unknown resistance is independent of the contact resistance even though it is present in the circuit. This is possible only if the two sets of ratio arms have equal values.

This additional ratio arms eliminate the effect of lead and contact resistances. The important condition for this bridge balance condition is that the ratio of the resistances of ratio arms must be the same as the ratio of the resistances of the second ratio arms.

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

1. Low voltage
2. High power
3. Medium resistance
4. High current

Explanation:-

The accuracy of measurements made using this bridge are dependent on a number of factors. The accuracy of the standard resistor is of prime importance. Measurement accuracy is also increased by setting the current flowing through standard resistance and Unknown resistance to be as large as the rating of those resistors allows. This gives the greatest potential difference between the innermost potential connections (R2 and b) to those resistors and consequently sufficient voltage for the change in a and b to have its greatest effect.

# 7. The relation between the ratio of resistance arms and ratio of resistance arms of the second bridge is ____

1. Unequal
2. Equal
3. Twice
4. One fourth

Explanation:-

Kelvin’s Bridge is also known as a Double bridge because it incorporates a second set of ratio arms as compared to the Wheatstone bridge. The second set of ratio arms is the resistances ‘a’ and “b with the help of these resistances, the galvanometer is connected to point ‘c’. The galvanometer gives a null indication when the potential of the terminal ‘c’ is the same as the potential of the terminal E.

The balanced equation of Kelvin’s bridge is

Rx = R1.R3 ⁄ R2

It indicates that the resistance of the connecting lead Ry, has no effect on the measurement, provided that the ratios of the resistances of the two sets of ratio arms are equal.

# 8. The range of accuracy of Kelvin bridge is of the order of ______

1. ±0.5 to ±2 %
2. ±0.05 to ±0.02 %
3. ±0.05 to ±0.2 %
4. ±0.005 to ±0.02 %

Explanation:-

The Kelvin bridge is used for measuring low resistance values. In a typical Kelvin’s double bridge the range of a resistance covered 1Ω to 10µΩ with an accuracy of+0.05% to +0.2%.

In a typical Kelvin bridge, the range of resistance covered is 1Ω to 10µΩ. The accuracies of the kelvin bridge are as under.

From 1000µΩ to 1Ω = 0.05%

From 100µΩ to 1000µΩ = 0.2% to 0.05%

From 10µΩ to 100µΩ = 0.5% to 0.2% (limited by thermoelectric emfs.)

# 9. Which of the following is/are the advantages of the kelvin bridge?

1. Eliminate the effect of lead and contact resistance
2. Can measure low Value of resistance
3. Low power consumption
4. All of the above

The advantages of the Kelvin double bridge are that the effects of contact a lead resistances are eliminated and that variations in the current through the unknown have no effect on the balance of the bridge. Also, the resistance of the unknown may be measured at its rated or working current hence the power consumption is less.

The Kelvin bridge is used for measuring low resistance values. In a typical Kelvin’s double bridge the range of a resistance covered 1Ω to 10µΩ with an accuracy of+0.05% to +0.2%.

# 10. The disadvantage of the Kelvin bridge is being

1. Overdamping of galvanometer
2. Thermoelectric E.M.F
3. Both 1 and 2
4. None of the above

Explanation:-

Overdamping of the galvanometer. In the Kelvin bridge, the galvanometer may become seriously overdamped as it may have only a few ohms in its external circuit. However, the currents in the bridge are fairly large under these conditions so that plenty of sensitivity is available and the critical damping resistance of the galvanometer may be reduced by reducing the strength of the magnetic field by means of an adjustable magnetic shunt across the poles of the galvanometer magnet.

Thermoelectric E.M.F:- Thermoelectric e.m.f.s occur at junctions of dissimilar metals. To reduce the effect of thermoelectric e.m.f.s it is necessary to reverse the direction of the current by reversing the battery connection and taking another measurement and average the values obtained for the unknown.

In using a Kelvin bridge, one must follow precautions similar to those given for the Wheatstone bridge. A rheostat is usually placed in series with the battery so that the bridge current can be conveniently limited to the maximum current allowable. This value of current, which affects the sensitivity of the bridge, is determined by the largest amount of heat that can be sustained by the bridge resistances without causing a change in their values. All connections must be firm and electrically perfect so that contact resistances are held to a minimum. The use of point and knife-edge clamps is recommended.

# 11. The application of Wheatstone bridge is/are

1. Used in Electrical conductor
2. Used in Generator
3. Used to measure transformer winding resistance
4. All of the above

Explanation:-

Kelvin’s double bridge principle is also used to eliminate the effects of resistances of leads and contact resistances in strain gauge bridge circuits. The temperature rise of the field windings of large generators can be calculated by measuring the small variation in their resistance with the help of Kelvin’s double bridge.

Kelvin’s double bridge is also used for the determination of conductivity of samples of electrical conductors to ascertain whether the composition and processing are as per specifications. From this, for a given length and cross-sectional area of the conductor, electrical resistivity is determined.

Kelvin bridge ohmmeter is suitable for the measurement of resistance of the series field windings and the commutating field windings of large motors, generators, resistances of transformer windings and large metering shunts mounted on a switchboard or in a bus bar structure, which cannot be moved to the laboratory.

# The resistance of connecting leads from standard to unknown resistance is 680 µΩ. The value of unknown resistance will be

1. 50Ω
2. 30Ω
3. 25Ω
4. 10Ω

Explanation:-

Given R3 (Standard arm) = 100.03 µΩ = 100.03 × 10−6Ω

l (additional arm) = 100.31 µΩ  = 100.31 × 10−6Ω

m (additional arm) = 200 µΩ = 200 × 10−6Ω

R1 (Outer arms) = 100.24µΩ = 100.24 × 10−6Ω

R2 (Outer arms) = 200 µΩ = 200 × 10−6Ω

Ry = 680 µΩ = 680 × 10−6Ω

We know that the value of unknown resistance is given by # 13.  ln the given Fig. the ratio of Ra, to Rb, is 1000Ω, R1 is 5Ω and R1 = 0.5 R2. What is the value of Rx.

1. 10Ω
2. 0.01Ω
3. 1.2Ω

Explanation:-

Under balanced conditions Resistance Rx can be calculated as

Rx/R2 = Rb/Ra

Therefore

Rx/R2 = Rb/Ra = 1/1000

Since

R1 = 0.5 R2

R2 = 5/0.5 = 10Ω

∴ Rx/10 = 1/1000

Rx = 0.01Ω

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