# 1. Which of the following is/are limitations of the Wheatstone bridge?

1. Can’t be used for high resistance measurement
2. Effect of lead resistance and contact resistance
3. High cost
4. All of the above

Limitations of Wheatstone bridge

Explanation:-

1. In case of measurement of low resistance the resistance of the leads and contacts becomes significant, resulting in the introduction of an error. This error can be eliminated by using Kelvin’s double bridge.

2. While measuring high resistances the resistance of the bridge becomes so large that the galvanometer becomes insensitive to imbalance. Consequently, a power supply has to replace the battery and a D.C. VTVM replaces the galvanometer. The Wheatstone bridge cannot be used for measuring high resistances in megaohms.

3. The change in resistance of the bridge arms due to the heating effect of current through the resistance-the temperature rise causes a change in the value of the resistance, and excessive current may cause a permanent change in value.

4. The cost is high because the resistance is used must be very precise having tolerance upto 1% or 0.1%.

# 2. Cost of maintenance of Wheatstone bridge is _______

1. Low
2. High
3. Depends on the components
4. Depends on the source e.m.f

Explanation:-

In a Wheatstone bridge, the resistance is used must be very precise having tolerance upto 1% or 0.1% hence its has a high initial cost as well as the maintenance cost. Voltage source does not add to the cost of maintenance of a Wheatstone bridge as it is uniform for all bridge circuits.

# 3. A Wheatstone bridge may not give accurate readings if ________

1. It is not balanced
2. It is balanced
3. The voltage drop across the circuit is maximum
4. Any of the above

Explanation:-

The Wheatstone bridge gives inaccurate readings if it is unbalanced. The upper range of the bridge can be increased with the help of the applied emf, and the lower range is limited by connecting the lead at the binding post. The bridge is balanced at the time of measurement by adjusting the resistances of the other arms. The output voltage obtained from the “unbalanced” Wheatstone bridge is a function of the amount of unbalance.

A Wheatstone bridge is said to be in a balanced condition when no current flows through the galvanometer. This condition can be achieved by adjusting the known resistance and variable resistance.

# 4. If the amount of voltage increase in the Wheatstone bridge then

1. It gives a more accurate reading
2. No effect is observed
3. Decrease bridge accuracy
4. Any of the above

Explanation:-

The Wheatstone bridge is usually portable because they require only a small, dc source to power the bridge, which is easily obtained from flashlight batteries. In those cases where an external supply voltage is desirable for the operation of the bridge, use the minimum voltage that will give a reliable indication by the galvanometer.

Increasing the supply voltage any further results in uncompensated thermal variations and decreased bridge accuracy. If greater bridge sensitivity is needed, use a galvanometer with greater sensitivity.

# 5. The damping used in galvanometer for Wheatstone bridge to achieve high accuracy is

1. Critically damping
2. Eddy current damping
3. Oil damping
4. No damping is required

Explanation:-

The galvanometer should not be subjected to false or erratic indications because of external magnetic fields. This requirement dictates the choice of a shielded meter mechanism.

It is also desirable to use a critically dampened meter movement to ensure decisive movement of the meter pointer during conditions of bridge unbalance. Thermal agitation sometimes produces voltages that interfere with the balancing of the bridge.

# 6. Wheatstone bridge usually includes a polarity-reversing switch in the detector circuit to reduce the effect of

1. Current agitation
2. Heating
3. Thermal Agitation
4. Resistance inaccuracy

Explanation:-

Thermal agitation sometimes produces voltages that interfere with the balancing of the bridge. For this reason, the Wheatstone bridge usually includes a polarity-reversing switch in the detector circuit.

When a measurement is required, note the reading for both positive and negative indications, and figure the average of both readings. With the exception of inaccuracies introduced by thermal variations (caused by excessive supply voltages), the accuracy of the Wheatstone bridge 1s, otherwise, independent of the value of supply voltages.

The units used in calibrating the galvanometer are unimportant to the accuracy of the bridge since a 0 indication is desired at the balanced condition.

# 7. Which of the following method is used to minimize the drift in the value of resistance measured using a Wheatstone bridge?

1. By using a high precision null detector
2. By keeping the lead wire resistances within 0.2 mΩ
3. By using an e.m.f source of minimum value
4. By minimizing the flow of current

Explanation:-

Resistance drift is a spontaneous increase in electrical resistance. One of the most elementary precautions concerning the use of a bridge, when measuring low resistance, is to tighten the binding posts securely so that the contact resistance between the binding posts and the resistance to be measured is minimum. Leakage paths between the resistor lead along the outside surface of the resistor body must be avoided when resistances greater than 0.1 megohms are measured.

Search for defective solder joints or broken strands in stranded wire leads; these defects can cause erratic galvanometer indications. In those cases where wire leads must be used to reach from the resistance under test to the bridge terminals, measure the ohmic value of those leads prior to further measurements.

The drift in the measured value of resistance in a Wheatstone bridge can be minimized by maintaining the lead wire resistances within 0.2 mΩ. As a result, the effect of lead wire resistance gets removed in the output value of the bridge circuit.

# 8. What is the effect of lead wires on the Wheatstone bridge?

1. No effect
2. Stabilizes the system
3. Destabilize the system
4. Increases the resistance of the circuit

Explanation:-

In order to measure the strain, the strain gauge needs to be connected to the Wheatstone bridge circuit. The strain gauge is connected to the bridge circuit through the lead wire. As discussed earlier, to obtain the maximum output signal, the Wheatstone required to have all four arms be the same resistance value. However, when connecting the strain gauge via lead wire to the bridge circuit, the resistance of the lead wire adds to the existing values of resistances across the ratio arms. As a result, the net value of resistance changes, thus destabilizing the system.

The lead wires are generally made of copper alloy, which has very less resistance however even such low resistance can influence the strain measurement. The lead wire resistance also adds to the total resistance and hence the bridge balance is affected. When the lead wire resistance is modest, the percentage loss of signal is approximately equal to the lead wire resistance to strain gauge resistance ratio.

# 9. What is the effect of a galvanometer on the measurement of high resistance?

1. No effect
2. Depends on the e.m.f source
3. Galvanometer becomes insensitive to an imbalance
4. Depends on the type of galvanometer used

Answer.3. Galvanometer becomes insensitive to an imbalance

Explanation:-

Measurement of resistance greater than 1 megohm becomes difficult because of two factors:

(1) The ratio of standard resistances RA and RB involve a ratio on the order of 1,000 to 1

(2) The voltage applied to the bridge must be substantially increased to obtain definite galvanometer action. The result is that an increase in the supply voltage increases the power dissipation (heat) of the bridge resistors.

(3) Use the minimum voltage that will give a reliable indication by the galvanometer. Increasing the supply voltage any further results in uncompensated thermal variations and decreased bridge accuracy. If greater bridge sensitivity is needed, use a galvanometer with greater sensitivity.

# 10. The Wheatstone bridge can’t be used for signal conditioning due to

1. Galvanometer
2. Excessive current flow
3. e.m.f source used

Explanation:-

Lead wire resistance adds to sensor resistance and cannot be completely compensated by a bridge connection. As a result, the bridge circuit is destabilized and can’t be used for signal conditioning.

# 11. What is the effect of heat on the resistances in a Wheatstone bridge?

1. No effect
2. Increases the voltage drop across the circuit
3. Decreases the current flowing through the circuit
4. Causes a permanent change in the resistance value

Answer.4. Causes a permanent change in the resistance value

Explanation:-

Heating effect: When the current passes through the resistances, due to the heating effect (I2R) the temperature increases. Hence the values of the resistances of the bridge arms change due to the heating effect. The excessively high current may cause a permanent change in the resistance values. This may cause a serious error in the measurement. To avoid this, power dissipation in the arms must calculate well in advance and currents must be limited to a safe value.

# Statement 1:- Resistances less than l ohm are difficult to measure accurately.Statement 2:- Resistance greater than 1 megaohm can be accurately measured by Wheatstone.

1. Both statements are correct
2. Both Statements are incorrect
3. Only Statement 1 is correct
4. Only statement 2 is correct

Answer.3. Only statement 1 is correct

Explanation:-

Resistance values ranging from 1 ohm to 1 megohm can be measured with an accuracy of approximately 0.1%.

However, difficulties are encountered when very high and very low resistances are measured.

Resistances less than 1 ohm is difficult to measure accurately because of uncertainty arising from the contact resistance present between the resistor to be measured and the binding posts of the bridge.

Measurement of resistance greater than 1 megohm becomes difficult because of two factors:

(1) The ratio of standard resistances RA and RB involve a ratio on the order of 1,000 to 1.

(2) The voltage applied to the bridge must be substantially increased to obtain definite galvanometer action.

This results in an increase in the supply voltage thus increasing the power dissipation (heat) of the bridge resistors.

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