Answer.4. All of the above

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%.

Answer.2. High

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.

Answer.1. It is not balanced

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.

Answer.3. Decrease bridge accuracy

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.

Answer.1. Critically damping

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.

Answer.3. Thermal Agitation

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.

Answer.2. By keeping the lead wire resistances within 0.2 mΩ

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.

Answer.3. Destabilize the system

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.

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 R_A and R_B 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.

Answer.4. Lead wires

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.