Answer.1. 1Ω to a few megaohms

Explanation:-

The Wheatstone bridge circuit is used to compare an unknown resistance with a known resistance. The bridge is commonly used in control circuits. The wheat stone bridge is the most accurate method available for measuring resistance and is popular for laboratory use.

The resistance with values ranging from 1Ω to 100Ω is called medium resistance. The resistors employed in electronic circuits usually are of medium resistance type.

This bridge circuit is used to determine medium resistance ranging from approximately an ohm (1Ω) to low mega ohms (MΩ).

Wheatstone bridge is also used to measure the resistance of cables and telephone lines for the locatıon of fault positions. For measuring low resistances, care should be taken to reduce the error in measurement. Resistances of connecting leads and contacts introđuce some errors in measurement. Errors due to connecting leads and contacts can be eliminated when we use Kelvin double bridge.

Answer.4. All of the above

Explanation

Errors in the Wheatstone bridge may occur because of the following:

(1). Limiting error:-  The main error is because of lımiting errors of the three known resistances. Hence very precise resistances are required having a tolerance of 1% or even 0.1%.

(2). Insufficient sensitivity of the null detector:- If the null detector is an ammeter, and if the imbalance current is few mA and if the meter is not able to measure the same, it will indicate null balance. If the detector is a voltmeter and if the potential difference is very small and the voltmeter is not able to read the same, a null indication will be given. This can result in errors in determining the value of the unknown resistance R.

(3). 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.

(4). The resistance of leads and contacts exterior to the actual bridge circuit adds extra resistance and is the major cause of the errors while measuring low resistance values.

(5). If the value of unknown resistance R, is very small, the resistances of the wires used for connections in the circuit and also contact resistance can affect the measurement.

(6). Thermal electromotive forces (EMFs) in the bridge circuit can also affect the balancing of the bridge. This arises because of dissimilar metals in contact with one another in making connections. The effect is significant for low values of R.

(7). Changes in the values of resistances in the arms of the bridge circuit because of a rise in temperature can also affect the measurement.

Answer.2. Using a more sensitive galvanometer

Explanation:-

Thermal EMFs in the bridge arms may cause serious trouble, particularly while measuring low resistances. Thermal emf in galvanometer circuits may be serious in some cases, so care must be taken to minimize those effects for precision measurements.

Some sensitive galvanometers employ all-copper systems (i.e., copper coils as well as copper suspensions) so that there is no junction of dissimilar metals to produce thermal emf. The effect of thermal emf can be balanced out in practice by adding a reversing switch in the circuit between the battery and the bridge, then making the bridge balance for each polarity and averaging the two results.

Answer.2. Limiting current to a safe 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.

Answer.3. Limiting errors of the three known resistances

Explanation:-

The measured value of the unknown quantity may be more than or less than the true value of the measurand. The manufacturer has to specify the deviations from the nominal value of a particular quantity. The limits of these deviations from the specified value are defined as Limiting errors or Guarantee errors.

The main source of error in the Wheatstone bridge is limiting errors of the three known resistances. Hence very precise resistances are required having a tolerance of 1% or even 0.1%.

Answer.1. Lead and contact resistance

Explanation:-

Lead Resistance:- In low-resistance measurements, the test leads have some resistance and can cause a significant error if the lead resistance is high enough. Sometimes lead resistance can be as high as hundreds of milliohms. This can result in an incorrect measurement.

The effect of lead resistance and contact resistance is very much significant while measuring low resistance.

Use of the null function or zero function of instruments, short leads, and four-probe measurements if possible can help to reduce the lead resistance errors.

Device Heating:- Device heating is one of the common errors that can greatly affect the measured resistance value for low-resistance materials especially temperature-sensitive materials. Power dissipation and temperature in the device can cause to change in the device resistance (usually the test currents used in low-resistance measurements are much higher than high-resistance measurements).

Answer.2. Insufficient sensitivity of the null detector

Explanation:-

Insufficient sensitivity of the null detector:- If the null detector is an ammeter, and if the imbalance current is few mA and if the meter is not able to measure the same, it will indicate null balance. If the detector is a voltmeter and if the potential difference is very small and the voltmeter is not able to read the same, a null indication will be given. This can result in errors in determining the value of the unknown resistance R.

Answer.4. When dissimilar metals come in contact

Explanation:-

Thermal EMFs in the bridge arms may cause serious trouble, particularly while measuring low resistances. In the galvanometer circuit, the disSimilar metals come in contact and generate the thermal e.m.f.s. Such thermal emf may cause the error while measuring low-value resistances.

Answer.2. Directly proportional

Explanation:-

The resistance of all pure metals increases linearly with an increase in temperature over a limited temperature range. At low temperatures, the ions are almost stationary. As the temperature increases, the ions inside the metal acquire energy and start oscillating about their mean positions. These vibrating ions collide with the electrons Hence resistance increases with an increase in temperatures.

Answer.4. Temperature increases due to power dissipation

Explanation

The power dissipation in the bridge circuit is defined as

P = I²R

The power dissipation as well as temperature in the device increases by the current. Therefore the effective way to minimize device heating is using the lowest current possible. If the current level cannot be reduced, using only a single, brief current pulse to the sample under test during the measurement cycle. Offset compensation could also be used to reduce device heating.