Concept: A Wheatstone bridge is used to measure an unknown resistance by balancing two arms of a bridge circuit. At balance condition: where is the unknown resistance. However, this method has limitations for very high and very low resistances.
Case 1: Very High Resistance
Current through the bridge becomes extremely small.
The galvanometer receives negligible current, making null detection difficult.
Leakage currents and insulation resistance introduce significant errors.
Result: Poor sensitivity and inaccurate measurement.
Case 2: Very Low Resistance
Contact resistance and resistance of connecting wires become comparable to the unknown resistance.
These additional resistances disturb the balance condition.
Small errors produce large percentage deviations.
Result: Large measurement errors.
Conclusion:
Very high resistance current too small, leakage errors.
Very low resistance contact and lead resistances cause large errors.
Hence, the Wheatstone bridge is not suitable for accurately measuring extremely high or very low resistances.