Question

Why can high or very low resistance not be measured accurately using the Wheatstone bridge principle?

Hint:

Wheatstone bridge works best for medium resistances. Use Kelvin bridge for low resistance and megohm meters for high resistance.

Answer 1

Concept: A Wheatstone bridge is used to measure an unknown resistance by balancing two arms of a bridge circuit. At balance condition: \[ \frac{P}{Q} = \frac{R}{X} \] where \( X \) 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 $\rightarrow$ current too small, leakage errors. 
Very low resistance $\rightarrow$ contact and lead resistances cause large errors. 
Hence, the Wheatstone bridge is not suitable for accurately measuring extremely high or very low resistances.