Wheatstone bridge can’t be used for measurement of

Question

Examoogle · Accepted Answer

High resistance

Explanation: The Wheatstone bridge is primarily designed for the measurement of medium resistances ranging from $1\Omega$ to $100k\Omega$. It is generally ineffective for measuring high resistances because the leakage currents across the insulation become comparable to the measured current, and it is unsuitable for low resistances due to the influence of contact and lead wire resistances.

Working Principle: The Wheatstone bridge operates on the principle of a null detector. When the ratio of the two arms is balanced, the galvanometer current becomes zero, represented by the condition $\frac{R_1}{R_2} = \frac{R_3}{R_x}$. In high resistance measurements, the shunt resistance of the insulation affects the balance, while for low resistance, the contact resistance of the terminals introduces significant errors.

Key Formulas: $R_x = R_3 \cdot \frac{R_2}{R_1}$ — Standard balance equation for an unknown resistance $R_x$

Key Points: Medium resistance range: $1\Omega$ to $100k\Omega$; Sensitivity decreases significantly when measuring very high resistance values due to leakage.; Low resistance measurements (below $1\Omega$) are better handled by Kelvin's Double Bridge.; High resistance measurements (above $1M\Omega$) are usually performed using Megger or Loss of Charge methods.

Advantages: Simple circuit configuration; High accuracy for medium-range resistance measurement; Provides a null-balance indication

Disadvantages: Affected by lead and contact resistance for low values; Affected by insulation leakage for high values; Cannot measure resistance under $1\Omega$ accurately

Applications: Resistance measurement of coils and windings; Calibration of laboratory standard resistors; Used in sensors where resistance change is in the medium range

Additional Info: The Megger (ohmmeter) is the standard instrument for high resistance.. Kelvin's Double Bridge is the standard instrument for low resistance.. Option B (Medium resistance) is the primary application, not a limitation.

Answer: A is correct — The Wheatstone bridge is unsuitable for high resistance measurements due to the dominance of insulation leakage currents over the test current.

Exam Tip: Always remember the classification: Low (100k Ohm) to determine which bridge instrument applies.

Related Questions

Discussion (0)