Bridges and Potentiometers

The Wheatstone bridge measures resistance: at balance no current flows through the detector and R₁/R₂ = R₃/R₄, so the unknown follows directly. Because balance is a null, the result is independent of the supply voltage.

AC bridges extend this to inductance and capacitance. The Maxwell bridge measures inductance using a capacitance standard; the Schering bridge measures capacitance and dielectric loss; the Hay bridge suits high-Q inductors.

Relation 1 —

Write this relation with symbols exactly as in A Course in Electrical & Electronic Measurements — AK Sawhney before substituting numbers. Examiners award partial marks for a correct setup even when arithmetic slips.

Relation 2 —

Write this relation with symbols exactly as in A Course in Electrical & Electronic Measurements — AK Sawhney before substituting numbers. Examiners award partial marks for a correct setup even when arithmetic slips.

Relation 3 —

Write this relation with symbols exactly as in A Course in Electrical & Electronic Measurements — AK Sawhney before substituting numbers. Examiners award partial marks for a correct setup even when arithmetic slips.

At AC-bridge balance both the magnitude and phase must match, giving two equations — one for the real part (resistance) and one for the imaginary part (reactance). Solving them yields both the unknown reactive element and its associated loss resistance.

The Kelvin double bridge eliminates lead and contact resistance errors when measuring very low resistances by adding a second ratio arm.

A potentiometer measures EMF by a null method: the unknown is balanced against a calibrated voltage drop with no current drawn at balance, so the source’s internal resistance causes no error — ideal for standard-cell calibration.

State assumptions explicitly before using any relation for bridges and potentiometers — steady state, uniform properties, linear elastic material, ideal gas, incompressible flow, etc., as applicable.

Wrong assumptions invalidate the entire solution even when the formula is correct. In Instrumentation viva and GATE descriptive questions, listing valid assumptions often earns separate marks.

1. Read the question and list given data with SI units (common in Instrumentation papers).
2. Draw a neat labelled diagram where applicable — examiners in Indian universities award diagram marks even when arithmetic slips.
3. Identify which relation from this topic applies to bridges and potentiometers.
4. Use equation 1: .
5. Use equation 2: .
6. Substitute values, compute, and verify units and sign (direction).
7. State conclusion in one line — e.g. safe/unsafe, stable/unstable, feasible/infeasible.

Bridges and Potentiometers appears in process control and labs. In Indian electrical curricula this topic is tested because it connects theory to measurement and transducers.

GATE and semester exams often combine bridges and potentiometers with earlier units — revise prerequisites before attempting mixed problems.

Industry interview panels sometimes ask: "Where did you use bridges and potentiometers?" — answer with a lab, mini-project, or plant visit example if possible.

• Writing the Wheatstone ratio the wrong way round (R₁/R₂ = R₃/R₄)
• Balancing only magnitude in an AC bridge (both real and imaginary parts must balance)
• Using a simple Wheatstone bridge for very low resistances (lead resistance dominates — use Kelvin)
• Forgetting the potentiometer draws no current at balance (why it is accurate)

Before attempting bridges and potentiometers problems, confirm you can:

1. AC bridges for L and C measurement (Hay, Owen, Schering)
2. Potentiometer: null method — no current drawn at balance
3. Guard terminals reduce leakage errors in high-R measurement

Revise the solved examples in A Course in Electrical & Electronic Measurements — AK Sawhney and one previous-year GATE or university paper for this unit.