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Meter Bridge Experiment: Measuring Unknown Resistance Using a Wheatstone Bridge Setup

The **meter bridge** (slide wire bridge) is a classroom implementation of the Wheatstone bridge principle, using a uniform 1-meter resistance wire to measure an unknown resistance by balancing lengths.

Objective

Determine the value of an unknown resistor X by comparing it with a known standard resistor R.

Materials

Meter bridge apparatus (1 m uniform wire AC on meter scale, gaps for resistors, galvanometer, jockey).
Battery / DC power supply (2–5 V) with key/switch.
Rheostat (variable resistor) – to limit current.
Resistance box (known standard R).
Unknown resistor X.
Connecting wires, sandpaper (clean contacts).

Procedure

Circuit Setup: Connect R in left gap, X in right gap. Battery across bridge wire ends A and C (via rheostat and key). Galvanometer between jockey (sliding contact on wire) and junction of R & X.
Initial Balance: Close key. Slide jockey along wire to find **null point** (galvanometer reads zero). Note balancing length l₁ from left end A (in cm).
Repeat: Swap R and X (or use different R). Find new null point l₂.
Multiple Readings: Use 3–5 known R values; average results.

Principle & Formula

At balance, R / X = l₁ / l₂ (since wire resistance ∝ length). Thus:

X = R × (l₂ / l₁)

Where l₁ = length from A to jockey (left segment), l₂ = (100 − l₁) cm (right segment).

Example

Known R = 10 Ω. Null at l₁ = 40 cm → l₂ = 60 cm.

X = 10 × (60 / 40) = **15 Ω**.

Precautions

Wire must be uniform (constant cross-section, material).
Tight connections; clean contacts.
Low current (rheostat) to avoid heating wire (changes resistivity).
Jockey contact: light touch, perpendicular; don't slide continuously while powered.
Balance point near center (40–60 cm) for best sensitivity.