Correct Answer

Concept & PrincipleElectricalMeasurement & Instrumentation

Option A

Double bridge

Quick Summary: Kelvin's bridge is referred to as a 'double bridge' because it employs two sets of ratio arms to eliminate the effect of connecting lead and contact resistance. It is specifically designed to measure very low resistances (below 1 \Omega) with high precision.

💡 Explanation

Kelvin's bridge is referred to as a 'double bridge' because it employs two sets of ratio arms to eliminate the effect of connecting lead and contact resistance. It is specifically designed to measure very low resistances (below 1 $\Omega)$ with high precision.

🔢 Key Formulas

$R = S \cdot \frac{P}{Q}$ — Balance equation for Kelvin bridge when potential drops across leads are compensated

$\frac{P}{Q} = \frac{p}{q}$ — Condition for eliminating the effect of lead resistance $r$

⚙️ Working Principle

In a standard Wheatstone bridge, lead resistances introduce significant errors when measuring low values. Kelvin's double bridge overcomes this by introducing an auxiliary set of ratio arms connected to the potential terminals of the unknown resistor, ensuring the voltage drop across the connecting leads does not influence the balance equation.

📌 Key Points

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Used for accurate measurement of resistances in the range of $10°{-6} \Omega$ to $1 \Omega$ .
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Uses four ratio arms: two main arms ( $P, Q$ ) and two inner arms ( $p, q$ ).
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The 'double' refers to the two sets of ratio arms used to account for contact resistance.

✅ Advantages

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Eliminates lead resistance errors
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High accuracy for low resistance measurements

❌ Disadvantages / Limitations

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Complex circuit compared to Wheatstone bridge
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Requires balancing two sets of arms

🛠️ Applications / Uses

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Measurement of resistance of copper conductors
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Calibration of low-value standard resistors

📄 Additional Information

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Wheatstone bridge is suitable for medium resistance ( $1 \Omega$ to $100 k\Omega$ ).
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Option B (Single bridge) refers to a basic Wheatstone arrangement, which fails at low resistance due to lead drop.

📊 Diagram / Illustration

✅

A is correct — Kelvin's bridge is termed a double bridge as it utilizes two pairs of ratio arms to minimize errors caused by lead and contact resistances.

Core Concepts Used

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Low resistance measurement Lead resistance compensation Bridge sensitivity

💡 EXAM TIP

Always remember: Wheatstone for Medium, Kelvin for Low, and Megger/Loss of Charge for High resistance measurements.

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