Which is the most popular method for measuring low resistance?

Question

Examoogle · Accepted Answer

Option C: kelvin double bridge method

Explanation & Concepts:
The Kelvin double bridge is the most accurate and popular method for measuring low resistance (typically below $1 \Omega$). It effectively eliminates the errors caused by lead and contact resistances that are significant when measuring small values.

Working Principle: The Kelvin double bridge is a modification of the Wheatstone bridge. It incorporates a second set of ratio arms to eliminate the effect of the resistance of the connecting leads between the low-resistance standard and the unknown resistor. By establishing a potential balance between the inner and outer ratio arms, the contact resistance is effectively excluded from the measurement circuit.

Key Formulas: $R = \frac{P}{Q} S$ — Balancing equation when $p/q = P/Q$; $R_x = \frac{P}{Q} S + \frac{wr}{p+q+r} (\frac{P}{Q} - \frac{p}{q})$ — General equation including lead resistance

Key Points: Used specifically for measuring resistances in the range of $10^{-6} \Omega$ to $1 \Omega$.; Uses four-terminal resistor connections to avoid the effects of lead resistance.; Eliminates the 'lead resistance' error which is problematic in standard Wheatstone bridges.; Requires a sensitive galvanometer for null detection due to low voltage drops.

Advantages: High accuracy for very low resistance values; Eliminates error due to contact and connecting lead resistance; Widely accepted standard for precision laboratory measurements

Disadvantages: Requires complex balancing of both ratio arms; Requires a high-sensitivity null detector (galvanometer); More expensive and complex setup compared to simple bridge circuits

Applications: Measuring the resistance of busbars and power cables; Calibration of low-value standard resistors; Resistance measurement of armature windings in large machines

Additional Info: The ammeter-voltmeter method (Option A) is unsuitable for low resistance because the contact resistance of the probes becomes significant relative to the unknown resistance.. The Kelvin double bridge is specifically designed to overcome the limitations of the Wheatstone bridge for low-resistance measurements.

Answer: C is correct — The Kelvin double bridge method is the industry standard for accurately measuring low-value resistances by eliminating errors caused by lead and contact resistances.

Exam Tip: Always remember that for high resistance, we use the Megger or loss-of-charge method, whereas for low resistance, Kelvin Double Bridge is the default choice in competitive exams.

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Option A: ammeter-voltmeter method

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Option B: potentiometer method

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Option D: ducter ohmmeter method

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