Wheatstone bridge measurement principle: In classical electrical measurements, a Wheatstone bridge is primarily used to determine which unknown quantity by balancing two resistor ratios and detecting a galvanometer null?

Introduction:
The Wheatstone bridge is one of the most important null-measurement circuits in electrical engineering. It enables highly accurate comparison of resistances by balancing two arms of a bridge and observing a zero-current (null) condition through a sensitive detector. Understanding what physical quantity the bridge is designed to find clarifies why it is ubiquitous in sensor interfacing and precision metrology.

Given Data / Assumptions:

• A four-resistor network arranged as a bridge: two known standard resistors, one variable/ratio resistor, and one unknown element.
• A sensitive null detector (galvanometer or differential amplifier) between the midpoints of the two arms.
• A stable DC source feeding the bridge.

Concept / Approach:

At balance, the ratio of the two resistors in one arm equals the ratio in the other arm. The null condition means no current flows through the detector, allowing a ratio equation to be written that directly solves for the unknown resistance. Because this is a ratio method at null, it minimizes the influence of source voltage drift and detector calibration errors, yielding high accuracy for resistance determination.

Step-by-Step Solution:

Verification / Alternative check:

By substituting any three known resistor values into the balance equation and computing RUNK, then re-checking the ratio equality, you verify the bridge principle. Practical bridges use precision ratio arms and variable resistors to dial in balance exactly.

Why Other Options Are Wrong:

• current: The detector monitors near-zero current at balance; current is not what is determined as an unknown.
• power: Power depends on both voltage and current and is not directly measured by the bridge balance condition.
• voltage: The source voltage cancels in the ratio; voltage magnitude is not the bridge’s unknown.
• frequency: A DC Wheatstone bridge is not a frequency-measuring device.

Common Pitfalls:

• Assuming you need the exact source voltage; you do not at balance because of the ratio method.
• Ignoring lead and contact resistances, which can disturb balance in low-ohm measurements; use Kelvin (4-wire) techniques when needed.

Final Answer:

resistance