A Wheatstone bridge is used to measure unknown resistance by placing resistors in a balancing circuit. Which of the following best describes this instrument?

• A. Wheatstone bridge
• B. Voltmeter
• C. Ohmmeter
• D. Amperemeter

Answer: (A)

Balancing a Wheatstone bridge relies on the null-balance principle to measure unknown resistance with high precision. In this setup, two voltage dividers share the same supply, and a detector (usually a galvanometer) is connected between the midpoints. When the bridge is balanced, no current flows through the detector, so the potential difference between the midpoints is zero. This balance occurs when the ratio of the resistances in one branch equals the ratio in the opposite branch, allowing you to solve for the unknown resistor using the known ones (for example, Rx = (R2 × R3) / R1, depending on how the resistors are arranged).

This approach is distinct from a voltmeter, which simply measures voltage, an ohmmeter, which measures resistance by directly applying current and reading the response, or an ammeter, which measures current. The Wheatstone bridge uses a delicate balance condition to infer the unknown resistance with minimal loading and higher accuracy.