Wheatstone Bridge & Meter Bridge — A/L Physics

Wheatstone Bridge Principle A circuit used to measure an unknown resistance by balancing two ratio arms. The bridge consists of four resistors arranged in a diamond shape (P, Q, R, S) with a galvanometer between the midpoints. When the galvanometer shows zero deflection (balanced condition), the ratio of resistances in one arm equals the ratio in the other: P/Q = R/S. This allows precise determination of an unknown resistor if three others are known. Real-world example: Used in strain gauges to measure small changes in resistance due to mechanical stress.

Balanced Condition No current flows through the galvanometer when the bridge is balanced. At balance, the potential difference between the two midpoints is zero. This implies that the voltage drop across P equals that across R, and across Q equals that across S. The condition is independent of the source voltage and galvanometer resistance, making it highly accurate. Intuition: Think of two parallel voltage dividers producing the same output voltage.