EASA / CAR Part-66 · Module 3 · 3.7 Resistance & Resistors · Cat B1/B2
A bridge of four resistors measures an unknown by balancing. Adjust R3 until the galvanometer reads zero, at balance R1/R2 = R3/Rx, so the unknown Rx can be found precisely.
The bridge is balanced when the two ratios match, then no current flows through the galvanometer and Rx = R3 × (R2 ÷ R1). Bridges are the basis of strain gauges, temperature sensors and many aircraft transducers.