EASA Module 3 · Measurement
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.