Bridgette

Meet Bridgette, a diamond of four resistors who can find an unknown resistance with great precision.

A Wheatstone bridge has two pairs of resistors connected side by side across a battery, with a sensitive meter joining the middle points of the two pairs. Current splits into the two branches, and the meter shows whether the voltages at its two ends are equal.

When the ratios match, so that R₁/R₂ = R₃/R₄, the two middle points sit at the same voltage and no current flows through the meter. The bridge is then balanced. If R₁ = 100 Ω, R₂ = 200 Ω and R₃ = 150 Ω, balance tells you that R₄ = 300 Ω.

The method is a null measurement. You adjust a known resistor until the meter reads zero, instead of trusting the exact reading of the meter itself, which makes it very accurate.

Bridges like this are still used in strain gauges and sensors, where a tiny change in one resistor shows up as a small current through the meter.