You can’t measure a tiny resistance by eye — but you CAN tell when two arms of a diamond are perfectly balanced, because a needle between them reads exactly zero. Tune a known resistor until the needle dies, and the unknown is pinned by a ratio. Slide the unknown and hunt the null.
Four resistors form a diamond; a galvanometer bridges the middle. The bridge voltage is Vᵢₙ·(Rₓ/(R₃+Rₓ) − R₂/(R₁+R₂)), which is exactly zero when R₁·Rₓ = R₂·R₃ — a ratio, independent of the supply voltage or its drift. With R₁=R₂=100Ω and R₃=200Ω the null lands at Rₓ=200Ω. A fail-loud self-check throws unless the bridge voltage is zero at balance and changes SIGN as Rₓ crosses it — the property that makes the null sharp and the measurement precise.
Ideal resistors, ideal galvanometer, no lead resistance — real bridges add Kelvin connections and guard the ratio arms. The null principle (measure by balancing to zero, not by reading a scale) is exact and is what makes it a precision instrument (Christie 1833, Wheatstone 1843).