Wheatstone bridge
Four resistors in a diamond configuration with a galvanometer across the middle. When the bridge is balanced — R1/R2 = R3/R4 — no current flows through the galvanometer. This simple circuit is the foundation of precision resistance measurement, strain gauges, and sensor interfaces.
The Wheatstone bridge was invented by Samuel Hunter Christie in 1833 and popularized by Sir Charles Wheatstone in 1843. It remains one of the most important circuits in electrical measurement.
The circuit has four resistors arranged in a diamond (or bridge) topology, with a voltage source across one diagonal and a galvanometer (sensitive current meter) across the other. When R1/R2 = R3/R4, the voltage at the two middle nodes is equal, so no current flows through the galvanometer. This is the balance condition.
To solve the circuit, we apply Kirchhoff's laws. The four nodes have voltages VA (top, between R1 and R3), VB (bottom, between R2 and R4), V+ (left, positive terminal), and V− (right, ground). By writing node equations and solving the linear system, we get the current through each branch, including the galvanometer.
Wheatstone bridges are everywhere: strain gauges use a bridge where one resistor changes with mechanical stress; RTD temperature sensors use bridge circuits for precision measurement; and audio equalizers use AC bridges for impedance matching.