Vol. IV — Electronics · Tool
Two voltage dividers face each other across the same supply. The mid-point difference is Vg. Change Vin and the four arms — Va, Vb, and Vg update as you type. At balance, solve for the unknown R4.
Left leg: R1 on top, R2 on the bottom. Mid-point A is Va = Vin × R2 / (R1 + R2). Right leg: R3 on top, R4 on the bottom. Mid-point B is Vb = Vin × R4 / (R3 + R4). The bridge output is Vg = Va − Vb.
Balance means no voltage between the mid-points: R1 / R2 = R3 / R4, or R1 · R4 = R2 · R3. When you know three arms and the bridge is balanced, the fourth is R4 = R3 × R2 / R1. That is how a strain gauge or an unknown resistor is measured against a known ratio.
Pair with the voltage divider for a single leg, resistor codes to pick parts, and the electronics bench.