How this calculator works
A Wheatstone bridge compares two divider ratios. At balance the midpoint voltages match, so a high-impedance detector reads zero and the unknown resistance follows from the known ratio. Away from balance, the output is the difference between divider voltages. The detector Thevenin resistance is the sum of the two branch parallel resistances after suppressing the ideal voltage source. A finite detector load reduces the measured voltage.
Formula
Vg = Vs[R2/(R1+R2)-Rx/(R3+Rx)]; at balance Rx = R2R3/R1; Rth = (R1 || R2)+(R3 || Rx).
Worked example
R1 = 100 ohm, R2 = 200 ohm and R3 = 300 ohm balance at Rx = 600 ohm. With R1 = R2 = R3 = 100 ohm, Rx = 300 ohm and Vs = 12 V, detector voltage is -3 V.
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Frequently asked questions
Does voltage polarity matter?
Yes. Positive means the left midpoint is above the right midpoint using the labelled topology.
Must supply voltage be known for balance?
No. It cancels from the equal-ratio condition.
Can I connect a low-resistance meter?
Its loading changes the result. This calculation assumes an open-circuit or high-impedance detector.