Across a 10 V DC supply, two branches are connected in parallel: R1 = 1 kΩ in series with R2 = 1 kΩ, and R3 = 1 kΩ in series with R4 = 1.5 kΩ (R1 and R3 on the positive side). What is the magnitude of the voltage between the R1–R2 junction and the R3–R4 junction? (Ignore any current drawn by the measurement.)

Answer: C. 1 V

The R1–R2 junction (relative to the negative terminal) is at 10 V × 1/(1 + 1) = 5 V, and the R3–R4 junction is at 10 V × 1.5/(1 + 1.5) = 6 V. The difference is 6 − 5 = 1 V, with the R3–R4 junction higher. The balance condition R1/R2 = R3/R4 is not met (1 ≠ 1/1.5), so a difference voltage appears. Bridge circuits such as strain-gauge bridges amplify this difference to read tiny resistance changes.

• A. 0 V would mean the bridge is balanced, but R1/R2 = 1 and R3/R4 ≈ 0.67 are not equal.
• B. 6 V is the voltage of the R3–R4 junction relative to the negative terminal.
• C. Correct. 6 V − 5 V = 1 V.