In a Post Office Box (a Wheatstone-bridge variant), if the ratio arms have $P = 100\ \Omega$, $Q = 10\ \Omega$ and $R = 250\ \Omega$ at balance, the unknown resistance $X$ is:
A25 Ω
B2.5 Ω
C2500 Ω
D250 Ω
Answer & Solution
Correct answer: A. 25 Ω
P.O. box: $X = RQ/P = 250 \times 10/100 = 25\ \Omega$. The ratio arms scale the measured resistance to fit any range.
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