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A Wheatstone bridge is balanced with 625 ohm in the third (known) arm, with P, Q, S in the 1st, 2nd and 4th arms. When P and Q are swapped, the third arm must be increased by 51 ohm to restore balance. Find the unknown resistance S in the fourth arm.

1. 650 ohm
2. 625 ohm
3. 676 ohm
4. 600 ohm

Correct answer: 650 ohm

Solution

Multiplying the two balance conditions gives S² = R*R' = 625*676, so S = 650 ohm.

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