In a balanced Wheatstone bridge with four arms P, Q, R, and S, what is the condition for null deflection (zero current) through the central galvanometer?

Answer and explanation

Correct answer: C. P / Q = R / S

  1. Step 1: In a Wheatstone bridge network, let the galvanometer be connected between nodes and , with branches (), (), (), and ().
  2. Step 2: For zero current through the galvanometer (), the electric potentials at nodes and must be equal ().
  3. Step 3: This requires the voltage drop across to equal that across () and the voltage drop across to equal that across ().
  4. Step 4: Dividing the two equations yields the balanced bridge ratio: .
Answer
P / Q = R / S
Why not the others: 'P Q = R S' does not equate branch voltage drops; 'P + Q = R + S' adds branch resistances; 'P - Q = R - S' is algebraic fiction.

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