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In the experimental set up of metre bridge shown in the figure, the null point is obtained at a distance of 40 cm from A. If a 10 Ω resistor is connected in series with R₁, the null point shifts by 10 cm. The resistance that should be connected in parallel with (R₁+10) Ω such that the null point shifts back to its initial position is [Figure: metre bridge with wire AB, R₁ in the left gap and R₂ in the right gap; galvanometer G from the junction of the gaps to a jockey on the wire; a cell and key across A and B.]

Asked in JEE Main 11th Jan 2nd Shift 2019 · Metre bridge

• (1) 40 Ω
• (2) 20 Ω
• (3) 60 Ω
• (4) 30 Ω

Step-by-step solution

Why the other options are wrong

• (1) No standard slip gives this; 40 Ω in parallel with 30 Ω gives 17.1 Ω, not the needed 20 Ω.
• (2) Comes from quoting R₁=20 Ω, the target value, instead of the parallel resistor needed.
• (4) Comes from quoting R₂=30 Ω or R₁+10 instead of the parallel resistor needed.

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