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In a meter bridge experiment, the initial balance point is at J with AJ = l (where R and X are the two resistances in the bridge, R != X). Consider the following changes and identify the correct statement(s). (A) Swapping the galvanometer and battery leaves the galvanometer current zero. (B) Doubling the radius of wire AB changes the balance length to l' = 2l. (C) Doubling the length of wire AB changes the balance length to l' = 2l. (D) Doubling both R and X and then interchanging them gives l' = l.

1. If in shown set-up the galvanometer and battery are inter changed current through galvanometer is zero.
2. radius of wire AB is double then l' = 2l
3. length of wire AB is doubled then l' = 2l
4. If the values of R and x both are double and then interchanged then l' = l

Correct answer: If in shown set-up the galvanometer and battery are inter changed current through galvanometer is zero.

Solution

A is correct (reciprocity theorem). B is wrong (radius change does not affect the balance ratio or absolute balance length). C is correct (doubling wire length makes l' = 2l in absolute terms). D: swapping 2R and 2X gives R/X -> X/R ratio, so new balance is at L-l, not l. D is wrong. Correct: A and C.

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