Two cells of emf's approximately 5 V and 10 V are to be accurately compared using a Potentiometer of length 400 cm :

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The battery of potentiometer can have a voltage of 15 V and R adjusted so that the potential drop across the wire slightly exceeds 10V.

Potentiometer is usually used for comparing resistances and not voltages.

The first portion of 50 cm of wire itself should have a potential drop of 10 V.

The battery that runs the Potentiometer should have voltage of 8 V

answer is B.

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Detailed Solution

Given that, emf of primary cells are 5 V and 10 V The potential drop across potentiometer wire must be slightly more than that larger emf 10 V. So, the battery should be of 15 V and about 4 V potential is dropped by using rheostat or resistances. So option (2) is correct.

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