In a potentiometer (see figure) a balance is obtained at a length of $400\ mm$ when using a known battery of emf $1.6\ V$. After removing this battery, another battery of unknown emf is used and balance is obtained at a length of $650\ mm.$ The emf of unknown battery is ............. $volt$
A$2.6$
B$1.6$
C$3.4$
D$4.7$
Medium
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A$2.6$
a $\frac{\ell_{2}}{\ell_{1}}=\frac{\mathrm{E}_{2}}{\mathrm{E}_{1}}$
$\frac{400}{650}=\frac{1.6}{E}$
$\mathrm{E}=2.6$
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