The $e.m.f.$ of a standard cell balances across $150\, cm$ length of a wire of potentiometer. When a resistance of $2\,\Omega $ is connected as a shunt with the cell, the balance point is obtained at $100\,cm$. The internal resistance of the cell is .............. $\Omega $
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(b) Using $r = R\,\left( {\frac{{{l_1}}}{{{l_2}}} - 1} \right)$ $ = 2\,\left( {\frac{{150}}{{100}} - 1} \right) = 1\,\Omega $
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