A Daniel cell is balanced on $125\,cm$ length of a potentiometer wire. Now the cell is short-circuited by a resistance $2\, ohm$ and the balance is obtained at $100\,cm$. The internal resistance of the Daniel cell is .............. $ohm$
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(a) $r = \left( {\frac{{{l_1} - {l_2}}}{{{l_2}}}} \right)\,R = \left( {\frac{{25}}{{100}}} \right)\,2 = 0.5\,\Omega $
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