A potentiometer wire of length $100\, cm$ has a resistance of $10\, ohm.$ It is connected in series with a resistance and an accumulator of emf $2\,V$ and of negligible internal resistance. A source of emf $10\, mV$ is balanced against a length of $40\, cm$ of the potentiometer wire. What is the value of external resistance :- ................. $\Omega$
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