A potentiometer wire of length 100 cm ha...

A potentiometer wire of length 100 cm has a resistance of `10Omega`.It is connected in series with a resistance and a cell of emf 2V having negligible internal resistance.A source of emf 10 mV is balanced against a length of 40 cm of the potentiometer wire. The value of external resistance is

`760Omega`

`640Omega`

`790Omega`

`840Omega`

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