An ideal battery of $4\, V$ and resistance $R$ are connected in series in the primary circuit of a potentiometer of length $1\, m$ and resistance $5\,\Omega $ . The value of $R$, to give a difference of $5\, mV$ across $10\, cm$ of potentiometer wire, is: ................ $\Omega$
JEE MAIN 2019, Medium
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