A potentiometer circuit has been set up for finding the internal resistance of a given cell. The main battery, used across the potentiometer wire, has an emf of $2.0 V$ and a negligible internal resistance. The potentiometer wire itself is $4$ m long. When the resistance $R$ is connected across the given cell, it has values of$\newline (a) \infty \quad (b) 9.5 \Omega \newline$The balancing lengths, on the potentiometer wire are found to be $3 m$ and $2.85 m$, respectively. The value of internal resistance of the cell is