A null point is obtained at 200 cm on potentiometer wire when cell in secondary circuit is shunted by $5 \Omega$. When a resistance of $15 \Omega$ is used for shunting, null point moves to 300 cm . The internal resistance of the cell is

To determine the internal resistance of a cell with potentiometer, when the cell is shunted by a resistance of $5 \Omega$ the balancing length is 250 cm . When the cell is shunted by $20 \Omega$, the balancing length of potentiometer wire is 400 cm . The internal resistance , of the cell is

Two cells $E_1$ and $E_2$ having equal e.m.f ' $E$ ' and internal resistances $r_1$ and $r_2\left(r_1>r_2\right)$ respectively are connected in series. This combination is connected to an external resistance ' R '. It is observed that the potential difference across the cell $E_1$ becomes zero. The value of $R$ will be