A null point is found at \( 200 \, \text{cm} \) is potentiometer when cell in secondary circuit is shunted by \( 5\,\Omega \). When a resistance of \( 15\,\Omega \) is used for shunting, null point moves to \( 300 \, \text{cm} \). The internal resistance of the cell is __________ \( \Omega \).
