Potentiometer: emf & internal resistance of batteries

A battery with internal resistance of $1\ \Omega$1, space, \Omega and an unknown emf drives a current across a $9\ \Omega$9, space, \Omega resistor.

When a potentiometer with a wire $\text{AB}$start text, A, B, end text is connected across the battery, the balancing length is, $\text{AE}=80\ \text{cm}$start text, A, E, end text, equals, 80, space, start text, c, m, end text.