Figure shows a potentiometer circuit for determining the internal resistance of a cell.  When switch S is open, the balance point is found to be at 76.3 cm of the wire.  When switch S is closed and the value of R is 4.0 $\Omega$, the balance point shifts to 60.0 cm.  The internal resistance of cell E' is.

$ 1.1 \Omega$

$ r = \frac{l - l'}{l'} R = \frac{76.3 - 60}{60} 4\Omega = 1.1\Omega$