16. In order to measure the internal resistance \( r_{1} \) of a cell of emf \( E \), a meter bridge of wire resistance \( R_{0}=50 \Omega \), a resistance \( R_{0} / 2 \), another cell of emf \( E / 2 \) (internal resistance \( r \) ) and a galvanometer \( G \) are used in a circuit, as shown in the figure. If the null point is found at \( \ell=72 cm \), then the value of