In order to measure the internal resistance r 1 of a cell of emf E , a meter bridge of wire resistance $\mathrm{R}_0=50 \Omega$, a resistance $\mathrm{R} / 2$, another cell of emf $\mathrm{E} / 2$ (internal resistance r) and a galvanometer G are used in a circuit, as shown in the figure. If the null point is found at $\mathrm{I}=72 \mathrm{~cm}$, then the value of $r_1=$ $\_\_\_\_$ $\Omega$.