The circuit diagram of potentiometer used to measure the internal resistance of a cell (E) is shown in figure. The key ' K ' is kept closed so as to…

The circuit diagram of potentiometer used to measure the internal resistance of a cell (E) is shown in figure. The key ' K ' is kept closed so as to send constant current through potentiometer wire. When key ' $\displaystyle \mathrm{K}_1$ ' is kept open the null point is found to be at $\displaystyle 120$ cm on the potentiometer wire. When the key ' $\displaystyle \mathrm{K}_1$ ' is closed the null point is shifted at $\displaystyle 80$ cm at the potentiometre wire. The internal resistance of the given cell is \_\_\_\_ $\displaystyle \Omega$.