Consider the two cells having emf $E_{1}$ and $E_{2}\left(E_{1}>E_{2}\right)$ connected as shown in the figure. A potentiometer is used to measure potential difference between $P$ and $Q$ and the balancing length of the potentiometer wire is $0.8\, m$. Same potentiometer is then used to measure potential difference between $P$ and $R$ and the balancing length is $0.2\, m$. Then, the ratio $E_{1} / E_{2}$ is