Two cells of emfs `E_(1)` and `(E_(2) (E_(1) gt E_(2))` are connected as shows in Fig. `6.45`.

When a potentiometer is connected between `A` and `B`, the balancing length of the potentiometer wire is `300 cm`. On connecting the same potentiometer between `A` and `C`, the balancing length is `100 cm`. The ratio `E_(1)//E_(2)` is