A potentiometer PQ is set up to compare two resistances as shown in the figure. The ammeter A in the circuit reads 1.0 A when two way key $K_3$ is open. The balance point is at a length $l_1$ cm from P when two-way key $K_3$ is plugged in between 2 and 1, while the balance point is at a length $l_2$cm from P when key $K_3$ is plugged in between 3 and 1. The ratio of two resistances $\frac{R_1}{R_2}$ is found to be :