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