The inductance of a coil is measured using the bridge shown in the figure. Balance $(D = 0)$ is obtained with $C_1= 1\; nF, R_1 = 2.2\; M\Omega, R_2 = 22.2\; k\Omega, R_4 = 10\; k\Omega$. The value of the inductance $L_x$ (in $\text{mH} $ is $\_\_\_\_\_\_$
