An air cored coil having a winding resistance of $10\: \Omega$ is connected in series with a variable capacitor $C_{x}$. The series circuit is excited by a $\text{10 V}$ sinusoidal voltage source of angular frequency $1000$ rad/$\text{s}$. As the value of the capacitor is varied, a maximum voltage of $\text{30 V}$ was observed across it. Neglecting skin-effect, the value of the inductance of the coil in millihenry is ___________.