An inductor of $10 \mathrm{mH}$ is connected to a $20 \mathrm{V}$ battery through a resistor of $10 \mathrm{k\Omega }$ and a switch. After a long time, when maximum current is set up in the circuit, the current is switched off. The current in the circuit after $1 \mathrm{\mu s}$ is $\frac{x}{100} \mathrm{mA}$. Then $x$ is equal to ______ . (Take $e^{-1}=0.37$)