An inductor of 10 mH is connected to a 20 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 $1 \mu \mathrm{~s}$ is $\frac{x}{100} \mathrm{~mA}$. Then $x$ is equal to $\_\_\_\_$ . (Take $\mathrm{e}^{-1}=0.37$ )