An inductor of inductance $L=400$ $mH$ and resistors of resistance ${R_1} = 2\Omega $ and ${R_2} = 2\Omega $ are connected to a battery of $emf$ $12$ $V$ as shown in the figure. The internal resistance of the battery is negligible. The switch $S$ is closed at $t=0.$ The potential drop across $L$ as a function of time is :