In an LR circuit with battery $\varepsilon$: current growth $I = \frac{\varepsilon}{R}(1 - e^{-t/\tau})$ where $\tau = L/R$ is the time constant. After one time constant: $I \approx 63.2\%$ of maximum. Current decay (battery removed): $I = I_0 e^{-t/\tau}$. The time constant $\tau = L/R$ governs how fast the circuit responds — large $L$ or small $R$ means slow response. The inductor acts as an open circuit at $t = 0$ (opposes sudden current change) and as a short circuit at $t = \infty$ (steady state, no change in current).