An inductor is a coil that resists a CHANGE in current. Switch it on and the current climbs rather than jumping; switch it off and the collapsing field will do something spectacular to your contacts if nothing is there to catch it.

$\tau = \dfrac{L}{R}$ — read aloud tau equals L over R. $\tau$ is the time constant in seconds, $L$ is the inductance in henries, and $R$ is the total circuit resistance in ohms. Note the shape and note it hard: the capacitor MULTIPLIED its two components, the inductor DIVIDES. It is the same $\tau$, the same seconds, the same 37%-after-one and gone-after-five — but the arithmetic is upside down, and mixing the two is this lesson's named trap. The reason is physical: resistance drags a capacitor's discharge out, but it hurries an inductor's current to its final value.

And the coil banks energy too, in its magnetic field rather than an electric one: $E = \tfrac{1}{2} L I^{2}$, E equals half L I squared, with $E$ in joules, $L$ in henries and $I$ the steady current through the coil, in amperes. The shape is the capacitor's energy relation with volts swapped for amps, ½ and square intact — which is the tidiest analogy in first-year circuits and worth carrying.

The units will catch the swapped time constant if you let them. Henries over ohms is seconds; henries TIMES ohms is not. Push the units through the rearrangement every time — if they refuse to cancel into the answer's units, the rearrangement is wrong, no appeal. Units that do work out never prove you right, though. The check runs one way.