The unit of inductance

When a battery is connected across a wire-coil inductor (or any kind of inductor), it takes a while for the current flow to establish itself throughout the inductor. The current changes at a rate that depends on the inductance: the greater the inductance, the slower the rate of change of current for a given battery voltage.
The unit of inductance is an expression of the ratio between the rate of current change and the voltage across an inductor. An inductance of one henry, abbreviated H, represents a potential difference of one volt across an inductor within which the current is increasing or decreasing at one ampere per second.
The henry is an extremely large unit of inductance. Rarely will you see an inductor anywhere near this large, although some power-supply filter chokes have inductances up to several henrys. Usually, inductances are expressed in millihenrys (mH), microhenrys (μH), or even in nanohenrys (nH). You should know your prefix multipliers fairly well by now, but in case you’ve forgotten, 1 mH = 0.001 H = 10^-3 H, 1 μH = 0.001 mH = 0.000001 H = 10^-6 H, and 1 nH = 0.001 μH = 10 ^-9 H.
Very small coils, with few turns of wire, produce small inductances, in which the current changes quickly and the voltages are small. Huge coils with ferromagnetic cores, and having many turns of wire, have large inductances, in which the current changes slowly and the voltages are large.