THE UNIT OF INDUCTANCE

When a source of dc is connected across an inductor, it takes awhile 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 is the rate of change of current for a given dc 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 1 henry (1 H) represents a potential difference of 1 volt (1 V) across an inductor within which the current is increasing or decreasing at 1 ampere per second (1 A/s).
The henry is an extremely large unit of inductance. Rarely will you see an inductor this large, although some power-supply filter chokes have inductances up to several henrys. Usually inductances are expressed in millihenrys (mH), microhenrys (μH), or 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 = 0.000000001 H = 10^-9 H.

Small coils with few turns of wire have small inductances, in which the current changes quickly and the voltages are small. Huge coils with ferromagnetic cores and many turns of wire have large inductances, in which the current changes slowly and the voltages are large.