Unit of Inductance:

Whenever a source of dc is associated across an inductor, it acquires awhile for the current flow to begin itself throughout the inductor. The current changes at a rate which depends on the inductance. The greater the inductance is, the slower is the rate of change of current for a known dc voltage. The unit of inductance is an expression of the ratio among the rate of current change and the voltage across an inductor. An inductance of 1 henry (1 H) symbolizes a potential difference of 1 volt (1 V) across an inductor in which the current is rising or reducing at 1 ampere per second (1 A/s).

The Henry is a very large unit of inductance. Very rarely will you see an inductor this large, though some power-supply filter chokes have inductances up to numerous henrys. Generally inductances are stated in millihenrys (mH), microhenrys (µH), or nanohenrys (nH). You should know your prefix multipliers pretty well by now, though in situation you have forgotten,

1 mH = 0.001 H = 10^-3 H, 1µH = 0.001 mH = 0.000001 H = 10^-6 H,

and 1 nH = 0.001 µH = 0.000000001 H = 10^-9 H.

Small coils with little turns of wire have small inductances, in which the current changes rapidly and the voltages are small. Vast coils with ferromagnetic cores and many turns of wire have large inductances, in which the current changes gradually and the voltages are large.