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.
