Unit of inductance

When the battery is connected across a wire coil inductor, it takes a while for the current flow to establish itself throughout inductor. The current changes at the rate which depends on the inductance: greater the inductance, slower the rate of change of current for the 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 1 henry, represented as H, represents the potential difference of 1 volt across an inductor within which current is increasing or decreasing at 1 ampere per second.

The henry is an extremely large unit of inductance. You will rarely see an inductor anywhere near this large, some power supply filter chokes have inductances up to several henrys. Generally, inductances are expressed in millihenrys, micro- henrys, or even in nanohenrys. You must know your prefix multipliers well by now, but in case you have forgotten, 1mH = 0.001  H=10^-3 H, 1 µH=0.001 mH = 0.000001 H=10^-6H, and 1 nH = 0.001 µH = 10^-9H.

Very small coils, having few turns of wire, produce small inductances,  in which current changes quickly and voltages are small. Huge coils having ferromagnetic cores, and having several turns of wire, have large inductances, in this the current changes very slowly and the voltages are large.