When a battery is first connected across an inductor, the current builds up at a rate that depends on the inductance. The greater the inductance, the slower the rate of current buildup for a given battery voltage. The unit of inductance is an expression of the ratio between the rate of current buildup 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 changing at the rate of 1 ampere per second (1 A/s).
The henry is a huge unit of inductance. You won’t often 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 by now, but in case you’ve forgotten:
1 mH = 0.001 H = 10−3 H
1 μH = 0.001 mH = 10−6 H
1 nH = 0.001 μH = 10−9 H
Small coils with few turns of wire produce small inductances, in which the current changes quickly and the induced voltages are small. Large coils with ferromagnetic cores, and having many turns of wire, have high inductances in which the current changes slowly and the induced voltages are large. The current from a battery, building up or dying down through a high-L coil, can give rise to a deadly potential difference between the end terminals of the coil—many times the voltage of the battery itself. This is how spark coils work in internal combustion engines. Be careful around them!