Inductance is only a momentary thing. Exactly how much of a momentary thing depends on the amount of inductance an inductor has. Inductance is measured in units called henrys.

The definition of one henry is simple: One henry is the amount of inductance necessary to induce one volt when the current in coil changes at a rate of one ampere per second.

As you might guess, one henry is a fairly large inductor. Inductors in the single-digit henry range are often used when dealing with household current (120 VAC at 60 Hz). But for most electronics work, you'll use inductances measured in thousandths of a henry (millihenrys, abbreviated mH) or in millionths of a henry (microhenrys, abbreviated μH).

Here are a few additional things to know about inductors and henrys:

• The plural of henry is henrys, not henries.

• The letter L is often used to represent inductance in formulas. Inductors in schematic diagrams are usually referenced by the letter L. For example, if a circuit calls for three inductors, they will be identified L1, L2, and L3.

• The henry is named after Joseph Henry, the person who discovered self-inductance and invented the inductor.

• This is the symbol used to represent inductors in a schematic diagram: