Inductors are usually represented by the letter "L" in schematics.
The unit of measure of inductance (the size of the inductor) is "Henries".
An inductor is made by winding many turns of insulated wire into a coil often with a ferrite or iron core.
When current flows through an inductor, it generates a magnetic field that tends to oppose any change in current.
Alternating currents are therefore impeaded by an inductor. Direct currents flow unimpeaded.
If a direct current in an inductor is suddenly interrupted by breaking the circuit with a switch,
the magnetic field will induce an extremely high voltage in an attempt to maintain the current, and
a high-voltage spark will be produced. This is how the spark is generated for spark plugs.
In some ways inductors act like the opposite of capacitors.
Capacitors provide a low impeadance to alternating currents and high impeadance to direct currents.
Inductors provide a high impeadance to alternating currents and a low impeadance to direct currents.
In electronics, inductors are often used in filters to oppose the flow of alternating currents.
The impeadance of an inductor is often refered to as reactance (with the symbol "X").
The impeadance of an inductor can be calculated as follows for sinusoidal alternating currents:
(where Pi = 3.1416, f = frequency and L is inductance in Henries)