The standard international unit used
to measure inductance is the henry. An inductor with a value of one
henry would be fairly large and heavy, especially if intended to carry
a lot of current (because it will need heavier wire). Typical inductors
are much smaller than one henry and fall in the millihenry (mH) and
micro Henry (μH) ranges.
Inductance is affected by how much wire is in the coil and how close
the turns are to each other. If you add more wire, by either making the
diameter of the coil larger or adding more turns of wire, the
inductance increases. Placing the turns closer together also increases
the inductance. Adding a soft iron1 core will further concentrate the
magnetic field and increase the inductance.
Iron cores may be made of ferrite or laminated plates of silicon steel.
Ferrite is a mixture of iron powder and a ceramic material that can be
molded into convenient shapes. Variable inductors often have a screw-
shaped ferrite core. The core is turned with a suitable tool and screws
in or out of the coil to change the inductance.
Inductance is affected by the following properties:
Inductance increases with more turns of wire
Inductance increases with a larger diameter of coil
Inductance increases by adding an iron core
Inductance decreases with greater distance between turns