To build an inductor, we would take a length of wire and
wrap it around a cylinder, like a coil. If we connect this inductor to a DC
power supply, then the flow of current through the wire will set up a strong
magnetic field through the center of the coil. (Remember the right-hand rule?)

Each turn in the coil reinforces and strengthens the
magnetic field. To DC, an inductor — remember, it’s simply a coil of wire — is
a direct short. It has no impedance other than the characteristic resistance of
the wire.

But if we connect the inductor to an AC source, something
very interesting happens. During the positive half cycle, the current sets up a
strong magnetic field in one direction. When the current reverses direction
during the negative half cycle, the magnetic field that was set up by the
positive half cycle does not collapse right away; it takes time.

During the time that the magnetic field is collapsing, it is
in direct opposition to the magnetic field that is trying to set up due to the
negative half cycle of current. Therefore, the inductor opposes the change of
current, providing an impediment to the free flow of current. It acts as a
“choke.”

After a short while, the magnetic field collapses completely
and the current flowing in the opposite direction sets up the magnetic field
again, but in the opposite orientation. Both the current and the magnetic field
are constantly changing directions, and the current is constantly impeded.

In our water–electricity analogy, an inductor may be thought
of as a large paddle wheel or a turbine blade in a channel of water. When the
water flows, it starts the paddle wheel turning, giving it momentum. If the
water current suddenly changes direction, the paddle wheel will resist it
because it’s turning the other way.

Once the reverse current overcomes the momentum of the wheel
it will begin to turn the other way. But it initially resists the change in
direction until the momentum is overcome. The same is true of an electrical
current. The magnetic field of the inductor is like the momentum in the paddle
wheel.

Inductance is measured in henrys, after the American
scientist Joseph Henry. But it is often represented in mathematic equations by
the letter “L,” after Heinrich Lenz, a Baltic German physicist who advanced the
study of inductance. The henry is a very large value; therefore, it is more
common for inductors to be measured in millihenries (10-3 henrys or 0.001
henrys).

In a vacuum, the value of an inductor depends on the
diameter of the wire or the wire gauge, the diameter of the coil, and the
number of turns in the coil. By inserting an iron core in the center of an
inductor, the inductance increases in direct proportion to the permeability of
the iron core, i.e., the more the magnetic field influences the core material,
the higher the inductance.

An inductor offers no impedance to the flow of DC (other
than the small resistance of the wire), but it does impede the flow of AC. As
the frequency of the alternating current in an inductor increases, so does the
impedance. The amount of impedance in an inductor is called inductive
reactance, XL, and it is measured in ohms. XL (ohms) = 2πfL, where XL is the
inductive reactance, π is pi (3.14), f is the frequency in hertz, and L is the
inductance in henrys.

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