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Most students of electricity begin their
study with what is known as direct current (DC),
which is electricity flowing in a constant direction, and/or
possessing a voltage with constant polarity. DC is the kind
of electricity made by a battery (with definite positive and
negative terminals), or the kind of charge generated by
rubbing certain types of materials against each other.
As useful and as easy to understand as DC
is, it is not the only "kind" of electricity in use. Certain
sources of electricity (most notably, rotary
electro-mechanical generators) naturally produce voltages
alternating in polarity, reversing positive and negative
over time. Either as a voltage switching polarity or as a
current switching direction back and forth, this "kind" of
electricity is known as Alternating Current (AC):
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