"AC" stands for Alternating Current,
which can refer to either voltage or current that alternates
in polarity or direction, respectively. These experiments
are designed to introduce you to several important concepts
specific to AC.
A convenient source of AC voltage is
household wall-socket power, which presents significant
shock hazard. In order to minimize this hazard while taking
advantage of the convenience of this source of AC, a small
power supply will be the first project, consisting of
a transformer that steps the hazardous voltage (110
to 120 volts AC, RMS) down to 12 volts or less. The title of
"power supply" is somewhat misleading. This device does not
really act as a source or supply of power, but rather
as a power converter, to reduce the hazardous voltage
of wall-socket power to a much safer level.