Mains Electricity - Alternating Current.
If an alternating current supply is connected
to a cathode ray oscilloscope (called a CRO),
the display shows a wave as the current changes
from maximum positive to maximum negative.
Two complete cycles of the wave are shown on the CRO.
Where the wave goes above the zero line,
the current flows forwards.
Where the wave goes below the zero line
the current flows backwards.
If a direct current supply is connected to the
the display shows a single straight line
which represents a constant current in one direction only.
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