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
CRO,
the display shows a single straight
line
which represents a constant
current in one
direction only.
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