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point it reverses and commences to flow in the opposite .
direction, rising gradually and then dying away again.
This is repeated a definite number of times per second;
when the current rises from zero, reverses and returns to
zero, it is said to pass through a cycle.
The part of the curved line from a to b in Figure 88 repre-
sents the first part of the current, when it is rising. From






FIG. 88. — Graphic Representation of a Direct and an Alternating Current:

b to c represents its fall. The point at which the curved line
crosses the straight line is zero. At c the current crosses the
line and commences to flow in the opposite direction until
it reaches d, at which point it dies away and again crosses
the line to flow in its original direction and repeat the cycle.

In electrical parlance, that part of the current from a to c
or from c to e is known as an alternation. From a to e is
called a cycle.

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