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cycles being so numerous that they are hardly distinguishable. And
this time it is fully modulated; the sample is sufficient to show two
whole cycles of modulation frequency. The local oscillation, which
is certain to be much stronger, and of course is unmodulated, is
shown at b. When a is added to it (c), nothing happens to it at the
troughs of modulation, where a is zero, but elsewhere the cycles of a
alternately strengthen and weaken those of b, causing its amplitude
to vary at the beat frequency, in the manner shown. The amplitude
of beating is proportional to the amplitude of and the frequency of
beating is equal to the difference