From this it is obvious that the rates of swing of the two pendulums can be
adjusted relatively to one another. If they are exactly equal, they are
said to be in unison, and under these conditions the instrument would trace
figures varying in outline between the extremes of a straight line on the
one hand and a circle on the other. A straight line would result if both
pendulums were released at the same time, a circle,[1] if one were
released when the other had half finished a swing, and the intermediate
ellipses would be produced by various alterations of "phase," or time of
the commencement of the swing of one pendulum relatively to the
commencement of the swing of the other.
[Footnote 1: It should be pointed out here that the presence of friction
reduces the "amplitude," or distance through which a pendulum moves, at
every swing; so that a true circle cannot be produced by free swinging
pendulums, but only a spiral with coils very close together.]
But the interest of the harmonograph centres round the fact that the
periods of the pendulums can be tuned to one another.
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