The edges of the hole had been rounded off
carefully to prevent fraying of the threads. (Fig. 177.) The over-all
length of the pendulum, reckoning from the point of suspension, is 20
inches. The weights of the lower pendulum are several in number, ranging
from l lb. to 3 lbs.
[Illustration: FIG. 177.--Suspension for lower weight of Twin Elliptic
Harmonograph.]
Working the Harmonograph.--A preliminary remark is needed here. Harmonies
are, as we have seen, a question of ratio of swing periods. The larger the
number of swings made by the more quickly moving pendulum relatively to
that of the slower pendulum in a given time, the higher or sharper is the
harmony said to be. Thus, 1:3 is a higher harmony than 1:2, and 2:3 is
lower or flatter than 3:8.
The tuning of a harmonograph with independent pendulums is a simple matter.
It is merely necessary to move weights up or down until the respective
numbers of swings per minute bear to one another the ratio required. This
type of harmonograph, if made of convenient size, has its limitations, as
it is difficult to get as high a harmonic as 1:2, or the octave with it,
owing to the fact that one pendulum must in this case be very much shorter
than the other, and therefore is very sensitive to the effects of friction.
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