XXXI. HOME-MADE HARMONOGRAPHS.
Have you ever heard of the harmonograph? If not, or if at the most you have
very hazy ideas as to what it is, let me explain. It is an instrument for
recording on paper, or on some other suitable surface, the figures
described by two or more pendulums acting in concert.
The simplest form of harmonograph is shown in Fig. 168. Two pendulums are
so suspended on points that their respective directions of movement are at
right angles to one another--that is, pendulum A can swing only north and
south, as it were, and pendulum B only east and west. On the top of B is a
platform to carry a card, and on the upper end of A a lever is pivoted so
as to be able to swing only vertically upwards and downwards. At its end
this lever carries a pen, which when at rest lies on the centre of the card
platform.
[Illustration: FIG. 168.--Simple Rectilinear Harmonograph.]
The bob, or weight, of a pendulum can be clamped at any point on its rod,
so that the rate or "period" of swing may be adjusted or altered. The
nearer the weight is brought to the point of suspension, the oftener will
the pendulum swing to and fro in a given time--usually taken as one minute.
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