It may thus
be made, without difficulty, to vary from two to three tenths of a
millimeter to three or four millimeters or more.
But it is not sufficient to cause the amplitude to vary; it is necessary
to measure it and to keep it constant at the value desired.
[Illustration: FIG. 5]
The measurement is effected by the aid of a very simple apparatus that I
have before described under the name of the _vibrating micrometer_. This
is a small square of paper carrving a design like that shown in Fig. 5,
and which is seen in Fig. 4 glued to one of the masses, M, which serve
to vary the number of the instrument's vibrations. This figure is in
fact, an angle, one of whose sides is graduated into millimeters, for
example, and the other forms the edge of a wide black band. The apex of
the angle is above and the divided side is perpendicular to the
direction of the vibrations.
Under such conditions, when the fork is vibrating, the apex of the
angle, by virtue of the persistence of impressions upon the retina,
_seems_ to advance along the graduation in measure as the amplitude of
the vibrations increases. If an angle has been drawn such that the slope
of one of its sides to the other is one-tenth, it is easy to see that
for each millimeter passed over _apparently_ by the apex of the angle,
the amplitude will increase by two-tenths of a millimeter.
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