In making a knife-edge, such as that shown in Fig. 172, put the finishing
touches on with a flat file drawn lengthwise to ensure the edge being
rectilinear. For the same reason the V slots in the ring support should be
worked out together. If they are formed separately, the chances are against
their being in line with one another.
Gimbals, or universal joints, giving motion in all directions, require the
employment of a ring which supports one pair of edges or points (Fig. 172),
and is itself supported on another pair of edges or points set at right
angles to the first. The cups or nicks in the ring should come halfway
through, so that all four points of suspension shall be in the same plane.
If they are not, the pendulum will not have the same swing-period in all
directions. If a gimbal does not work with equal freedom in all ways, there
will be a tendency for the pendulum to lose motion in the direction in
which most friction occurs.
By wedging up the ring of a gimbal the motion of the pendulum is changed
from universal to rectilinear. If you are making a harmonograph of the type
shown in Fig.
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