One-way suspensions are easily made. Two types, the point and knife-edge
respectively, are shown in Fig. 168 and the top part of Fig. 172. The point
suspension is most suitable for small rods and moderate weights; the
knife-edge for large rods and heavy weights which would tend to crush a
fine point.
[Illustration: FIG. 171.--Gimbal giving universal movement: point
suspension.]
Points should rest in cup-shaped depressions in a metal plate; knife-edges
in V-shaped grooves in a metal ring.
[Illustration: FIG. 172.--Knife-edge universal-motion gimbal.]
Screws turned or filed to a sharp end make convenient points, as they can
be quickly adjusted so that a line joining the points lies exactly at right
angles to the pendulum. The cups to take the points should not be drilled
until the points have been thus adjusted. Make a punch mark on the
bedplate, and using this as centre for one of the points, describe an arc
of a circle with the other. This will give the exact centre for the other
cup. It is evident that if points and cup centres do not coincide exactly
there must be a certain amount of jamming and consequent friction.
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