The prime essential in a well working harmonograph is a properly
constructed universal joint. Where such a joint is made with
pivots for its bearings, one pair of pivots are very liable to
have more friction than the other, which retards the movement and
causes the harmonograph to undergo a continuous change of axis. To
obviate this difficulty, the joint should be made similar to those
used on scales. The general appearance of such a joint is shown in
the first illustration, Fig. 1. Stirrups A and B are made of 7/8
by 1/4-in. metal. Holes are drilled in each end of these stirrups
and filed out as shown at C. The two holes shown in the center of
the stirrup A are drilled to fasten the apparatus to the ceiling.
Two corresponding holes are drilled in B to fasten the long
pendulum F to the joint. The cross of the joint D has the ends
shaped as shown at E. The rounded shoulder on E is to prevent the
cross from becoming displaced by a jar or accident. The ends of
the cross are inserted through the holes C of the stirrups, then
slipped back so the knife edges engage in the V-shaped holes of
the stirrups.
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