A second solution and the one adopted here is to introduce a Charron
ring; this is the large brass ring our pendulum strikes near the end of
each swing. Ideally the
Charron ring would stop the pendulum completely and then release it from rest so that the motion is again like that shown in Fig. 2. Such an ideal ring would not
affect the Coriolis turning of the plane of oscillation because the Coriolis force acts only while the bob is in motion and produces most of its deflection of the bob as
the bob passes near its equilibrium position. A real Charron ring however cannot stop the pendulum; it can only slow it down by a combination of impact and friction
forces. Now if the bob strikes the ring very gently, friction between bob and ring will be inadequate to reduce the tangential motion of the bob substantially and the
elliptic motion will remain. If the bob is made to strike the ring very hard in an attempt to enhance the damping of the elipticity then one cannot guarantee the large
impact forces do not on their own cause a deflection of the orbit and thus give a contribution to the precession of the pendulum.
A compromise must be struck which leaves an elipticity in the motion
with b/a ratio considerably larger than that shown in Fig. 2. As
a result, the restoring force on
the bob does have a substantial component perpendicular to the plane of oscillation and since this force is not precisely harmonic, it will lead to an additional
deflection of the bob and an additional precession. Now Crane's experimental discovery in 1981 was that to eliminate this residual precession one need only modify
the force law to make it harmonic on average. He also noted that this could be done very simply with a pair of magnets, one on the bob and one on a fixed support
underneath. The magnitude of the force modification is adjusted by changing the magnet separation.
This extra magnetic force has very little effect on the mean precession
of the pendulum which is approximately we'
as driven by the Coriolis force. The essential point
is that elipticity driven by the anisotropy alternates between positive and negative values as the pendulum sweeps between the two axes where the frequency takes
on its extreme values w1 and w2. Then, as the elipticity alternates, so too does the additional precession resulting from the anharmonic restoring forces. The total
precession rate is alternately greater than and less than we' and the magnetic force is adjusted to minimize these fluctuations and allow the true Coriolis precession
we' to be observed at any time of day.
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