PDF Version - The American Watchmakers-Clockmakers Institute

horological time bases
by Jean-Felix Perotto, CSEM, Neuchâtel, Switzerland
slave pendulum, itself synchronized with the master
pendulum. The transmission of time to users was
electrically ensured by the slave pendulum.
Shortt’s pendulum equipped chronometry observatories
the world over and transmitted the most
accurate time in its day. Its stability, around 0.3
seconds per year, was such that for the first time
in history it became possible to measure the fluctuations
of the Earth’s annual rotation speed on its
axis. Using the relation (4), the quality factor of this
pendulum clock can be estimated at several tens of
millions, indeed far superior to that of quartz resonators
housed in our electronic watches.
The pendulum clock, as an horological time base,
displayed two major defects: its resistance to miniaturization,
and the difficulty of transportation due
to its sensitivity to vibration and dependence on its
geographical positioning (the Earth is not a sphere
and it rotates). Therefore, from 1930 the chronometric
pendulum clock was replaced by more stable
versions and relegated to the ranks of decorative
clocks.
Torsion pendulums and balance-springs
The 19th century saw the introduction of clocks with
a time base constituted by a torsion pendulum. This
type of resonator no longer relied upon the Earth’s
gravity but used a rotating heavy weight attached
to a torsion wire. The Q-factor of such resonators
was excellent, although inferior to that of Shortt’s
pendulum clock, as the energy dissipation in the
torsion wire cannot be entirely cancelled.
In 1928, the Frenchman Jean-Léon Reutter built
the first model of the torsion pendulum, the Atmos
Clock, on the basis of his patent. Still manufactured
today, this mythical clock has the particularity of
drawing its energy source from infinitesimal temperature
and atmospheric pressure variations. Such
a performance, totally ecological before its day,
is made possible according to the relation (3), on
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Horological Times November 2010 22