Hi Tom:
One problem with adjusting the rate of a pendulum clock is that you
don't want to stop it to make fine adjustments. That's why the tray for
the micro weights. In the case of the Western Union Self Winding Clocks
you can slow down the pendulum by dropping candle wax on the bob.
http://www.prc68.com/I/SWCC.shtml
Have Fun,
Brooke Clarke
http://www.PRC68.com
Tom Van Baak wrote:
How does the pressure change the frequency?
Assuming a fixed mechanical impulse, an increase in
pressure will result in a slight decrease in amplitude
due to the increased friction of the air. Then because
of circular error this will result in a very slight change
in period (by a few tens to hundreds of ppm). Thus
you can use pressure as a fine-adjustment of rate.
Equivalently, if one does not maintain the pressure to
some level of stability, it will affect the stability of the
amplitude which will affect the stability of the period
which will affect the timekeeping performance of the
clock. The exact numerical relationship among these
factors depends on the design of the clock.
So as you can imagine open-case pendulum clocks
make good barometers as well as thermometers. And
that's one reason why all the best pendulum clocks
make use of a sealed, low pressure, chamber.
That does raise an interesting issue. How would you fine tune a
pendulum?
Typically one adjusts the effective length by placing
micro-weights to slightly move the center of gravity
of the bob. Larger adjustments are made by raising
or lowering the bob itself with the rating nut.
These technique is also used for vacuum pendulum
clocks but one does not want to do it more than once
or twice.
/tvb
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