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 _______________________________________________ time-nuts mailing list -- [email protected] To unsubscribe, go to https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts and follow the instructions there.
