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Accurate Clock Pendulums$
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Robert J. Matthys

Print publication date: 2004

Print ISBN-13: 9780198529712

Published to Oxford Scholarship Online: January 2010

DOI: 10.1093/acprof:oso/9780198529712.001.0001

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Time error due to air pressure variations

Time error due to air pressure variations

(p.205) Chapter 30 Time error due to air pressure variations
Accurate Clock Pendulums

Robert James Matthys

Oxford University Press

The rate of a pendulum clock is affected by air pressure. As the air pressure increases, the clock slows down, and vice versa. The basic cause is that the pendulum floats in a sea of air, and when the density of the air changes, the effective weight of the pendulum changes by a small but significant amount. A pendulum's sensitivity to air pressure depends on bob shape and density, and is in the range of 0.2-0.4 second per day per inch of mercury. A pendulum clock is normally set to run true over some length of time, meaning a nominally zero time error is obtained at the average air pressure during that time period. This chapter shows that a clock's time error varies considerably with location. In addition, the predominant effect of air pressure is long-term time error, not short term, as any effects of one year or more in duration are considered long term.

Keywords:   pendulum clock, air pressure, clock rate, time error, location, bob

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