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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

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

Robert James Matthys

Publisher:
Oxford University Press
DOI:10.1093/acprof:oso/9780198529712.003.0030

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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