Editing Pendulum (section)

=== Atmospheric pressure ===
The effect of the surrounding air on a moving pendulum is complex and requires [[fluid mechanics]] to calculate precisely, but for most purposes its influence on the period can be accounted for by three effects:<ref name="BritannicaP540" /><ref>[https://books.google.com/books?id=TL4KAAAAIAAJ&pg=PA13 Poynting & Thompson, 1907, p.13-14]</ref>
* By [[Archimedes' principle]] the effective [[weight]] of the [[bob (physics)|bob]] is reduced by the buoyancy of the air it displaces, while the [[mass]] ([[inertia]]) remains the same, reducing the pendulum's acceleration during its swing and increasing the period. This depends on the air pressure and the density of the pendulum, but not its shape.
* The pendulum carries an amount of air with it as it swings, and the mass of this air increases the inertia of the pendulum, again reducing the acceleration and increasing the period. This depends on both its density and shape.
* Viscous [[air resistance]] slows the pendulum's velocity. This has a negligible effect on the period, but dissipates energy, reducing the amplitude. This reduces the pendulum's [[Q factor]], requiring a stronger drive force from the clock's mechanism to keep it moving, which causes increased disturbance to the period.
Increases in [[barometric pressure]] increase a pendulum's period slightly due to the first two effects, by about {{convert|0.11|s/day/kPa|s/day/inHg s/day/torr|lk=out|abbr=none}}.<ref name="BritannicaP540" /> Researchers using pendulums to measure the [[Gravity of Earth|acceleration of gravity]] had to correct the period for the air pressure at the altitude of measurement, computing the equivalent period of a pendulum swinging in vacuum. A pendulum clock was first operated in a constant-pressure tank by Friedrich Tiede in 1865 at the [[Berlin Observatory]],<ref>{{cite journal
  | last = Updegraff
  | first = Milton
  | title = On the measurement of time
  | journal = Science
  | volume = 15
  | issue = 371
  | pages = 218–219
  | date = February 7, 1902
  | url = https://books.google.com/books?id=O44CAAAAYAAJ&q=tiede+clock+observatory&pg=RA1-PA219
  | pmid = 17793345| doi = 10.1126/science.ns-15.374.218-a
  | s2cid = 21030470
  | access-date = 2009-07-13}}</ref><ref>{{cite book
  | last = Dunwoody
  | first = Halsey
  | title = Notes, Problems, and Laboratory Exercises in Mechanics, Sound, Light, Thermo-Mechanics and Hydraulics, 1st Ed
  | publisher = John Wiley & Sons
  | year = 1917
  | location = New York
  | page = 87
  | url = https://books.google.com/books?id=ZDe5XCIug_0C&pg=PA87
  }}</ref> and by 1900 the highest precision clocks were mounted in tanks that were kept at a constant pressure to eliminate changes in atmospheric pressure. Alternatively, in some a small [[aneroid barometer]] mechanism attached to the pendulum compensated for this effect.