Editing Pendulum (section)

=== Clock pendulums ===
{{main|Pendulum clock}}
{{multiple image
| align    = right
| image1   = Pendulum-with-Escapement.png
| caption1 = Pendulum and [[anchor escapement]] from a [[grandfather clock]]
| width1   = 100
| image2   = Anchor escapement animation 217x328px.gif
| caption2 = Animation of [[anchor escapement]], one of the most widely used escapements in pendulum clocks
| width2   = 140
}}

Pendulums in clocks (see example at right) are usually made of a weight or [[bob (physics)|bob]] ''<span style="color:red;">(b)</span>''  suspended by a rod of wood or metal ''<span style="color:red;">(a)</span>''.<ref name="Milham1945" /><ref>{{cite book
| last=Glasgow
| first=David
| title=Watch and Clock Making
| year=1885
| publisher=Cassel & Co.
| location=London
| pages = [https://archive.org/details/watchandclockma00glasgoog/page/n265 279]–284
| url=https://archive.org/details/watchandclockma00glasgoog}}</ref>  To reduce [[air resistance]] (which accounts for most of the energy loss in precision clocks)<ref>{{cite book
  | last = Matthys
  | first = Robert J.
  | title = Accurate Pendulum Clocks
  | publisher = Oxford Univ. Press
  | year = 2004
  | location = UK
  | page = 4
  | url = https://books.google.com/books?id=Lx0v2dhnZo8C&pg=PA4
  | isbn = 978-0-19-852971-2}}</ref>  the bob is traditionally a smooth disk with a lens-shaped cross section, although in antique clocks it often had carvings or decorations specific to the type of clock. In quality clocks the bob is made as heavy as the suspension can support and the movement can drive, since this improves the regulation of the clock (see [[#Accuracy of pendulums as timekeepers|Accuracy]] below). A common weight for [[seconds pendulum]] bobs is {{convert|15|lb|kg}}.<ref>[https://books.google.com/books?id=_78S_w3EBmAC&dq=matthys+%22common+bob+size&pg=PA13 Mattheys, 2004, p. 13]</ref>  Instead of hanging from a [[wikt:pivot|pivot]], clock pendulums are usually supported by a short straight [[spring (device)|spring]] ''<span style="color:red;">(d)</span>'' of flexible metal ribbon. This avoids the friction and 'play' caused by a pivot, and the slight bending force of the spring merely adds to the pendulum's [[restoring force]]. The highest precision clocks have pivots of 'knife' blades resting on agate plates. The impulses to keep the pendulum swinging are provided by an arm hanging behind the pendulum called the ''crutch'', ''<span style="color:red;">(e)</span>'', which ends in a ''fork'', ''<span style="color:red;">(f)</span>'' whose prongs embrace the pendulum rod. The crutch is pushed back and forth by the clock's [[escapement]], ''<span style="color:red;">(g,h)</span>''.

Each time the pendulum swings through its centre position, it releases one tooth of the ''escape wheel'' ''<span style="color:red;">(g)</span>''. The force of the clock's [[mainspring]] or a driving weight hanging from a pulley, transmitted through the clock's [[Wheel train (horology)|gear train]], causes the wheel to turn, and a tooth presses against one of the pallets ''<span style="color:red;">(h)</span>'', giving the pendulum a short push. The clock's wheels, geared to the escape wheel, move forward a fixed amount with each pendulum swing, advancing the clock's hands at a steady rate.

The pendulum always has a means of adjusting the period, usually by an adjustment nut ''<span style="color:red;">(c)</span>'' under the bob which moves it up or down on the rod.<ref name="Milham1945" /><ref>[https://books.google.com/books?id=Lx0v2dhnZo8C&pg=PA91 Matthys 2004], p.91-92</ref>  Moving the bob up decreases the pendulum's length, causing the pendulum to swing faster and the clock to gain time. Some precision clocks have a small auxiliary adjustment weight on a threaded shaft on the bob, to allow finer adjustment. Some [[Turret clock|tower clocks]] and precision clocks use a tray attached near to the midpoint of the pendulum rod, to which small weights can be added or removed. This effectively shifts the centre of oscillation and allows the rate to be adjusted without stopping the clock.<ref>[https://books.google.com/books?id=OvQ3AAAAMAAJ&pg=PA48 Beckett 1874], p.48</ref><ref>{{cite web
  | title = Regulation
  | website = Encyclopedia of Clocks and Watches
  | publisher = Old and Sold antiques marketplace
  | year = 2006
  | url = http://www.oldandsold.com/articles02/clocks-r.shtml
  | access-date = 2009-03-09}}</ref>

The pendulum must be suspended from a rigid support.<ref name="Milham1945" /><ref>[https://books.google.com/books?id=OvQ3AAAAMAAJ&pg=PA43 Beckett 1874], p.43</ref>  During operation, any elasticity will allow tiny imperceptible swaying motions of the support, which disturbs the clock's period, resulting in error. Pendulum clocks should be attached firmly to a sturdy wall.

The most common pendulum length in quality clocks, which is always used in [[grandfather clock]]s, is the [[seconds pendulum]], about {{convert|1|m|in|abbr=off}} long.  In [[mantel clock]]s, half-second pendulums, {{convert|25|cm|in|abbr=on}} long, or shorter, are used. Only a few large [[turret clock|tower clocks]] use longer pendulums, the 1.5 second pendulum, {{convert|2.25|m|ft|abbr=on}} long, or occasionally the two-second pendulum, {{convert|4|m|ft|abbr=on}} <ref name="Milham1945" /><ref>[https://archive.org/details/watchandclockma00glasgoog/page/n268 <!-- pg=282 --> Glasgow 1885], p.282</ref> which is used in [[Big Ben]].<ref>{{cite web
 |url=http://www.parliament.uk/about/livingheritage/building/big_ben/facts_figures/great_clock_facts.cfm 
 |title=Great Clock facts 
 |date=13 November 2009 
 |website=Big Ben 
 |publisher=UK Parliament 
 |access-date=31 October 2012 
 |location=London 
 |url-status=dead 
 |archive-url=https://web.archive.org/web/20091007101459/http://www.parliament.uk/about/livingheritage/building/big_ben/facts_figures/great_clock_facts.cfm 
 |archive-date=7 October 2009 
}}</ref>