Editing Pendulum (section)

=== Coupled pendulums ===
[[File:Coupled oscillators.gif|frame|Two pendulums with the same period coupled by suspending them from a common support string. The oscillation alternates between the two.]]
[[File:Huygens synchronization of two clocks (Experiment).jpg|thumbnail|left|100px|Repetition of Huygens experiment showing synchronization of two clocks]]
{{main|Injection locking}}
In 1665 Huygens made a curious observation about pendulum clocks. Two clocks had been placed on his [[mantlepiece]], and he noted that they had acquired an opposing motion. That is, their pendulums were beating in unison but in the opposite direction; 180° [[out of phase]]. Regardless of how the two clocks were started, he found that they would eventually return to this state, thus making the first recorded observation of a [[Coupled oscillation|coupled oscillator]].<ref>{{cite web
 | last=Toon | first=John | date=2000-09-08
 | url=http://gtresearchnews.gatech.edu/newsrelease/PENDULUM.html
 | title=Out of Time: Researchers Recreate 1665 Clock Experiment to Gain Insights into Modern Synchronized Oscillators
 | publisher=Georgia Tech | access-date=2007-05-31 }}</ref>

The cause of this behavior was that the two pendulums were affecting each other through slight motions of the supporting mantlepiece. This process is called [[entrainment (physics)|entrainment]] or [[mode locking]] in physics and is observed in other coupled oscillators. Synchronized pendulums have been used in clocks and were widely used in [[gravimeter]]s in the early 20th century. Although Huygens only observed out-of-phase synchronization, recent investigations have shown the existence of in-phase synchronization, as well as "death" states wherein one or both clocks stops.<ref>{{cite journal|author1=A.L. Fradkov|author2=B. Andrievsky|title=Synchronization and phase relations in the motion of two-pendulum system|journal=International Journal of Non-linear Mechanics|volume=42|date=2007|issue=6 |pages=895–901|doi=10.1016/j.ijnonlinmec.2007.03.016 |bibcode=2007IJNLM..42..895F }}</ref><ref>{{cite book|author=I.I. Blekhman|title=Synchronization in science and technology|publisher=ASME Press|location=New York|date=1988}}</ref>