The Foucault pendulum (pronounced /fuːˈkoʊ/ "foo-KOH"), or Foucault's pendulum, named after the French physicist Léon Foucault, was conceived as an experiment to demonstrate the rotation of the Earth.

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The experimental apparatus consists of a tall pendulum free to oscillate in any vertical plane.

The direction along which the pendulum swings rotates with time because of Earth's daily rotation.

the experimental apparatus consists of a tall pendulum free to oscillate in any verical plane.

the direction along which the pendulum swings rotates with time because of earths daily rotation.

实验装置由一个高大自由振荡摆在任何垂直平面。该方向的钟摆摆动旋转随着时间的推移，因为地球每天自转。

第一次公开展出的福柯摆发生在1851年2月在经络室巴黎天文台。几周后，福柯，他最有名的钟摆暂停时，他的28公斤鲍勃的67米长的电线从圆顶在巴黎先贤祠。这架飞机的钟摆的摆动顺时针旋转11 ° ，每小时作出充分循环三十二点七小时

The first public exhibition of a Foucault pendulum took place in February 1851 in the Meridian Room of the Paris Observatory. A few weeks later, Foucault made his most famous pendulum when he suspended a 28-kg bob with a 67-metre wire from the dome of the Panthéon in Paris. The plane of the pendulum's swing rotated clockwise 11° per hour, making a full circle in 32.7 hours.

In 1851 it was well known that Earth rotated: observational evidence included Earth's measured polar flattening and equatorial bulge.

1851年，众所周知，地球旋转：包括地球观测证据的测量极地和赤道扁平隆起。

然而，福柯的钟摆是第一个动态证明轮换一个容易看到的实验，并创造了轰动学术界和整个社会

However, Foucault's pendulum was the first dynamic proof of the rotation in an easy-to-see experiment, and it created a sensation in the academic world and society at large.

At either the North Pole or South Pole, the plane of oscillation of a pendulum remains fixed with respect to the fixed stars while Earth rotates underneath it, taking one sidereal day to complete a rotation. So relative to Earth, the plane of oscillation of a pendulum at the North or South Pole undergoes a full clockwise or counterclockwise rotation during one day, respectively. When a Foucault pendulum is suspended on the equator, the plane of oscillation remains fixed relative to Earth. At other latitudes, the plane of oscillation precesses relative to Earth, but slower than at the pole; the angular speed, α (measured in clockwise degrees per sidereal day), is proportional to the sine of the latitude, φ:

http://upload.wikimedia.org/math/4/d/9/4d94bc5d46b8701a52c39f8f30d7f179.png

Here, latitudes north and south of the equator are defined as positive and negative, respectively. For example, a Foucault pendulum at 30° south latitude, viewed from above by an earthbound observer, rotates counterclockwise 180° in one day.

In order to demonstrate the rotation of the Earth without the philosophical complication of the latitudinal dependence, Foucault used a gyroscope in an 1852 experiment. The gyroscope's spinning rotor tracks the stars directly. Its axis of rotation is observed to return to its original orientation with respect to the earth after one day whatever the latitude, not subject to the unbalanced Coriolis forces acting on the pendulum as a result of its geometric asymmetry.

A Foucault pendulum requires care to set up because imprecise construction can cause additional veering which masks the terrestrial effect. The initial launch of the pendulum is critical; the traditional way to do this is to use a flame to burn through a thread which temporarily holds the bob in its starting position, thus avoiding unwanted sideways motion. Air resistance damps the oscillation, so Foucault pendulums in museums often incorporate an electromagnetic or other drive to keep the bob swinging; others are restarted regularly. In the latter case, a launching ceremony may be performed as an added show.