As is usual in any short explanation we do not tell the whole story of what are the complicating factors. Yet part of the fascination of the Foucault pendulum is how it
can measure what is a very, very tiny effect. Thus we could not resist telling you in the bottom half of the display what are some of the difficulties of making a
working Foucault pendulum and what are some of the clever ideas people have applied to circumvent these difficulties.

In 1851 the astronomer Jean Bernard Leon Foucault, while working on a conical pendulum clock to regulate a telescope drive, realized that with an ideal pendulum
supported at the north pole "the motion of the earth, which forever rotates from west to east, will become appreciable in contrast with the fixity of the plane of
oscillation". Although he further wrote that "when our latitudes are approached, the phenomenon becomes complicated in a way that is rather difficult to appreciate",
he did intuit that the effect would be proportional to the sine of the latitude. His first experiments were with a 2 m pendulum in his basement but he quickly went on to
construct a 67 m pendulum at the Pantheon in Paris. This pendulum created a sensation at the Paris exhibition in 1851 and generated both a flood of experiments
around the world and an enormous scientific literature. In the autumn of 1995, the 28 kgm iron sphere used by Foucault was dusted off and again rehung from the
Pantheon dome as in 1851.

In describing his experiments in 1851 Foucault noted he was aware of a 1837 paper on the deviation of projectiles in which Poisson used the calculations of his
student Coriolis from 1831 on accelerations in rotating frames of reference. Poisson thought the effect unobservable but Foucault clearly understood that "a
pendulum has the advantage of accumulating the effects". The experiment was crucial to the development of mechanics; it established the force concept of Coriolis as useful and the subsequent work cleared up misconceptions people had on the effect of the rotation of the earth on air flow in the atmosphere.

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