Arago`s rotations

Arago`s rotations is an observable phenomenon and effect discovered by Fran├žois Arago in 1824. In the effect, as a copper disc is rotated in its own plane, and if a magnetic needle be freely suspended on a pivot over the disc, the needle will rotate with the disc. If on the other hand the needle is fixed it will tend to retard the motion of the ....
Found on http://en.wikipedia.org/wiki/Arago`s_rotations
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