Arago's Disk
   In this device, a copper disk is rotated rapidly with a hand crank and a step-up pulley system. Balanced on a pivot above the center of the disk is a compass needle. The motion of the needle relative to the highly conducting copper disk induces eddy currents in the disk. In turn, these eddy currents produce a torque on the magnetic needle, which starts to rotate. The presence of eddy currents may be inferred from the fact that a copper disk with radial slots cut in it produces little effect; the slots interrupt the eddy currents.

   The inverse effect also occurs: a spinning bar magnet will cause a suspended copper disk to rotate. 

   The French physicist, Dominique Arago (1786-1853) discovered this effect in 1825 while studying the observation that the proximity of a piece of copper to a compass needle reduces the effect of the earth's magnetic field on the needle.

   The Maynooth apparatus is by Yeates & Son of Dublin, and is ca. 1877. The Smithsonian apparatus is by Queen and cost $16.50 in 1889.

                  St. Patrick's College, Maynooth, Ireland 
 
                             Smithsonian Institution
   The example at the right of Arago's Wheel from the Garland Collection of Classical Physics Apparatus at Vanderbilt University is unmarked. The copper disk is 22 cm in diameter and the compass needle is 15 cm long. The base is made of walnut.
                                                                                        Vanderbilt University
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