| 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 |
