Foucault's Disk

   Metallic conductors passing rapidly through magnetic fields have Eddy Currents induced in them. Since the conductors have a finite resistivity, there will be ohmic heating of the conductors and their temperature will increase.

   This effect was studied by Leon Foucault (1819-1868), who is better known for his 1851 demonstration of the rotation of the earth by what is now called Foucault's Pendulum, his measurement of the relative speeds of light in air and water (the subject of his doctoral thesis submitted in 1853) and his invention of the gyroscope in 1852.

   In Foucault's experiments, a geared-up copper wheel was rotated in the gap of an electromagnet. With no current through the magnet coils the wheel rotated easily. As soon as the magnet was energized, resistance to rotation was felt and the copper disk started to heat up. In one experiment, the temperature of the disk rose from 10ºC to 61ºC. In the 19th century, eddy currents were sometimes called Foucault currents.

   The apparatus in the Washington and Jefferson collection is by Max Kohl of Chemnitz.
                          Washington and Jefferson College                                                  Smithsonian Institution

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