Thompson's Apparatus
   This apparatus was invented by the American scientist Elihu Thompson (1853-1937) about 1900. He was one of the original developers of alternating current power systems, and his Thompson-Houston Company later merged with the Edison Company to form General Electric.

   The tall coil of the apparatus is driven by an alternating current. Conducting rings placed around this ring have eddy currents induced in them. In the picture the slanted aluminum ring will feel a repulsive force, and flies off unless restrained by the two black strings. A ring with a break in it will not have induced currents, and experiences no force.

   Eddy currents in water placed in the toroidal tank placed around the ring heats the water to boiling. The resulting steam causes a cork placed in the end of the curving silver pipe to pop out. 

   This apparatus, which has no maker's name, is in the museum of St. Patrick's College, Maynooth, County Kildare, Ireland.

   The catalogue published by Max Kohl of Chemnitz, Germany ca. 1900 listed this as an "Apparatus according to Elihu Thomson with complete accessories for demonstrating the repulsion which is exercised over well conducting metal masses placed in a strong magnetic field, which is produced by an alternating current ... 120 Marks [about $30]". The apparatus was described by Thomson in articles published in 1890 and 1891.

   This apparatus is in regular use in the physics lecture halls at the University of Texas in Austin.

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