Double Revolving Magnet
   Paraphrasing Davis in the 1842 edition of the Manual of Magnetism, pp 106-107: 

   In this instrument there are two semicircular electro-magnets of the same size, both of which have freedom of motion. The lower semicircle is supported by a pivot entering the upright pillar below it; its own axis is hollowed to receive the pivot on which the upper semicircle revolves. The two white disks are commutators furnishing alternating connections to the two electromagnets.

   When the apparatus is connected to a battery, the two coils will spin in opposite directions. 

   This instrument, which cost $6.00 when new, is in the Collection of Historical Scientific Instruments at Harvard University.

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