Electro-Dynamic Revolving Ring
   "This instrument consists of a U-shaped steel magnet, fixed on a stand, in a vertical position, and a circular coil of insulated copper wire C, so arranged as to revolve on a vertical axis between the magnetic poles. The rotation is effected in a different manner from any previously mentioned. The polarity of the ring is reversed twice in each revolution, by means of a contrivance of Dr. Page's called a pole-changer, which is employed in many of the instruments to be hereafter described." (from the 1842 edition of the Manual of Magnetism, pp 96-97. 

   The pole-changer, shown at P in the cut, is our familiar split-ring commutator used on all small, two-pole direct-current motors today.

   The lettering on the other side of the magnet reads "Daniel Davis, Jr." and "Manufacturer". 

   Charles Grafton Page described this apparatus in an 1838 article in American Journal of Science. This particular piece of apparatus is at Harvard University.
   This later version of Page's Electro-Dynamic Revolving Ring seems not to be listed in the 1851 and earlier Manual of Magnetism. It is in the Harvard Collection of Historical Scientific Instruments.

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