| "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".
| 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.