| "The points of the wheel, R, dip into mercury contained
in a groove hollowed in the stand. A more rapid revolution will be obtained
if a small electro-magnet be substituted for a steel magnet, as is shown
in the cut. The electro-magnet is fixed to the stand, and included in the
circuit with the spur-wheel, so that the current flows through them in
succession. Hence the direction of the rotation will not be changed by
reversing that of the current; since the polarity of the electromagnet
will also be reversed."
(from the 1842 edition of the Manual of Magnetism, pg 94)
Other examples of Barlow's Wheel are on another page.
Both the single and double wheel models are in the Harvard University Collection of Historical Scientific Instruments.
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