Barlow's Wheel
   "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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