Electro-dynamic Revolving Ring
   "The mutually attractive and repulsive action of currents may be made to produce a revolution analogous to some of those strictly called electro-magnetic; as in the instrument represented [here], which consists of a coil of insulated wire B fitted to rotate on a vertical axis within a larger one A, mounted on a brass pillar. The inner coil has a pole-changer [commutator] fixed to its axis of motion for the purpose of reversing the current twice in each revolution. The current may traverse the two coils in succession, or be divided between them [serial or parallel connections], but its direction must be changed only in B. 

   The coil B being placed at right angles to A, and the cups on the stand connected with the galvanic battery, the faces of each coil immediately exhibit north and south polarity... and B is obliged to make a quarter of a revolution in order to bring its north pole within the
north pole of A, the two coils corresponding in direction. As soon as B reaches this position, the current is reversed by means of the pole-changer, and its south pole now being within the north pole of A, it continues to move on in the same direction" -- from the 1841 Manual of Magnetism, pp 130-131.

   Davis also follows through on the opening part of the quote, and explains how the forces between two parallel, current-carrying wires will cause the rotation. The physics of the situation is more obvious in the electro-dynamic revolving rectangle.

   The apparatus cost $6.00 in 1842, and is in the Middlebury College collection.It is missing its inner coil.

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