Revolving Armature Engine
   "In this instrument there are several armatures [iron bars] fixed on the circumference of a vertical wheel, parallel to its axis. In [these examples] three are represented... On the poles of the electromagnet is secured a brass plate, from which arise two pillars to support the axis of the wheel: as the wheel turns, the iron bars in succession pass over the poles with their extremities very near to the them. ...On the shaft of the wheel, but not insulated from it, is the break-piece [commutator] consisting of a small metallic disk, from which projects, in a lateral direction, several pins, equal in number to the iron bars;... A silver spring connected with one end of the wire surrounding the electro- magnet, plays upon these pins...; the other end of this wire is soldered to the iron of the magnet, which brings it into metallic communication with the shaft by means of the brass plate and pillars. 

   The break-piece is arranged in such a manner, that the electro-magnet is 
charged when any of the iron bars is brought near it by the motion of the wheel. This bar is then attracted toward the poles; when it arrives at the plane of the magnet, the current is cut off, in consequence of the corresponding pin ... releasing the silver spring from its bearing. The armature being no longer attracted, the wheel moves on by its momentum until the next bar comes into the same position..."  (from the 1842 edition of the Manual of Magnetism, pp 110-111)

   The device at the right is at the Smithsonian Institution, and that at the left is at Oberlin College.

   This apparatus is usually called Froment's Motor, after Gustav Froment, a Parisian instrument maker, who designed similar motors from 1844 to 1862.

   The photograph below shows a piece of apparatus in the collection at Washington and Lee University. It is not in the 1842 or 1851 editions of the Manual of Magnetism, but shows typical Davis design features. It may be by Thomas Hall or Palmer and Hall, successors to Daniel Davis.

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