Demonstration Apparatus, Rotator - coils rotate between arms of horseshoe magnet, simple commutator, brass fittings. This unit is similar to 1989.0743.340 and 1988.0288.01. Univ. of Virginia property marking: "H-10" Ref: Daniel Davis, Manual of Magnetism, (Boston 1847), 211-213, fig. 146.
From Davis, 1847: "Revolving Electro-Magnet. In the instrument represented in Fig. 146, a steel U-magnet is fixed in a vertical position, and a small straight bar of soft iron, enclosed in a helix, is so arranged as to revolve between its poles. The two extremities of the insulated wire surrounding this electromagnet, are connected respectively with the segments of a pole-changer on the shaft. The silver springs, which press upon the pole-changer, are attached to two stout brass wires, passing through the brass arch surmounting the U-magnet, but insulated from it by the intervention of ivory or horn; each of these wires supports a brass cup for connection with the battery. These springs must be so placed with regard to the segments, that the poles of the revolving bar shall be reversed at the moment when it is passing the poles of the fixed magnet.
On making connection with the battery, when the bar is at right angles to the plane of the magnet, it immediately acquires a strong polarity. Its north pole is then attracted by the south pole of the steel U-magnet and repelled by the north pole. The south pole of the bar, on the contrary, is repelled by the similar pole of the upright magnet, and attracted by its opposite pole. These four forces conspire in bringing the electro-magnet between the poles of the U-magnet. When it reaches this position, each segment of the pole-changer leaves the spring with which it was in contact, and passes to the other. As the bar is moving past the poles by the momentum it has gained, its magnetism is destroyed for a moment, and immediately restored in the opposite direction. Each pole of the bar is now repelled by that pole of the permanent magnet which it has just passed, and attracted by the opposite one; it consequently moves on, the polarity being reversed twice in each revolution."
National Museum of American History