This “differentially controlled” arc lamp consisted of two slabs of carbon between which the arc played. In the original lamp the carbon slabs were mounted on pieces of wood held in place by bolts, adjustment being made by hitting the upper carbon slab with a hammer. This lamp is in the collection of the Smithsonian Institution.] [Illustration: WALLACE-FARMER DYNAMO, 1875. This was the first commercial dynamo used in the United States for arc lighting. This dynamo is in the collection of the Smithsonian Institution.] About 1875 William Wallace of Ansonia, Connecticut, made an arc light consisting of two rectangular carbon plates mounted on a wooden frame. The arc played between the two edges of the plates, which lasted much longer than rods. When the edges had burned away so that the arc then became unduly long, the carbon plates were brought closer together by hitting them with a hammer. Wallace became associated with Moses G. Farmer, and they improved this crude arc by fastening the upper carbon plate to a rod which was held by a clutch controlled by a magnet. This magnet had two coils in one, the inner winding in series with the arc, and outer one in shunt and opposing the series winding. The arc was therefore differentially controlled. [Illustration: WESTON’S ARC LAMP, 1876. This lamp is in the collection of the Smithsonian Institution.] They also developed a series wound direct current dynamo. The armature consisted of a number of bobbins, all connected together in an endless ring. Each bobbin was also connected to a commutator bar. There were two sets of bobbins, commutators and field poles, the equivalent of two machines in one, which could be connected either to separate circuits, or together in series on one circuit. The Wallace-Farmer system was commercially used. The arc consumed about 20 amperes at about 35 volts, but as the carbon plates cooled the arc, the efficiency was poor. The arc flickered back and forth on the edges of the carbons casting dancing shadows. The carbons, while lasting about 50 hours, were not uniform in density, so the arc would flare up and cast off soot and sparks. Edward Weston of Newark, New Jersey, also developed an arc lighting system. His commercial lamp had carbon rods, one above the other, and the arc was also differentially controlled. An oil dash pot prevented undue pumping of the carbons. His dynamo had a drum-wound armature, and had several horizontal field coils on each side of one pair of poles between which the armature revolved. The system was designed for about 20 amperes, each are taking about 35 volts. [Illustration: BRUSH’S DYNAMO, 1877. This dynamo was used for many years for commercial arc lighting.] [Illustration: DIAGRAM OF BRUSH ARMATURE. The armature was not a closed circuit. For description of its operation, see text.]