Charles F. Brush made a very successful arc lighting system in 1878. His dynamo was unique in that the armature had eight coils, one end of each pair of opposite coils being connected together and the other ends connected to a commutator segment. Thus the armature itself was not a closed circuit. The machine had two pairs of horizontal poles between which the coils revolved. One end of the one pair of coils in the most active position was connected, by means of two of the four brushes, in series with one end of the two pairs of coils in the lesser active position. The latter two pairs of coils were connected in multiple with each other by means of the brushes touching adjacent commutator segments. The outside circuit was connected to the other two brushes, one of which was connected to the other end of the most active pair of coils. The other brush was connected to the other end of the two lesser active pairs of coils. The one pair of coils in the least active position was out of circuit. The field coils were connected in series with the outside circuit. [Illustration: BRUSH’S ARC LAMP, 1877. The carbons were differentially controlled. This lamp was used for many years. This lamp is in the collection of the Smithsonian Institution.] Brush’s arc lamp was also differentially controlled. It was designed for about 10 amperes at about 45 volts. The carbons were copper plated to increase their conductivity. Two pairs of carbons were used for all-night service, each pair lasting about eight hours. A very simple device was used to automatically switch the arc from one to the other pair of carbons, when the first pair was consumed. This device consisted of a triangular-shaped piece of iron connected to the solenoid controlling the arc. There was a groove on each of the outer two corners of this triangle, one groove wider than the other. An iron washer surrounded each upper carbon. The edge of each washer rested in a groove. The washer in the narrow groove made a comparatively tight fit about its carbon. The other washer in the wider groove had a loose fit about its carbon. Pins prevented the washer from falling below given points. Both pairs of carbons touched each other at the start. When current was turned on, the solenoid lifted the triangle, the loose-fitting washer gripped its carbon first, so that current then only passed through the other pair of carbons which were still touching each other. The further movement of the solenoid then separated these carbons, the arc starting between them. When this pair of carbons became consumed, they could not feed any more so that the solenoid would then allow the other pair of carbons to touch, transferring the arc to that pair. [Illustration: THOMSON-HOUSTON ARC DYNAMO, 1878. This dynamo was standard for many years. This machine is in the collection of the Smithsonian Institution.]