Elihu Thomson and Edwin J. Houston in 1878 made a very successful and complete arc light system. Their dynamo was specially designed to fit the requirements of the series arc lamp. The Thomson-Houston machine was a bipolar, having an armature consisting of three coils, one end of each of the three coils having a common terminal, or “Y” connected, as it is called. The other end of each coil was connected to a commutator segment. The machine was to a great extent self-regulating, that is the current was inherently constant with fluctuating load, as occurs when the lamps feed or when the number of lamps burning at one time should change for any reason. This regulation was accomplished by what is called “armature reaction,” which is the effect the magnetization of the armature has on the field strength. Close regulation was obtained by a separate electro-magnet, in series with the circuit, which shifted the brushes as the load changed. As there were but three commutator segments, one for each coil, excessive sparking was prevented by an air blast. [Illustration: DIAGRAM OF T-H ARC LIGHTING SYSTEM.] The “T-H” (Thompson-Houston) lamp employed the shunt feed principle. The carbons were normally separated, being in most types drawn apart by a spring. A high resistance magnet, shunted around the arc, served to draw the carbons together. This occurred on starting the lamp and thereafter the voltage of the arc was held constant by the balance between the spring and the shunt magnet. As the carbon burned away the mechanism advanced to a point where a clutch was tripped, the carbons brought together, and the cycle repeated. Both the T-H and Brush systems were extensively used in street lighting, for which they were the standard when the open arc was superseded by the enclosed. OTHER AMERICAN ARC LIGHT SYSTEMS [Illustration: THOMSON-HOUSTON ARC LAMP, 1878. This is an early model with a single pair of carbons.] [Illustration: THOMSON DOUBLE CARBON ARC LAMP. This later model, having two pairs of carbons, was commercially used for many years. This lamp is in the collection of the Smithsonian Institution.] Beginning with about 1880, several arc light systems were developed. Among these were the Vanderpoele, Hochausen, Waterhouse, Maxim, Schuyler and Wood. The direct current carbon arc is inherently more efficient than the alternating current lamp, owing to the fact that the continuous flow of current in one direction maintains on the positive carbon a larger crater at the vaporizing point of carbon. This source furnishes the largest proportion of light, the smaller crater in the negative carbon much less. With the alternating current arc, the large crater is formed first on the upper and then on the lower carbon. On account of the cooling between alternations, the mean temperature falls below the vaporizing point of carbon, thus accounting for the lower efficiency of the alternating current arc. [Illustration: MAXIM DYNAMO.