The arc was controlled by an electro-magnet which held an iron tube to which the upper carbon was fastened.] [Illustration: SLATER AND WATSON’S ARC LAMP, 1852. Clutches were used for the first time in this arc lamp to feed the carbons.] In the same year (1852) Slater and Watson obtained an English patent on an arc lamp in which the upper carbon was movable and held in place by two clutches actuated by electro-magnets. The lower carbon was fixed, and normally the two carbons touched each other. When current was turned on, the electro-magnet lifted the clutches which gripped the upper carbon, pulling it up and striking the arc. This was the first time that a clutch was used to allow the carbon to feed as it became consumed. Henry Chapman, in 1855, made an arc in which the upper carbon was allowed to feed by gravity, but held in place by a chain wound around a wheel. On this wheel was a brake actuated by an electro-magnet. The lower carbon was pulled down by an electro-magnet working against a spring. When no current was flowing or when the arc went out, the two carbons touched. With current on, one electro-magnet set the brake and held the upper carbon stationary. The other electro-magnet pulled the lower carbon down, thus striking the arc. None of these mechanisms regulated the length of the arc. It was not until 1856 that Joseph Lacassagne and Henry Thiers, Frenchmen, invented the so-called “differential” method of control, which made the carbons feed when the arc voltage, and hence length, became too great. This principle was used in commercial arc lamps several years afterward when they were operated on series circuits, as it had the added advantage of preventing the feeding of one arc lamp affecting another on the same circuit. This differential control consists in principle of two electro-magnets, one in series with, and opposing the pull of the other which is in shunt with the arc. The series magnet pulls the carbons apart and strikes the arc. As the arc increases in length, its voltage rises, thereby increasing the current flowing through the shunt magnet. This increases the strength of the shunt magnet and, when the arc becomes too long, the strength of the shunt becomes greater than that of the series magnet, thus making the carbons feed. [Illustration: DIAGRAM OF “DIFFERENTIAL” METHOD OF CONTROL OF AN ARC LAMP. This principle, invented by Lacassagne and Thiers, was used in all arc lamps when they were commercially introduced on a large scale more than twenty years later.]