Arc Lamp

The method first devised for producing the electric light consisted in bringing the carbon terminals of a sufficiently powerful generator into contact and then separating Brush Arc Lamp. them. The current continues to flow acro the gap, developing, in consequence of the great resistance there, a most intense heat, which raises the terminals to vivid incandescence. If the carbons be horizontal, the upward current of air causes the luminous stream to a ume an arched form; whence the name "electric arc." The electric are lamp is a device for holding the carbons of an are light and for maintaining the light constant by means of suitable mechanism. The latter function it is which has given the name "regulator" to such lamps. Two distinct operations must be performed by the regulator; first, it must separate the carbons to produce the light; and second, it must feed these carbons forward as fast as they are consumed, so as to preserve the light from extinction and to keep the length of the arc constant. It may be also nece ary, as in the case of focusing lamps, for the regulator to perform a third function; viz., that of keeping the arc constant in position. Moreover, lamps must be capable of operating independently, so as to be used in a circuit with others without being effected by their variations. In commercial lighting, lamps are usually arranged in series, and the Brush lamp may well serve as a type of this cla . In this a ring clutch, surrounding the upper carbon holder, and controlled by an electro-magnet in series with the carbons, lifts this holder as soon as the current pa es, and thus separates the carbons and produces the light. Moreover, this magnet is antagonized by a second one in shunt circuit with it, for the purposes of regulation; since, whenever the arc becomes too long and is in danger of extinction, the increased resistance thus developed throws more current into the antagonizing shunt magnet, enabling it to overcome the main magnet and so to allow the carbons to feed together. Since the result is due to the differential action of two magnets, one situated in the main and the other in the shunt circuit, such lamps are generally known as shunt differential lamps. As the positive carbon burns away about twice as rapidly as the negative one, an additional device must be applied in focusing lamps to feed the former twice as fast as the latter. Of arc lamps those used for search-lights are the most powerful. For this purpose the arc is placed in the focus of a parabolic IMG:content-0250.jpg:[graphic] mirror.