176 Introduction to the Study of Science 



Types of arc lamps. The carbons are arranged in various 

 ways. They may be vertically superposed as in the old arc 

 lamp and in some of the latest flaming arc lamps, or they may 

 converge (Fig. 52) as the two sides of an acute triangle; or 

 again they may be made, as in the electric 

 lamp of a stereopticon (Fig. 50), to meet at 

 an angle of 45. The light of the flaming arc 

 when produced by carbons that incline and 

 converge is thrown downward rather than 

 horizontally. The need of horizontal distribu- 

 F i G 5 2 A n tion nas l 6 ^ to the invention of the flaming arc 

 arrangement of the with the carbons, as in the old-style arc lamp, 

 flaming arcTight C Composition of carbons. The carbons used 

 in the flaming arc light differ from those of the 

 ordinary arc in one important respect. Certain metals are com- 

 bined with the carbons for the flaming arc, which give a different 

 quality and intensity of light. The most common of the 

 flaming arc lights is the " yellow." This is produced by making 

 the core of the carbon of calcium salts. The usual yellow flam- 

 ing arc gives from 2500 to 3000 candle power. The so-called 

 red light is produced by a core composed of strontium salts ; 

 and the pearl white light by barium salts. In these arcs the 

 metals are vaporized and furnish about thirty per cent, and the 

 vapor of the carbon about seventy per cent of the light. In 

 one style of the arc lamp the upper rod is made of hard copper 

 instead of carbon and lasts from 7000 to 8000 hours. The 

 lower rod is an iron tube filled with the mineral magnetite, 

 which burns for about 200 hours. 



The recent improvements in the tungsten incandescent 

 lamp have made it a competitor in street illumination with the 

 arc lamp of all types. In many cities the artistic lamp stand 

 with a group of five or more lamps is rapidly replacing the glar- 

 ing arc and giving a lighting effect which the most highly im- 

 proved arc cannot produce. 



70. Industrial illumination. The illumination of modern 



