THE PROGRESS OF THE ELECTRIC LIGHT 9 



a small crater which is raised to an intense white heat : the 

 negative carbon burns to a sharp point and only a very 

 small area on this point is at a white heat ; the bulk of 

 the light therefore comes from the positive carbon. In 

 an alternating current lamp both carbons are cratered and 

 the light is emitted practically equally from each. The enclosed 

 arc lamp differs from the open arc in that the carbons are 

 enclosed in a fairly air-tight globe which, as they burn, becomes 

 filled with carbon dioxide : in this atmosphere the carbons 

 burn away much more slowly and the ends of both carbons 

 are flat instead of pointed. A pair of carbons burning in such 

 an enclosure will last ten or twelve times as long as similar 

 carbons burning in the open air. 



The improvement in arc lamps until the production of the 

 flame lamp was but slight. As more experience was gained in 

 the manufacture of carbons their quality was steadily improved, 

 the ordinary commercial carbon of to-day being a very high- 

 grade article both as regards purity and homogeneity, the two 

 principal desiderata. At the same time there was naturally a 

 steady improvement in the mechanism of the lamp by which the 

 feeding of the carbons is controlled in order to maintain a 

 constant length of arc and the length of arc regulated to correct 

 slight fluctuations in the pressure of the supply. The only 

 radical improvement was the introduction, fairly early in the 

 history of arc lighting, in place of a homogeneous solid rod, of 

 the cored carbon, in which a central canal in the carbon is filled 

 with a special coring material consisting of finely ground carbon 

 and potassium silicate. Being far more volatile than carbon, the 

 potassium silicate vaporises into the arc, lowering its resistance 

 and producing a marked steadying effect. The general practice 

 for open arcs is to use a cored positive with a solid negative 

 carbon for direct current and two cored carbons for alternating 

 current : for direct current enclosed lamps two solid carbons 

 are used and for alternating current one cored and one solid. 



Compared with that of the carbon filament lamp, the efficiency 

 of the arc lamp is high. The consumption in watts per candle 

 is about 1*4 for open type arcs using standard quality carbons 

 but by using the highest grade carbons containing a large 

 percentage of lamp-black the consumption is reduced to about 

 ii watts per candle. The enclosed arc is less efficient, the 

 consumption being about 2-3 watts per candle. The enclosed 



