ARC IN AIR BETWEEN OTHER SUBSTANCES 57 



The great advantage of these arcs is that they give much 

 more light for the power consumed than carbon arcs. 

 They also have two disadvantages. One of these is the 

 color of the light given. With the " white" flaming-arc 

 carbons now on the market this difficulty has been largely 

 obviated, the light from these being nearly the same as 

 that from common carbons. However, what is thus 

 gained in color is lost to some extent in efficiency, the 

 watts per candle for the "white" flaming arc being some- 

 what greater than those for the "yellow" flaming arc. 



The chief disadvantage of the flaming arc is that the 

 carbons are more rapidly consumed than those of the com- 

 mon carbon arc, and very much more rapidly than those 

 of the enclosed arc, and require more frequent renewal. 

 For this reason the flaming arc has not been widely used 

 in this country where the cost of labor is high. In Europe 

 they have been very extensively used. 



Considerable progress is, however, being made in obviat- 

 ing this difficulty by special forms of lamps. In many of 

 these the air is partially excluded, as in the enclosed arcs, 

 and ventilating flues which keep the solid products of com- 

 bustion from being deposited on the surrounding globe are 

 provided. The life of the electrodes has by this means 

 been increased to 80 or even 100 hours. 



The life of the electrodes can be increased by making 

 either the electrodes longer or their diameters greater. In- 

 crease in length, however, leads to an unsightly and clumsy 

 lamp, while increase in diameter leads to unsteadiness. 

 Increased life can also be attained by using carbons which 

 approach in composition more nearly to the common car- 

 bon, but such increase leads to a decided loss in efficiency. 



The potential difference needed to maintain the flaming 



