426 



ALTERNATING CURRENTS 



hot vapor becomes a conductor of electric current. A large 

 amount of heat energy is developed in a very small space, result- 

 ing in a very high temperature. This heats the carbon tips to 

 incandescence and results in their emitting a very white light. 

 Because of its very high temperature, the arc is a very efficient 

 illuminant. 



The resistance of the arc itself varies nearly inversely as its 

 cross-section. If the carbons were connected directly across the 

 line, a slight increase of current would result in a greater cross- 

 section of arc which would reduce its resistance. The arc would 

 then take more current resulting in a still less resistance, etc. 



D.C. Supply 



FIG. 383. Direct-current arc and candlepower distribution curve. 



which would ultimately produce such an extremely low value of 

 resistance as to be practically a short-circuit. To prevent this 

 instability in multiple lamps, a series resistance R, called the 

 ballast, is necessary. The power loss in the ballast, reduces the 

 over-all efficiency of the lamp. In a 110-volt lamp, the drop 

 across the ballast is about 50 volts. 



Figure 383 shows the early type of open, direct-current arc. 

 A crater, formed in the positive carbon, becomes filled with 

 molten carbon. This electrode is at a higher temperature than 

 the negative one. Most of the light comes, not from the arc 

 itself, but from this hot incandescent positive crater, so that 

 the upper carbon should always be the positive one. Figure 

 383 indicates the light-distribution curve of this type of lamp. 



