CH. XIII] USE OF BALLAST WITH ARC LAMPS 541 



the potential difference across the arc remains nearly the same 

 regardless of how much current is flowing. 



The commercial electric supply is designed to furnish current for 

 incandescent lamps, and is maintained at a nearly constant voltage 

 no matter how much current is used. The arc lamp, on the other 

 hand, is to be supplied by a constant current. If one were to 

 attempt to connect an arc directly to the terminals of the supply 



FIG. 295. LATERAL AND FACE VIEW OF THE RIGHT-ANGLE CARBON ARC WITH 

 20 AMPERES OF DIRECT CURRENT. 



No color screen was used with the lateral view so that the arc stream would 

 show. In the front view a color screen was used to bring out clearly the large 

 positive crater above and the small negative crater below. 



This figure is for comparison with the alternating current arc in fig. 296. 



To bring out the carbons, an additional exposure was made as for fig. 292- 

 293- 



line without an intermediate rheostat, as soon as the two electrodes 

 were brought in contact an extremely large current would flow. 

 Theoretically, this current would be infinite, but practically the 

 flow is limited by the very small resistance of the supply wires and 

 the capacity of the dynamo. In a modern installation the current 

 would be immediately interrupted by the circuit breakers and burn- 



