CH. XIII] 



USE OF BALLAST WITH ARC LAMPS 



543 



be 10 amperes, V/O = A. Now suppose the arc length were 

 changed say by bringing the electrodes in contact. In this case 

 there would be the full line voltage, no volts across the rheostat 

 and the current would be 1 10/6 = 18.3 amperes. Suppose the arc 

 length were increased until the potential at the arc was 60 volts. 

 The potential across the rheostat would then be no 60 = 50 

 volts. The current would then be 50/6 = 8.2 amperes. In this 

 example the conditions are what is known as stable, that is, as the 

 arc length is decreased the current is increased, but does not reach 

 an infinite value, and as the arc length is increased the current 

 decreases but it does not become zero. 



FIG. 297. 



LATERAL AND FACE VIEWS OF AN INCLINED CARBON ARC WITH 20 

 AMPERES OF DIRECT CURRENT. 



This picture shows that with the inclined carbons in proper position, the 

 positive crater on the upper carbon faces toward the condenser. It is evident 

 also that as the carbon burns away the crater will get farther and farther above 

 the principal axis of the projection apparatus. 



An additional exposure was made to bring out the carbons as with fi'g. 292- 

 293- 



