540 USE OF BALLAST WITH ARC LAMPS [Cn. XIII 



With a metallic wire, the resistance is nearly constant, and the 

 potential difference is greater the greater the current flowing. Any 

 change in resistance is due to the rise of temperature when a current 

 is flowing. The higher the temperature, the greater the resistance. 

 An arc, on the other hand, has no definite resistance, but its resist- 

 ance varies with the current flowing. This variation is such that 



A B 



FIG. 294. FACE AND LATERAL VIEWS OF THE RIGHT-ANGLE CARBON ARC 

 WITH 10 AND WITH 2O AMPERES OF DIRECT CURRENT. 



A With 10 amperes, B with 20 amperes of direct current. 



The size of the crater in the two cases is very strikingly brought out. 



The middle figures had an additional exposure to bring out the carbons (see 

 fig. 292-293), while the lateral views above and the front views below had only 

 an instantaneous exposure. 



The positive crater above and the negative crater below are clearly brought 

 out in all the pictures (see fig. 292). 



