538 



THE ELECTRIC ARC 



[Ca. XIII 



the change in voltage is almost entirely due to the change in the 

 length of the arc stream. 



When the arc is of medium length, as for use in projection, the 

 potential difference between the two carbons averages about 55 

 volts. This would mean that there is a drop of 32 volts between 

 the positive carbon and the upper end of the arc stream, a drop of 

 14 volts between the upper and lower ends of the arc stream, and 9 

 volts between the lower end of the arc stream and the negative 

 carbon. 



If the electrodes are made of other substances than carbon, the 

 potential drop is differently distributed. Thus in the "Luminous" 



FIG. 293. 



SIDE VIEW OF THE RIGHT-ANGLE CARBON ARC WITH 10 and WITH 

 20 AMPERES OF DIRECT CURRENT 



A 10 ampere arc, B 20 ampere arc. The size of the positive crater is 

 markedly larger with the higher amperage. 



The lower pictures were made by an instantaneous exposure. 



The upper pictures were made by a double exposure, that is, an instantaneous 

 exposure with the current on, to show the craters and the arc stream, and then 

 an additional exposure of 90 seconds with the current off to bring out the car- 

 bons. For the second exposure a 40 watt, mazda lamp was used for illuminat- 

 ing the carbons. 



