550 LIGHT FROM THE ARC CH. XIII 



its direction the crater is negative. The heating effect of the 

 current is small and the carbon tip continues to cool until the 

 current has again died out. This cooling still continues until the 

 current has again reversed its direction, and increased to a con- 

 siderable positive value. 



The temperature of an alternating current arc crater is at no 

 instant higher than that of a direct current arc crater with the same 

 amperage, and, as part of the time its temperature is much lower 

 than this, the average temperature will be lower than with a 

 direct current crater, hence the light will be less and of a yellower 

 color. 







I 



FIG. 300. SOME POSITIONS OF THE CARBON ELECTRODES USED IN PROJECTION 



LAMPS. 



A Vertical carbons. This position gives the least light along the principal 

 optic axis. 



B Inclined carbons. 



C Horizontal carbons. This arrangement is common for the search light, 

 and for the reflectors used in projection (see fig. 95). 



D The usual arrangement for the carbons when at right angles. The 

 upper or horizontal carbon is positive with direct current. '1 he crater on it is 

 in the optic axis and serves as the source of light with both direct and alter- 

 nating current. 



E Right-angle carbons in which the horizontal, positive carbon is below. 

 This is an unusual arrangement. 



V V-arrangement of the carbons for alternating current. With this 

 arrangement both craters supply light for the projection of lantern slides or 

 opaque objects. 



THE ARC LAMP AS AN ILLUMINANT 



753. The arc lamps suitable for projection purposes may have 

 the carbons in any one of five positions. 



1. With inclined carbons (fig. 297). 



2. With carbons at right angles (fig. 295). 



3. With converging carbons (fig. 300). 



4. With vertical carbons (fig. 292). 



5. With horizontal carbons along the axis (fig. 300). 



