CH. II] MAGIC LANTERN WITH ALTERNATING CURRENT 75 



must see to it that the carbons are brought toward each other 

 occasionally by turning the proper screws. With moderate cur- 

 rents the lamp will run from five to ten minutes without feeding, 

 but the greater the amount of current the oftener must the carbons 

 be fed together. As stated above, the noise increases with the 

 length of the arc ; therefore the carbons should be brought nearer 

 together every two to four minutes. 



1 18. Shadows on the screen. All the defects indicated under 

 "troubles" in chapter i (83) for the direct current light are liable 

 to appear when using alternating current. This is somewhat 

 complicated by the presence of an equally brilliant crater on both 

 the upper and the lower carbons. As with direct current, there is 

 less trouble with right-angled carbons than with vertical or inclined 

 ones. With right-angled carbons the defect is greatest when the 

 lower carbon is too high, thus shading the upper carbon, as in fig. 

 25 A (for the shadows see fig. 24-25, 27-29). As with the direct 

 current, the greater the aperture of the projection objective, the 

 less marked is the screen defect of a slight mal-position of the car- 

 bons. (See also Ch. Ill, 127, Ch. IX, 417, and Ch. X, 488 for 

 the arc lamp with small carbons to be used on the house lighting 

 system). 



