24 MAGIC LANTERN WITH DIRECT CURRENT [Cn. I 



magnetic release will allow the carbons to come in contact and 

 separate slightly so that the arc will be of the correct length. 



If the lamp is of the hand-feed type the operator must start it 

 by bringing the carbons in contact and then separating them a 

 short distance (3 to 4 mm.; }/% in.). This is done by turning the 

 feed screws by hand (fig. 3, F. S.). 



31. Managing the arc lamp during the exhibition. For an 



automatic lamp, the operator has only to close the switch to start, 

 and to open the switch to stop the lamp. The automatic mechan- 

 ism is supposed to keep the lamp burning in the best manner. 

 From the uneven burning of the carbons it is sometimes necessary 

 to make slight adjustments by hand even with automatic lamps. 

 This is easily accomplished by turning the proper screws present 

 for the purpose (fig. 3, F. S., L. A., V. A.). 



For the hand-feed lamp the operator must bring the carbons 

 closer together every four to five minutes or oftener by turning the 

 feed screws. If this is not done the distance between the carbons 

 soon becomes too great for the current to pass, and the lamp will go 

 out. Allowing the lamp to go out when it should not is one of 

 the things for the operator to avoid. 



32. Amount of current to use. This depends upon the kind 

 of arc lamp used (Ch. XIII), the screen distance, and the character 

 of the lantern slides. For dark lantern slides or long distances 

 more current must be used than for clear lantern slides and short 

 distances. 



For a screen distance up to 10 meters (33 ft.) and a right-angled 

 arc lamp (fig. 1-3) one will rarely need more than 12 amperes. 

 For a screen distance of 1 5 to 25 meters (50 to 80 ft.), 1 5 or at most 

 20 amperes should suffice. If more than 20 amperes are needed to 

 give the proper brilliancy to the screen images something is wrong 

 with the slides, the room, or the lantern itself, or more probably 

 with the management of the lantern. (See under Ammeter 7). 



33. When to light the lamp. The room should never be 

 totally dark during an exhibition. The incandescent lamp men- 



