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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 mechanism 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