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12 MAGIC LANTERN WITH DIRECT CURRENT [Cn. I
§ 8. Double-pole switch. — It is important to have a double-pole switch near the lamp. By its means the operator can at any time turn the current on or off the lamp. When the switch is open no current can reach the lamp (fig. 1-3).
§ 9. Arc Lamp ; automatic type. — The lamp is needed to hold the carbons, and to provide a mechanism for moving them toward each other as they burn away (see § 12). The lamp may be of the automatic type in which there is a magnetic release or motor for the mechanism, so that the carbons are brought nearer together whenever the arc gets too long. If it is properly designed and constructed, the lamp will burn continuously as long as the switch is closed, and the carbons last. There should also be a hand-feed mechanism in these arc lamps, so that slight modifications may be made by hand when necessary ; furthermore, there must be arrangements for moving one or both carbons separately to correct any irregularity in the wasting away of the carbons.
§ 10. Fine adjustments. — There must be adjusting screws by 'means of which the lamp can be slightly raised or lowered, or moved to the right or to the left, to enable the operator to keep the crater of the positive carbon exactly in the axis. This is to compensate for the slight change in position of the crater as the carbons burn away (fig. 3).
§ 11. Arc lamp, hand-feed type. — In this form of arc lamp the operator must work the mechanism by hand. The carbons usually have to be moved nearer together every four or five minutes. As with the automatic type, one or both carbons should be movable independently, and there should be fine adjustments (§ 9, 10).
§ 12. Carbon Terminals. — As a light source for projection, carbon terminals or electrodes are used in the arc lamp. With a direct current the carbons burn away unequally, the upper, positive carbon, wasting about twice as fast as the lower, negative carbon. If the carbons are of equal size and quality, the mechanism of the lamp must move the upper carbon about twice as fast as the lower one. Sometimes a lamp with equal motion for the upper and lower