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 when- 

 ever the arc gets too long. If it is properly designed and con- 

 structed, 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 arrange- 

 ments 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. Some times a lamp with equal motion for the upper and lower 



