CH. XIII] LIGHT FROM THE ARC 549 



FIG. 299. SIDE AND FRONT VIEWS OF THE INCLINED CARBON ARC WITH 15 

 AMPERES OF DIRECT CURRENT (EWON'S AUTOMATIC LAMP). 



The upper carbon (+c) is soft-cored and 18 mm. in diameter; the lower 

 carbon ( c) is solid and 12 mm. in diameter. 



This is to illustrate an automatic lamp with a magnet (m) to control the 

 magnetic blow; the use of a large, cored upper carbon (+c) 18 mm. in diame- 

 ter; and a small solid lower or negative carbon ( c) 12 mm. in diameter. 



Incidentally there is shown the wandering of the crater in the right hand 

 lower picture. When the crater wanders in this way the source of light is 

 outside the principal optic axis. 



Photographed with an instantaneous exposure for the arcs and with an 

 additional exposure of 90 seconds for the carbons and the blow magnet (see fig. 

 292-293). 



light is furnished by the incandescent gases of the arc stream. 

 Flame-arc carbons are not ordinarily used in projection. 



For purposes of projection, only the light from the positive crater 

 of the direct current arc, or usually from only one of the craters of 

 an alternating current arc need be considered. The large objective 

 of the rragic lantern utilizes the light from both carbons with 

 alternating current and this is important. 



752. The alternating current arc. Most conducting materials 

 when used as the terminals of an arc lamp will not allow a reversal 

 or even a very short interruption of the current without going out. 

 This property is used in the mercury arc rectifier. 



When carbon electrodes are used, however, the current may be 

 interrupted for a short interval, or the current may be reversed 

 without putting out the arc. 



When the alternating current is used, first one carbon and then 

 the other is positive. Craters of equal intensity are formed on both 

 carbons, but neither is as bright nor as large as is the single positive 

 crater when direct current of the same amperage is used. 



The light from a single crater is not steady but is intermittent. 



The process during one cycle can be described as follows : 



When the current is reversed so that, say, the upper carbon 

 becomes positive, the crater is fairly cool. For the short time it is 

 the positive crater, its temperature rises very rapidly. Whether 

 or not it momentarily reaches the temperature which it would if 

 permanently the positive crater is uncertain. The current dies 

 out and the crater cools rapidly. When the current has reversed 



