CH. XIII] 



THE ELECTRIC ARC 



537 



FIG. 292. 



THE VERTICAL CARBON ARC WITH 20 AMPERES OF DIRECT 

 CURRENT. 



a Vertical carbons with the positive carbon above and the negative carbon 

 below. This shows that the large crater is on the positive carbon and the small 

 crater on the negative carbon. Between the two craters extends the arc stream 

 of hot gases. 



This photograph was made with an exposure of i/ioo second, the aperture 

 being F/22. A color screen was used to cut out most of the violet, so that the 

 arc stream would not obscure the craters. A subsequent exposure of 90 seconds 

 was made without a color screen and with an aperture of F/8. The illumina- 

 tion during this exposure was by means of a 40 watt, mazda lamp. 



b Vertical carbons with a 20 ampere direct current. No color screen. 

 Exposure i/ioo sec.; opening F/22. 



This shows the size of the two craters ; it also shows the conical arc stream 

 almost as light as the craters. This is because the violet light which has 

 relativelv little effect in illumination has a great effect on the photographic 

 plate. 



This picture shows how the carbons, the craters and the arc stream appear 

 in an instantaneous view to the photographic plate, while the one at the left 

 (a) gives much more nearly the appearance to the human eye. with an instan- 

 taneous view. 



