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 carlxm 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 1/100 second, the aperture b>eing F/22. A color screen was used to cut out most of the violet, so that the ^.rc 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 cf a 40 watt, mazda lamp. b Vertical carbons with a 20 ampere direct current. No color screen. Expo sure 1/100 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 relatively 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.