PHOTOMETRY OF THE ELECTRIC ARC 



FIG. 50. 



In addition the spectra of different parts of the arc show 

 decided differences. The arc has indeed been divided into 

 two or three different regions by those who 

 have examined its spectrum. This is more or 

 less arbitrary but will help us to gain some 

 idea of the colors in different parts. For ex- 

 ample, Foley 1 divided the carbon arc into 

 three parts, as shown in Fig. 50. The first 

 region is violet, the second is blue and the 

 third yellow. At times the first region ex- 

 tends entirely across from the anode to the 

 cathode. At other times it extends only 

 part way as shown in the diagram. 



The spectra of all parts show a great many lines. 

 Many of these belong to the carbon spectrum, but even 

 when the purest carbon is used, there are also many lines 

 which can be identified as lines of various metals. There 

 are also many lines about which there is still more or less 

 dispute. It can not be definitely stated whether these are 

 carbon lines or metallic lines which do not ordinarily show 

 in the spectrum, but are brought out by the peculiar con- 

 ditions of the arc. 



The number of lines in the spectrum of the first region 

 is much greater than in those of the other two. This is 

 due chiefly to the fading out of carbon bands in the second 

 and third regions, but the lines of metals also show the 

 same tendency. Foley found the third part to be composed 

 largely of vapor of calcium and iron. 



Miss Baldwin 2 found that the carbon bands showed more 

 clearly at the positive pole and the metallic lines at the 



1 Phys. Rev., 5, 129; 1897. 



2 Phys. Rev., 3, 370; 1895. 



