THE CARBON ARC. 73 



EMISSON SPECTRUM OF THE CARBON ARC. 



The emission spectrum of carbon was previously examined by the 

 writer in connection with the emission spectra of metals and salts of metals 

 in the carbon arc. It was found that the violet part of the carbon arc had 

 several emission lines near 1 fx, followed by a weak continuous spectrum 

 with a maximum at about 2.5 it. There seemed to be a slight emission 

 band at 4.52 ti, which on subsequent examination could not be reproduced. 

 The writer, therefore, concluded that in the combustion of the carbon 

 electrodes no C0 2 or CO is formed, but that the carbon passes off as a 

 vapor. 



Furthermore, it was found that when salts of the alkali metals (Na, 

 K, Li) were present, even in small quantities, there was a strong emission 

 line at 4.52 fi. 



The radiometer used was not very sensitive, while the conclusion that 

 the carbon arc was mainly vapor of carbon did not seem satisfactory, 

 hence the work was repeated, using electrodes of ordinary gas carbon and 

 also of pure Acheson graphite. The latter is difficult to burn even when 

 the length of the arc is only 2 mm. long. 



In the present examination a Rubens thermopile was used. The 

 galvanometer period was 10 seconds, and its sensitiveness was about 

 2.5 Xio~ 10 amperes. The spectrometer slit was 0.1 mm. wide and 2 mm. 

 long, and an image of the blue vapors of the arc was projected upon it by 

 means of a short focus mirror. The carbon electrodes were of the usual 

 diameter, 12 mm., while two sizes (5 and 10 mm. diameter) of graphite 

 electrodes were used. An examination was also made using one carbon 

 and one graphite electrode. 



In fig. 50 are given the emission curves of the carbon arc, a and b for 

 a solid electrode with soft core, using 15 amperes (D. C), while curve c is. 

 for hollow electrodes (2 mm. holes). The latter was examined to learn 

 whether the air, drawn in through the hole, had any effect which appears 

 negative. 



In curves d and e are given the emission curves observed at various 

 times, under various conditions, when the hollow carbons contained the 

 salts of the metals NaCl and KC1. The length of the arc was from 5 to 

 7 mm. and in some instances was as much as 12 mm. On the whole, 

 whatever the current from 5 to 15 amperes there is always an emis- 

 sion band at 4.5 pt when salts of the alkali metals are in the arc, while the 

 pure carbon (on 15 amperes) had little if any selective emission in this 

 region. 



An examination was then made of the emission of pure graphite (Ache- 

 son's). The direct-current arc was maintained with difficulty and could 

 not be made longer than 4 to 5 mm. 



In fig. 51 are given the observed results for the emission of pure graphite 



