No. 6.] 



STUDY OF ARC SPECTRA. 



455 



Remarks. 



As has been noted in a previous paragraph, the Hnes of the 

 ordinary arc which are strongest at the negative carbon increase 

 in intensity in the outer sheaths, while the reverse is true with 

 the lines from the positive carbon. When the metals K, Na, Li, 

 Ba, Sr, and Ca are introduced, the lines from the positive carbon 

 are weakened. The effect is most marked in the case of potas- 

 sium. Barium reduces the carbon bands the most. 



The metals Cu and Ag, from the group I. b, give the spectrum 

 in about its normal condition, with the lines due to the especial 

 metal added ; while the metals Cd and Zn increase the intensity 

 of the bands and diminish the strength of the metallic lines 

 originally present near the negative pole. 



It has been frequently observed by others who have worked 

 with metallic spectra by this method, that it is necessary to have 

 added resistance in the circuit which supplies the arc, when metals 

 are used, in order to maintain a constant length of arc. The 

 amount that had to be added in the course of these experiments 

 varied very much with the different metals. 



In a circuit of such electromotive force that 3 ohms resistance 

 were necessary to give the ordinary arc its normal voltage (48 v), 

 it was found that the following resistances would restore the arc 

 to its proper condition : 



When the metals were used, the temperature of the carbons 

 was apparently much less than with the pure carbon points. 

 The temperature of the positive carbon was most affected, and it 

 did not waste so rapidly in proportion to the negative as when 

 no metallic salt was in the core. 



