PHOTOMETRY OF THE ELECTRIC ARC 145 



The matter is one of much importance from a theoretical 

 point of view, as almost nothing is known concerning the 

 reason why very different spectra are given by the same 

 element under different circumstances. A knowledge of 

 the cause for this difference would undoubtedly throw much 

 light on the construction of the atom and its behavior when 

 disturbed by electrical discharge. Even if the phenomena 

 are not as simple as was supposed by Lenard, there still is 

 no question but that the spectra of elements differ in 

 different parts of the arc and a further 'study of these 

 variations may throw light on this question. 



An examination of the spectra of the arc under different 

 pressures has been made by Humphreys. 1 He found that 

 as the pressure increased the width of the lines increased, 

 some, however, more than others. The metallic lines were 

 somewhat displaced; the carbon lines were not. 



The statement has been made by Steinmetz 2 that in 

 general the spectrum of the arc is that of the cathode 

 material. This, however, would appear to be true only 

 when the anode is not heated to a temperature sufficiently 

 high to vaporize it. With a carbon anode in air, or with 

 the majority of metals in air this does not appear to be the 

 case. Cady and Arnold, for example, found that with 

 ordinary currents the spectrum of the anode was very 

 apparent. 3 



1 Astro-phys. Journ., 26, 18 and 36; 1907. 



2 Trans. Intern. Elec. Cong., St. Louis, 2, 711; 1904. 



3 Further work has been done by the following: 



Snow, Phys. Rev., I, 108; 1893. 



Duffield, Astro-phys. Journ., 26, 375; 1900. 



Hallwachs, Ann. d. Phys., 13, 38; 1904. 



Crew, Astro-phys. Journ., 20, 274; 1904. 



Crew and Spence, Astro-phys. Journ., 22, 199; 1905. 



