ON THE SOURCES OF LUMINOSITY IN THE 

 ELECTRIC ARC* 



By Henry Crew and Olin H. Basquin. 



Presented by Charles R. Cross, March 9, 1898. 



When one considers any actual source of luminous energy, many 

 possible causes of luminosity suggest themselves. ' In the case of an 

 ordinary tallow candle, we have a considerable development of heat at a 

 temperature high enough to melt platinum, while at the same time hydro- 

 carbons are vaporized, decomposed, and again united with elements of 

 the atmosphere, to form the well known gaseous products of combustion. 

 Is the light of the candle flame immediately due to the high temperature 

 or to the chemical action which is going on ? Evidently these chemical 

 processes do not take place except at high temperatures. This fact, how- 

 ever, only makes high temperature a necessary, not a sufficient, condition 

 of luminosity. Kayser and Runge have shown that a large part of the 

 spectrum of the carbon arc is due to the union of carbon and nitrogen, — 

 perhaps to form CN. Runge and Paschen find that an atmosphere of 

 oxygen is almost a sine qua non for obtaining the compound line spec- 

 trum of S and Se. 



In the Plixcker tube, in the electric arc, and in the electric spark, we 

 have, beside chemical affinity and high temperature, still another possible 

 cause of luminosity, viz, the electric and magnetic field. 



"Why does the oscillatory discharge f give the blue spectrum of argon, 

 and the deadbeat discharge the red spectrum ? Is the temperature so 

 different in these two cases as to account for the different spectra, or does 

 the rapidity with which the electric potentials vary determine to some 

 extent the kind of atomic shock produced ? That the atomic motions are 

 affected by a magnetic field is a part of the recent brilliant discovery of 

 Zeeman.t 



* The following investigation has been made possible through financial aid 

 extended by the Trustees of the Rumford Fund. 



t Trowbridge and Richards, Amer. Jour. Sci., Vol. III. pp. 15-20 (1897). 



X Work of Graham at Berlin shows that the rate of variation of potential in 

 the Geissler tube keeps pace with the variation of luminosity, — as R. W. Wood 

 observed to be the case with temperature. 

 VOL. xxxiii. — 22 



