VISIBLE AND NEAR-VISIBLE RADIATION 



187 



The most used source of this type is the mercury arc. Efforts have 

 been made recently by the Illuminating Engineers' Society to produce 

 mercury arcs which will serve as rough standards of monochromatic 

 radiation. The intensities of the Unes for these arcs have been measured 

 and agreement secured between a number of laboratories to about 

 5 per cent. Such sources are, however, subject to variabiHty, not only 



XJ 



-^.^-AmAa. 



1.0 Oyu 



1.10 



1.20 



1.30 



J. /w 



u 



1 



J[L^ 



140^ 



1.50 1.60 1.70 1.80 



Fig. 28. — Near infra-red spectrum from a horizontal mercury arc, viewed end-on, operated 

 at 86 volts and 3.8 amp. {From data of McAlister, 49.) 



because of the difficulty of maintaining constant operating conditions of 

 voltage and temperature, but also owing to the variability resulting from 

 the changing transmissivity of quartz with time of operation, and the 

 gradual deposition of metal on the surfaces because of the gradual destruc- 

 tion of the electrodes. 



Absolute-intensity data, obtained by McAlister (50), on such an arc 

 at two different conditions, (a) at "high intensity," 143.5 volts potential 



