THE ELECTRIC ARC 



141 



mercury vapor emits energy of only the characteristic wavelengths of 

 mercury, comparable to those shown in Fig. IV-6. 



2600 



2800 



3000 

 Wavelengths in A 



3200 



3400 



Fig. IV-6. Distribution of energy curve of the " Sunlight Lamp " from data by 

 A. H. Taylor [1931]. 



The following average values for the ultraviolet output are obtain- 

 able when the arc strikes in a mixture of mercury vapor and argon gas 

 enclosed by a Corex D bulb of about 1-mm thickness: 



Relative Ultraviolet Output 



Wavelength 

 Output 



3657 

 198 



3130 

 100 



3024 

 28 



2967 A 

 17 



If a shallow oxidized aluminum reflector, about 13 in. in diameter, is 

 used over the bulb, then at 30 in. a minimum perceptible erythema is 

 produced on average untanned skin by exposures of 8 to 10 minutes. 

 At a distance of 1 meter this type of lamp emits an energy flux one third 



o 



as great as midday summer sunlight in the spectral range 2800 to 3100 A. 



o 



The Corex D glass transmits about 59 per cent of the 3000-A wavelength. 



The Electric Arc 



Carbon arcs produce the highest available artificial temperatures, 

 4000° K being obtainable. The positive crater of a direct-current arc 

 is used as a source of continuous radiation. By introducing salts into 



