ARTIFICIAL SOURCES OF ULTRAVIOLET RADIATION 139 



proper temperature and to store up the mercury used in starting the 

 burner to arc. 



The arc is started by tilting the burner so that a series of mercury 

 globules runs from the cathode to the anode and back to the cathode 

 chamber. The arc first strikes between these globules, and when the 

 device is operating with sufficient resistance in series (it is a low-pressure 



80 



60 



40 



20 



2300 2500 2700 2 o 900 



Wavelengths A 



3100 



3300 



Fig. IV-5. Distribution of the energy among the wavelengths emitted by a 

 quartz mercury-vapor arc. The right-hand axis shows the relative erythemal 

 effectiveness of the various wavelengths at 50 cm from the arc. 



arc) the light column appears to fill the whole arc tube uniformly. In 

 this condition its spectrum shows only the strongest mercury lines and 

 no continuous spectrum is visible. When the arc is adjusted for opera- 

 tion at high intensity, it starts as a low-pressure arc but, as it heats, 

 changes to the high-pressure condition. In this state, indicated by an 

 apparent concentration of the light into a narrow thread of great inten- 

 sity in the axis of the arc tube, a continuous spectrum is superimposed 

 upon the mercury line spectrum, several additional mercury lines 

 become visible, and the ultraviolet radiation is greatly intensified. 



The quartz mercury arc from which the data for Fig. IV-5 were 

 obtained was rated and operated at 143 volts and 4.5 amperes. It 

 radiated considerable energy shorter than 2537 A, and, notwithstanding 

 the low intensity of these short wavelengths, considerable erythema can 

 be produced by them. In substituting this arc for ultraviolet sunlight, 

 the radiations shorter than 3150 A are of primary importance. The 

 quartz mercury arc radiates strongly in the following groups of wave- 



