184 BIOLOGICAL EFFECTS OF RADIATION 



oration produces: (o) An increase in resistance of the filament due to loss 

 of metal by evaporation, thus changing the operating characteristics of 

 the lamp. (6) An accumulation of metalhc deposit upon the walls, thus 

 introducing effectively a filter of high absorbing power which varies 

 markedly in its characteristics with wave-length. Furthermore, it is 

 not satisfactory to use the specified temperature of a lamp for certain 

 operating conditions as a basis for assignment of corresponding curves 

 representing radiation as a function of wave-length. It is necessary that 

 the actual operating conditions be determined at the time of the exposure, 

 unless a special mode of procedure has been carefully followed. A lamp 

 for standardization purposes should be seasoned for a considerable length 

 of time; second, its characteristics for different operating conditions 

 should be accurately determined by a competent laboratory; third, it 

 should be operated at a relatively low temperature; and fourth, it should 

 be very little used. It is necessary, therefore, for a laboratory under- 

 taking careful radiation work to maintain a number of lamps to serve as 

 secondary and tertiary standards used only during brief intervals of 

 exposure. Refer to Bureau of Standards pubhcation on radiation stand- 

 ards (23). Table 15 gives data on a number of commercial lamps. 

 For further information, the reader is referred to the original articles (30). 



CARBON ARC 



Carbon arcs offer the highest available artificial temperatures, 

 4000°K. being easily attainable. The positive crater is used as a source 

 of continuous radiation. The objections to these sources are: (a) They 

 are difficult to maintain under steady operating conditions, owing to the 

 gradual deterioration of the crater. (6) Absorption and emission inevi- 

 tably occur, owing to the hot gases surrounding the crater. By intro- 

 ducing salts into the core of the carbon, selective emission in the hot gases 

 may be taken advantage of to enrich different parts of the spectrum. 

 Extensive literature is furnished by the manufacturers, on wave-length 

 distribution of available impregnated carbons. 



SOURCES OF SELECTIVE EMISSION 



Electrical discharge through gases furnishes a class of sources of great 

 value in radiation work. Such sources produce relatively pure mono- 

 chromatic radiation at a number of particular wave-lengths. Thus, 

 spread out in a spectrum, lines appear corresponding to radiation gener- 

 ally extending over only a few Angstroms of wave-length range, and often 

 over only a fraction of an Angstrom. For photochemical purposes, 

 these may be considered as ideally monochromatic, their width being 

 in most cases far below the resolution of a monochromator for irradiation 

 purposes. 



