188 BIOLOGICAL EFFECTS OF RADIATION 



drop across the arc, 4.5 amp. current, and (&), at "low intensity," 44 volts 

 across the arc and 3 amp. current, are given in Table 16, together with 

 data on a new arc (Column 5) for comparison. The line intensities have 

 also been show^n graphically for the high -intensity condition in Fig. 8, 

 those for the low intensity in Fig. 27. Comparison of the data for high- 

 and low-intensity operation indicates the drastic change in both absolute 

 and relative distribution resulting when the current is dropped from 4.5 to 

 3 amp. This brings out the absolute necessity of maintaining accurately 

 controlled operating conditions. 



Figure 28 show^s the distribution in the infra-red for another type of 

 mercury arc, viewed end-on. For complete wave-length and intensity 

 data, the reader is referred to the original paper (49). 



Since lines are available only at certain definite wave-lengths, if a 

 wide range of representative points is to be secured in any photochemical 

 study, it is necessary to use other sources which emit radiation at different 

 wave-lengths. Another source which is available commercially is the 

 helium arc. It produces a brilliant yellow line and a moderately strong 

 red line. Recently, sodium arcs have been placed on the market. These 

 are chiefly valuable because of the intense radiation of the yellow D 

 lines. Another valuable source is the hydrogen discharge tube, pro- 

 ducing an intense red line, and other weaker lines in the blue and violet. 

 The chief objection here is that such sources are not available for high- 

 intensity operation in a commercial lamp. If the line spectrum referred 

 to above is to be obtained, the lamp must be operated with the hydrogen 

 in atomic condition. In order to obtain hydrogen in atomic condition, 

 it is necessary to operate the tube with a continuous flow of wet hydrogen, 

 usually supplied by electrolysis. Only that part of the positive column 

 remote from the electrodes can be used. If hydrogen is present in molec- 

 ular form, a band spectrum presenting a multiplicity of lines is obtained. 

 These lines cannot be readily isolated, so that such a source is not of any 

 value for monochromatic irradiation. 



Only in case of the mercury arc have sources been constructed suffi- 

 ciently standardized to make intensity determinations of any great 

 value. A wide range of sources furnishing other lines can be used. For 

 ultra-violet work, some of the most valuable are given in Table 17 

 (63, cf. pages 507-508). 



The arc lines are obtained when operated in a low-voltage arc and 

 are due to the nonionized metal. The spark lines are obtained at high 

 voltage or spark discharge and are due to the once-ionized element. 



Figure 29 (56, cf. page 185) shows the sources commonly used for 

 purposes of spectroscopic calibration. For a more complete list of the 

 stronger lines, see Smithsonian Physical Tables 616, 617, and 618 (63, 

 cf. pages 509-510). 



