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April, 1930]
RADIATION CHARACTERISTICS
415
weakened by the absorption of the glass, and there is but little radiation below 3000 A. In Fig. 13 the broken line represents the distribution of energy in the spectrum, both arc and filament being included. The solid line marked "I" is the filament and electrode radiation, and the height between the solid line and the broken line is the arc radiation. A comparison of the regions about 3000 A. in Figs. 5 and 13 will show the effects of the glass. Later a comparison will be made to point out how this weakened spectrum compares with the sun in ultra-violet.
The growth of arc radiation with current is seen by Fig. 14 to be
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FIG. 13. Complete radiation from Type S-l.
slight in amount. It has been found that mercury arcs in general are apt to have both currents and voltages that give maximum outputs, and the relative uniformity of arc output is therefore in line with previous lamps. The incandescent output is shown in Fig. 14 to be nearly constant between 24 and 27 amperes and to rise rapidly at 30 amperes. This rather unique characteristic for tungsten is caused by the dual nature of the lamp. The negative resistance characteristic of the arc lowers the voltage across the filament as the current increases from 24 to 27 amperes, and the light from the filament decreases. At the same time the electrodes become more luminous, giving a small net increase of tungsten light. From 27 to 30