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April, 1930]
RADIATION CHARACTERISTICS
409
region, and a third grouping about 5500 A., the center of the visible region. This coincidence of energy with three regions where man's interest is concentrated makes the mercury arc one of the most interesting of lamps.
The lower limit of the therapeutic action is at about 2800 A. and it is sometimes desirable to eliminate shorter wave-lengths. This is done by an optical filter, as is illustrated in Fig. 6, for the therapeutic group of lines, where the filter passes those lines of most value in
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FIG. 5. Energy in spectrum of C. H. Uviarc burner.
therapeutic effect and eliminates much of the valueless short wave energy.
In order to attain the maximum intensity in the 2800 A. to 3200 A. region, the current and voltage must be held within certain limits. The radiation was found to show a pronounced maximum at 3.75 amperes when the voltage was held at 72.5 volts, and with a fixed current of 3.75 amperes a practical maximum was found at 72.5 volts. These figures are practically the specified operating figures, and attempts to force the output by overloading the lamp will result in a decrease in the output of the desired therapeutic energy.
The depreciation rate of the quartz mercury arc is always slow, and in this respect it is similar to the low pressure arc in having a long life with radiation that is not rapidly changed by the blackening of the tube. Thus tests have shown a 24 per cent depreciation in