2 SMITHSONIAN MISCELLANEOUS COLLECTIONS VOL. 8/ 



curves of the quartz mercury arc using an effective slit width of 

 200 A. and lOO A. at A 4,000 A. respectively. Hulburt supplemented 

 his thermopile measurements with photocell observations and obtained 

 a smaller effective slit width. ( 10 A. at A 2.500 A., 20 A. at A 

 3,000 A.). The present work was done with an effective slit width 

 of 2 A. at A 2,300 A. which increased to 12 A. at A 4,000 A. Since 

 the present experimental work was completed the text " Lichtelek- 

 trische Zellen " by Simon and Suhrmann (7) has been published. 

 In it appear energy curves of the mercury arc spectrum obtained by 

 Suhrmann which apparently have not been published elsewhere. The 

 resolution attained is nearly identical with the writer's, and the 

 measurements are given as absolute. The relative intensities agree 

 fairly well with the present work, even though the arcs were of 

 different type. However, the absolute values disagree by more than 

 two orders of magnitude. This will be discussed later. 



EXPERIMENTAL 



These measurements were made on the Cooper-Hewitt 220 volt 

 D.C. quartz arc in the spectral region 2,100 A. to 7,000 A. Observa- 

 tions were taken on four " vertical " arcs and one " horizontal " arc. 

 Two sets of measurements were made: i, the arcs running under 

 normal power consumption of 4.5 amperes with 150-volt drop across 

 the arc, and 2, a low power consumption of 3.0 amperes and 44 volts. 

 Subsequently these will be called " high intensity " and " low in- 

 tensity " operating conditions respectively. Radiation from a 20-mm 

 length of the arc midway between electrodes was measured with the 

 monochromator and thermocouple combination, care being taken that 

 no scattered light of any consequence was included. Filter measure- 

 ments with a bare (windowless) thermocoui^le were taken of the 

 radiation from this 20-mm midsection, as well as the radiation from 

 the total arc. These filter observations provide a measure of the 

 energy in different spectral regions with which the summation of the 

 line intensities can be compared. Also they provide a factor that can 

 be used — with no serious error — to reduce the line intensities for the 

 midsection of the arc to " total arc " intensities. A bank of storage 

 cells was used as a source of current for the low-intensity observa- 

 tions. A D.C. motor-generator outfit — regulated with a .synchronous 

 motor — was used for the high-intensity work. In both cases the choke 

 coil and series resistance furnished by the manufacturer were used 

 to steady the arc. 



A single-junction vacuum thermocouple provided with a crystal 

 quartz window and of a type described elsewhere (8) was used for 



