NO. 17 ULTRA-VIOLET INTENSITIES McALISTER I3 



2.5 per cent higher than the monochromator vahie i,i8o microwatts 

 per cm' given at the bottom of column 7 in Table 2. For the low- 

 intensity arc the filter measurement is 44.8 microwatts per cm ^ at 

 250 mm from the arc. The monochromator value given at the bottom 

 of column 8 in Table 2 is 43.6 microwatts per cm ^ which is 2.7 per 

 cent lower than the filter observation. This is a fairly satis- 

 factory agreement between the monochromator sums and the filter 

 measurements. 



From Table 3 we obtain the factor 600/84 (or 19.8/2.8) =7.1 

 which can be used to reduce the intensities for the midsection of the 

 arc to total arc intensities. This factor applies to Figures i and 2 

 and to the data in Table 2. 



Coblentz (4) gives a value of 623 microwatts per cm ' for the 

 intensity of radiation of wave lengths less than and including A 3,130 

 A. at 60 cm from a 260-watt arc of this type. Assuming his arc to 

 be equally efficient in producing radiation in this region and neglecting 

 the absorption of air this reduces to 582 microwatts per cm ^ This 

 is 3 per cent lower than the value 600 microwatts per cm ^ given in 

 Table 3. Hulburt (6) gives a value for the radiation from a 650-watt 

 arc in the region 2,000 A. to 6,500 A. His value is given as 22.0 mm 

 deflection (with a thermopile sensitivity of 0.97 microwatts per cm ^ 

 for I mm deflection) measured at a distance of 240 cm from the 

 arc. Of the 45 mm length of his arc, 4 mm was exposed. Assuming 

 this 4 mm length to be typical and that the arcs are equally efficient 

 we can reduce this to " total arc " at a distance of 25 cm to compare 

 with the monochromator sum in Table 2 at the bottom of column 4. 

 The calculation gives a value of 23,000 microwatts per cm ^ which is 

 6 per cent lower than the monochromator sum 24,500 microwatts 

 per cm ^ (3,450x7.1). This agreement is better than could be ex- 

 pected because of the uncertainty in calculating the radiation from 

 the " total arc " from the ratio of the total length to the length 

 exposed. Suhrmann's measurements (7) are given as absolute but 

 differ by two orders of magnitude from the present work. The mono- 

 chromator he employed has a numerical aperture of 12.5 and the 

 entrant slit (5 mm by 0.20 mm) was placed 25 mm from the arc. 

 Thus probably less than i/ioo of the radiation coming through the 

 monochromator slit passed through the collimating lens and was 

 measured. This very likely explains his very low values. 



To obtain an " overall " value of the probable errors involved in 

 the intensity measurements given in Table 2 a hypothetical line 

 whose intensity is the average of all the lines measured has been 

 assumed. Although the probable error calculated for it will be some- 



