. Radiometric investigation of water of crystallization, light filters and standard absorption bands . per cent at *1, 67 per cent at , 20 percent at ^, and per cent at It may therefore beassumed that a 2-cm layer of a 2 per cent solution (2 g CuCl2in 98 cc H20) of cupric chloride absorbs all the infra-red and transmits from 80 to 85 per cent at ^ and about70 per cent at *, which is in the region of the yellow mercuryline. In separating the yellow from the green or blue lines it is, ofcourse, necessary to combine the absorption cell of cupric c
. Radiometric investigation of water of crystallization, light filters and standard absorption bands . per cent at *1, 67 per cent at , 20 percent at ^, and per cent at It may therefore beassumed that a 2-cm layer of a 2 per cent solution (2 g CuCl2in 98 cc H20) of cupric chloride absorbs all the infra-red and transmits from 80 to 85 per cent at ^ and about70 per cent at *, which is in the region of the yellow mercuryline. In separating the yellow from the green or blue lines it is, ofcourse, necessary to combine the absorption cell of cupric chloridewith a monochromatic red, green, or blue (for the blue, the cop-per sulphate solution may be sufficient) glass. This, of course,reduces the intensity still further, but it will hardly fall below50 per cent of its original value. When we consider that thesensitivity of a bolometer is closely proportional to the squareroot of the surface and that a linear bolometer is never largerthan 1 by 15 mm (usually mm), while a surface bolo- 66o Bulletin of the Bureau of Standards [vol. 7, No. 4. .6 .8 Fig. Yl.—Cupnc Chloride, CuCl2+2H20 Cobient=\ Water of Crystallization 661 meter is at least 40X50 mm, the advantage of the latter com-bined with an absorption cell is apparent. Take, for example,the determination of the mechanical equivalent of light, in con-nection with which the present question of absorption screenswas undertaken. The determination of the mechanical equiva-lent of light requires an accurate measurement of the candle-power of the light used and its value in energy units. Thedetermination of the candlepower is an easy matter. The deter-mination of the energy equivalent is beset with such difficultiesthat measurements made with the best instruments (absolutebolometers and calorimeters) for measuring radiation in abso-lute units differ by 5 to 10 per cent. Until some of these radio-metric difficulties have been overcome it is, of course, futile t
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