Tables 378 and 379 347 



STANDARD WAVE LENGTHS 



TABLE 378. — Primary Wave-Length Standard. Definition of Angstrom 



The wave length of the red cadmium line in dry air, 15 C (hydrogen 

 thermometer), 760 mm of Hg pressure, gravity at latitude 45 being 980.67, 

 shall be taken as 



6438.4696 Angstroms 



The cadmium light shall be produced by a high-voltage, internal electrode 

 vacuum tube, volume greater than 25 cm 3 , exciting current less than 0.05 am])., 

 temperature not higher than 320 C. When connected to usual high voltage 

 the tube shall be nonluminous at room temperatures. (Trans. Int. Union Solar 

 Res., 2, 20, 1907. Trans. Int. Astron. Union, 2, 40, 1925.) 



TABLE 379. — International Secondary Standards. Iron Arc Lines 



The wave lengths are observed in air at 15 C, 760 mm pressure. The arc 

 should have its anode below, consisting of a bead of iron oxide supported in 

 the hollowed upper end of a rod of iron or copper at least 10 or 15 mm 

 diameter. The cathode is to be a rod of steel 6 or 7 mm in diameter having a 

 massive cylinder of brass or copper fitted close to the end, so that only 2 or 

 3 mm of the rod protrude. The arc is to be not less than 12 mm long, pre- 

 ferably 15 to 18 mm. The line voltage may be no or more and the current 

 strength 5 amperes or less. A horizontal central cone at right angles to the 

 axis of the arc not exceeding 1.5 mm in vertical dimension is to be used. (See 

 Trans. Int. Astron. Union, 3, n, 1929, for further details.) 



The wave lengths are in International Angstroms. They have been newly 

 referred to the red cadmium line. The results indicate the need of a slight 

 revision of the standards formerly adopted upon which all wave lengths in the 

 International System hitherto made have been based. The corrections to be 

 applied to the previously adopted standards and measures based upon them 

 to reduce them to the new standards are : 



Significance of small letters in following table : (a) Low-temperature lines; 

 always sharp and symmetrical ; energy level low ; pressure displacement small ; 

 limits of upper terms for Fe 19,700 to 32,500 cm" 1 , (b) Symmetrical under 

 pressure, but showing a slight dissymmetry toward red, or an unsymmetrical 

 reversal under high pressure and in the high-current arc ; energy level and 

 pressure displacement medium; limits of upper terms for Fe 32,500 to 41,500 

 cm" 1 , (d) High-temperature lines; asymmetrical toward violet: pole-effect 

 large and negative; energy level high; pressure displacement large; limits of 

 upper terms for Fe 53,500 to 55,000 cm" 1 . (Carnegie Publ. 396, Mt. Wilson 

 Obs.) The letters r and R indicate narrow and wide reversals, respectively, 

 as observed by Burns. 



Smithsonian Tables 



